U.S. patent application number 12/001792 was filed with the patent office on 2008-06-26 for plastic electronic component package.
Invention is credited to Jacob Shverdin, Keith Smith, Michael A. Zimmerman.
Application Number | 20080150064 12/001792 |
Document ID | / |
Family ID | 39512337 |
Filed Date | 2008-06-26 |
United States Patent
Application |
20080150064 |
Kind Code |
A1 |
Zimmerman; Michael A. ; et
al. |
June 26, 2008 |
Plastic electronic component package
Abstract
A plastic package for an image sensor or other electronic
component which comprises a plastic body, preferably of LCP
material, molded around a leadframe and defining a cavity in which
the image sensor is to be disposed. A lid assembly is provided
having a transparent glass lid retained in a plastic lid frame
which is weldable or otherwise bondable to the plastic body of the
package to enclose the image sensor mounted in the cavity. The
leadframe is usually composed of copper or a copper alloy, or a
ferrous alloy having a copper coating. An interfacial layer is
formed on the surfaces of the leadframe at least in those portions
which are in contact with the plastic body which serves to provide
substantially improved adhesion between the leadframe and the
plastic material to achieve a hermetic bond between the metal and
plastic materials. The interfacial layer is composed of a cuprous
oxide base layer formed on a surface of the leadframe, and a cupric
oxide layer formed on the cuprous oxide layer. The cupric oxide
outer layer has an acicular structure which provides an
interlocking mechanism for adhesion to the plastic material molded
thereto in forming the package.
Inventors: |
Zimmerman; Michael A.;
(North Andover, MA) ; Smith; Keith; (Methuen,
MA) ; Shverdin; Jacob; (Swampscott, MA) |
Correspondence
Address: |
WEINGARTEN, SCHURGIN, GAGNEBIN & LEBOVICI LLP
TEN POST OFFICE SQUARE
BOSTON
MA
02109
US
|
Family ID: |
39512337 |
Appl. No.: |
12/001792 |
Filed: |
December 12, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60874450 |
Dec 12, 2006 |
|
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Current U.S.
Class: |
257/433 ;
257/676; 257/E21.506; 257/E23.031; 257/E23.066; 257/E31.11;
277/590; 428/633; 438/123 |
Current CPC
Class: |
H01L 2924/15747
20130101; H01L 2924/12041 20130101; Y10T 428/12569 20150115; H01L
24/48 20130101; H01L 2924/181 20130101; Y10T 428/31678 20150401;
H01L 2224/48091 20130101; H01L 2224/4899 20130101; H01L 2924/15747
20130101; H01L 2924/3025 20130101; H01L 23/49861 20130101; H01L
2924/12041 20130101; H01L 27/14618 20130101; H01L 2924/00014
20130101; H01L 2924/00014 20130101; H01L 33/486 20130101; H01L
2924/09701 20130101; H01L 2924/16195 20130101; H01L 27/14683
20130101; H01L 2224/48091 20130101; H01L 2224/48227 20130101; Y10T
428/12618 20150115; H01L 23/3142 20130101; H01L 2924/00 20130101;
H01L 2924/00014 20130101; H01L 2924/00 20130101; H01L 2924/00014
20130101; H01L 2924/01079 20130101; H01L 2924/01012 20130101; H01L
2924/207 20130101; H01L 2924/00 20130101; H01L 2224/45015 20130101;
H01L 2224/45099 20130101; H01L 2924/181 20130101; H01L 23/10
20130101; H01L 2924/0102 20130101; H01L 2924/01078 20130101 |
Class at
Publication: |
257/433 ;
438/123; 257/676; 277/590; 428/633; 257/E23.031; 257/E21.506;
257/E31.11 |
International
Class: |
H01L 31/0203 20060101
H01L031/0203; H01L 23/495 20060101 H01L023/495; H01L 21/60 20060101
H01L021/60 |
Claims
1. An image sensor package comprising: a plastic frame molded
around a metal leadframe and defining a cavity; the leadframe
having a central portion in the cavity and a plurality of leads;
the central portion of the leadframe in the cavity adapted to have
an image sensor mounted thereon and connected to the plurality of
leads; a lid assembly having a transparent lid retained in a
plastic lid frame; and the lid frame being weldable to the plastic
frame to enclose an image sensor mounted in the cavity.
2. The image sensor package of claim 1 wherein the plastic frame
and the lid frame are each a high temperature thermoplastic
material.
3. The image sensor package of claim 2 wherein the plastic frame
and the lid frame are each a high temperature LCP material.
4. The image sensor package of claim 2 wherein the LCP material has
a melting temperature greater than about 300.degree. C.
5. The image sensor package of claim 2 wherein the LCP material has
a melting temperature greater than about 200.degree. C.
6. The image sensor package of claim 2 wherein the LCP material
contains filler particles to provide a CTE compatible with that of
the metal lead frame.
7. The image sensor package of claim 6 wherein the CTE of the LCP
material is in the range of about 6-25 ppm/.degree. C.
8. The image sensor package of claim 6 wherein the filler particles
are of nano size.
9. The image sensor package of claim 6 wherein the filler particles
are in the range of about 10-17% by weight.
10. The image sensor package of claim 2 wherein the leadframe is
copper.
11. The image sensor package of claim 2 wherein the leadframe is a
copper alloy.
12. The image sensor package of claim 2 wherein the leadframe is a
ferrous metal having copper surfaces.
13. A method of fabricating an electronic component package, the
method comprising the steps of: providing a metal leadframe;
providing on at least a portion of the surfaces of the metal
leadframe an interfacial layer of oxide material; molding a plastic
frame around the portion of the metal leadframe having the
interfacial layer, the frame defining a cavity; the leadframe
having a central portion in the cavity and a plurality of leads;
mounting an electronic component on the central portion of the
leadframe in the cavity and connecting the component to the
plurality of leads; providing a lid assembly having a transparent
lid bonded to a plastic lid frame; and welding the lid frame to the
plastic frame to enclose the component.
14. A hermetically sealable electronic component package
comprising: a high temperature plastic frame molded to a metal
leadframe and defining a cavity; the leadframe having a central
portion in the cavity and a plurality of leads; the central portion
of the leadframe adapted to have an electronic component mounted
thereon and connected to the plurality of leads; a lid assembly
having a lid of high temperature plastic retained in a lid frame of
high temperature plastic; and the lid frame being weldable to the
plastic frame to enclose an electronic component mounted in the
cavity.
15. The package of claim 14 wherein the metal leadframe has on the
surfaces thereof which are molded to the plastic frame an
interfacial layer of material effective to improve the adhesion of
the high temperature plastic frame to the metal leadframe.
16. The package of claim 14 wherein the leadframe is composed of
copper; and wherein an interfacial layer is formed on at least the
surfaces of the leadframe in contact with the plastic frame, the
interfacial layer composed of a cuprous oxide layer formed on the
surface of the leadframe and a cupric oxide layer formed on the
surface of the cuprous oxide layer.
17. The package of claim 15 wherein the interfacial layer has: a
first layer formed on a surface of the metal leadframe; and a
second layer formed on a surface of the first layer, the second
layer having a surface bonded to the high temperature plastic
frame.
18. The package of claim 15 wherein the leadframe has a copper
surface; the interfacial layer has a first layer of cuprous oxide
formed on the copper surface of the leadframe; and a layer of
cupric oxide formed on a surface of the cuprous layer, the cupric
layer having an acicular structure bonded to the high temperature
plastic frame.
19. For use in fabricating a hermetically sealed electronic package
having a high temperature plastic frame molded to a copper
leadframe and defining a cavity, a leadframe comprising: a first
layer of cuprous oxide formed on the surfaces of the leadframe at
least at the portions to be molded to the high temperature plastic
frame; and a second layer of cupric oxide formed on the first layer
and having an acicular structure bondable to the high temperature
plastic frame.
20. A hermetic seal between a metal element and a plastic element
comprising: an intermediate layer between the metal element and the
plastic element, the intermediate layer having: a layer of a first
oxide formed on a surface of the metal element; and a layer of a
second oxide formed on the layer of a first oxide, the second oxide
layer having an acicular structure bondable to the plastic
element.
21. The hermetic seal of claim 20 wherein: the first oxide is an
oxide of the metal material of the metal element; and the second
oxide is a different oxide form of the first oxide.
22. The hermetic seal of claim 21 wherein: the metal element has a
copper surface at least in the area bondable to the plastic
element; the first oxide is cuprous oxide and the second oxide is
cupric oxide.
23. For use in a hermetically sealable plastic electronic component
package having a plastic frame molded around a metal leadframe and
defining a cavity in which an electronic component is mountable, a
lid assembly comprising: a lid sized and configured to cover the
cavity of the plastic frame of the package; and a plastic lid frame
bondable to the lid and weldable to the plastic frame of the
package to enclose and hermetically seal the package cavity.
24. A method of constructing a lid assembly having a transparent
lid bonded to a plastic lid frame, the method comprising the steps
of: sawing a transparent lid to a predetermined size; injection
molding a plastic lid; and attaching the transparent lid to the
plastic lid by thermal bonding.
25. A method of constructing a lid assembly having a transparent
lid bonded to a plastic lid frame, the method comprising the steps
of: sawing transparent lid to a predetermined size; and injection
molding a plastic lid directly onto the transparent lid by
overmolding.
26. A method of thermally bonding a transparent lid to a plastic
lid frame, the method comprising the steps of: heating the
transparent lid in a fixture; heating the plastic lid in a mating
fixture; mating the transparent lid and the plastic lid to a fixed
displacement such that the material of the plastic lid contacts the
transparent lid and locally melts to it; and cooling the assembly
in place.
27. The method of claim 13 wherein the step of welding the lid
frame to the plastic frame comprises ultrasonic welding of the lid
frame to the plastic frame; and wherein the ultrasonic welding is
applied only to the perimeter of the lid frame.
28. A method of fabricating an electronic component package, the
method comprising the steps of: providing a metal leadframe;
providing on at least a portion of the surfaces of the metal
leadframe an interfacial layer of oxide material; molding a high
temperature thermoplastic frame around the portion of the metal
leadframe having the interfacial layer, the frame defining a
cavity; the leadframe having a central portion in the cavity for
mounting an electronic component, and a plurality of leads
connectable to the electronic component; providing a lid assembly
having a transparent lid bonded to a high temperature thermoplastic
lid frame; and welding the lid frame to the plastic frame to
enclosed the component mounted in the cavity.
29. The method of claim 28 wherein the LCP material contains
hydroquinine (HQ), teraphalic acid, isophalic acid, 2, 6 naphalene,
dicarboxyclic acid, 4-hydrobenzoic acid (HBA), bisphenol or
biphenol (BP), and hydroxynaphthoic acid; and filler particles of
at least one of the group consisting of talc, glass fiber, milled
glass and flake glass.
30. The method of claim 29 wherein the filler particles are in the
range of 10-50% by weight.
31. The method of claim 30 wherein at least some of the filler
particles have a particle size in the range of about 50-350
microns.
32. The method of claim 28 wherein the step of providing an
interfacial layer includes providing a first oxide on at least a
portion of the surfaces of the metal leadframe, and providing a
second oxide on the first oxide.
33. The method of claim 32 wherein the step of providing the
interfacial layer includes providing the interfacial layer on the
entire surface of the leadframe; and further including the step of
removing the exposed portions of the interfacial layer from the
leadframe after molding the high temperature thermoplastic
frame.
34. The method of claim 28 wherein the metal leadframe is copper
and wherein the interfacial layer is copper oxide material.
35. The method of claim 28 wherein the step of molding comprises
injection molding the high temperature thermoplastic frame; wherein
the step of providing a lid assembly comprises injection molding
the lid frame and bonding the transparent lid thereto.
36. The method of claim 28 wherein the step of welding comprises
ultrasonic welding the lid frame to the plastic frame.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit under 35 U.S.C.
.sctn.119(e) of U.S. Provisional Patent Application No. 60/874,450,
filed on Dec. 12, 2006, the disclosure of which is incorporated by
reference herein.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] N/A
BACKGROUND OF THE INVENTION
[0003] Image sensors such as those used in digital cameras and
other optical or image sensing equipment are conventionally housed
in a ceramic package. The ceramic package includes a ceramic frame
which is epoxy bonded to a glass lid or cover. The ceramic package
is expensive and not readily adapted to manufacture in strip form
or other multiple unit form, as is widely employed in the
semiconductor packaging industry. In addition, the use of epoxy as
a bonding agent presents several problems such as moisture
penetration through the epoxy bond, outgassing of the epoxy which
can contaminate the semiconductor device, and air leakage which
limits the ability to hermetically seal the ceramic package.
Further, it is difficult to accurately align the glass cover to the
ceramic frame so that the glass cover is parallel to the image
sensor surface. This alignment difficulty is caused by an inability
to control the thickness of an epoxy bead which is commonly
employed to seal the glass lid to the ceramic frame.
[0004] A conventional ceramic sensor package is shown in sectional
elevation view in FIG. 1. This package includes a two layer ceramic
substrate composed of an outer ceramic substrate 10 and an inner
ceramic substrate 12, on the upper surface of which is disposed a
CCD or CMOS image sensor 14. A ceramic frame 16 defines the cavity
18 in which the image sensor is located. The ceramic frame is
hermetically sealed to the peripheral surface of the substrate 10
and a boro-silicate glass lid or window 20 is sealed to the upper
surface of frame 16 by a UV curable adhesive. The contacts of the
image sensor are wire bonded to contacts 22 provided on the inner
surface of substrate 10. These contacts 22 are electrically
connected to outer contact pads 24 by conductive feedthroughs or
vias provided in substrate 10. The feedthroughs 26 are typically
plated with gold to enhance the electrical conduction between the
inner and outer contact areas. A UV curable adhesive is usually
employed to bond the glass window to the ceramic package frame to
prevent exposure of the semiconductor sensors to high temperatures
which are needed to cure other types of epoxy adhesives but which
can degrade or destroy the semiconductor sensors. The requirement
for UV curable adhesive materials limits the range of available
epoxies which can be employed in the conventional ceramic package,
since most epoxy adhesives are curable at elevated
temperatures.
[0005] The reliability of semiconductor and other electronic device
or component packages including image sensor packages is related to
the "airtightness" or hermeticity of the package. Hermeticity is a
measure of an ability of the package to protect the semiconductor
or other device housed in the package from an entrance of fluids
and moisture. Moisture or corrosive gases on or near a
semiconductor device can cause corrosion of the metallic traces on
the semiconductor device, and can lead to failure. Traditional
hermetic packages are made from metal, ceramic, or vitreous
materials. These materials have such low permeabilities that
moisture and fluids typically are impeded by these materials, and
cause a condensation on the semiconductor device or contamination
by corrosive gasses.
[0006] In addition to a permeation of fluids or moisture, fluids or
moisture can penetrate the image sensor package though "leaks" at
several interfaces. The interfaces for a ceramic package include a
metal/ceramic interface, along with epoxy/glass and epoxy/ceramic
interfaces. Any small openings at these interfaces allow fluids or
moisture to seep inside the image sensor package.
BRIEF SUMMARY OF THE INVENTION
[0007] The image sensor package according to the present invention
eliminates the need for ceramic components and employs a plastic
material which preferably is a high temperature liquid crystal
polymer (LCP) material. The package is useful not only for image
sensors but also for other light sensing or light emitting
semiconductor or other devices or components. A package in
accordance with the invention can also be used to contain
non-optical devices or components.
[0008] The image sensor package comprises a plastic body or frame,
preferably of LCP material, molded around a metal leadframe and
defining a cavity in which the image sensor is to be disposed. The
leadframe has a central portion in the cavity on which the image
sensor is mounted, and a plurality of leads which are connectable
to contact areas of the sensor. A lid assembly is provided having a
transparent glass lid retained in a lid frame which is also made of
plastic, preferably LCP material. The lid frame is weldable or
otherwise bondable to the plastic frame of the package to enclose
the image sensor mounted in the cavity. The leadframe is usually
composed of copper or a copper alloy, or a ferrous alloy having a
copper coating. An interfacial layer is formed on the surfaces of
the leadframe at least in those portions which are in contact with
the plastic frame. This interfacial layer serves to provide
substantially improved adhesion between the leadframe and the
plastic material and to achieve a hermetic bond between the metal
and plastic materials. The interfacial layer is composed of a
cuprous oxide base layer formed on a surface of the leadframe, and
a cupric oxide layer formed on the cuprous oxide layer. The cupric
oxide outer layer has an acicular structure which provides an
interlocking mechanism for adhesion to the plastic material molded
thereto in forming the package.
[0009] In another aspect of the invention a hermetic seal and
sealing technique is provided between a metal element and a plastic
element which utilizes an interfacial or intermediate layer between
the metal element and plastic element and which comprises a first
oxide base layer for providing strong adhesion to the metal
material and a second oxide layer formed on the first oxide base
layer and having an acicular structure for strong adhesion to the
plastic material.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0010] The invention will be more fully described in the following
detailed description taken in conjunction with the accompanying
drawings in which:
[0011] FIG. 1 is a cutaway elevation view of a ceramic package of
conventional construction;
[0012] FIG. 2A shows an image sensor package made in accordance
with the invention;
[0013] FIG. 2B shows the bottom of the package of FIG. 2A;
[0014] FIG. 3 shows a leadframe strip used in the package of the
invention;
[0015] FIG. 4A is a cutaway perspective view of the lid
assembly;
[0016] FIG. 4B is a perspective view of the lid assembly;
[0017] FIG. 4C is a cutaway elevation view of the assembly of FIG.
4A;
[0018] FIG. 5 is a cutaway elevation view of the ultrasonically
welded image sensor package;
[0019] FIG. 6 are photomicrographs showing the interfacial layer
between the copper leadframe and package frame material;
[0020] FIG. 7 is a diagrammatic elevation view of the interfacial
layer;
[0021] FIG. 8A is a flowchart illustrating the steps of package
fabrication;
[0022] FIG. 8B is a flowchart illustrating the steps of lid
assembly fabrication;
[0023] FIG. 9 is a flowchart of the steps typically employed by a
manufacturer in attaching a device in the package;
[0024] FIG. 10 is a photomicrograph illustrating dye leaking
through a conventional LCP package which is non-hermetic;
[0025] FIG. 11 is a photograph of the top and bottom of a hermetic
LCP package of the invention, after leakage testing; and
[0026] FIG. 12 is a graph illustrating the permeability of
packaging materials.
DETAILED DESCRIPTION OF THE INVENTION
[0027] The package, materials and method of package fabrication are
described in a preferred embodiment for an image sensor. The
invention is not to be limited to image sensor packages or packages
for other optical devices, but is more broadly useful for housing
other semiconductor, electrical and electronic devices, components
or circuits.
[0028] The package configuration can be of various forms to suit
particular packaging requirements. The package configuration may
vary in size and shape and can include electrical lead
configurations of many different forms. The invention is not to be
limited to any particular package type or configuration. The
invention will be described in the context of a package for a
semiconductor image sensor chip such as used in digital cameras and
other digital imaging systems and devices.
[0029] The image sensor package in accordance with the invention
comprises a high temperature thermoplastic body or frame,
preferably an LCP material, molded around a metal leadframe. An
image sensor chip is mounted in a cavity of the package on a
central portion of the leadframe and the chip is wirebonded or
otherwise connected to leads of the leadframe. A lid assembly
comprised of a glass lid retained in a high temperature
thermoplastic frame, also preferably an LCP material, is attached
to the package frame after the sensor chip has been mounted in the
cavity of the package. The frame of the lid assembly is
ultrasonically welded to the plastic frame of the image sensor
package to provide a hermetically sealed enclosure for the sensor.
The glass lid is chemically bonded to the plastic frame of the lid
assembly, preferably by thermal bonding. Alternatively, the glass
lid can be insert molded to the lid assembly frame. The glass is of
optical quality and is transmissive in the optical spectrum of
interest. For a photographic sensor, the glass is transmissive to
the visible light spectrum. For other purposes, such as for certain
LED packages, the glass is transmissive to UV light.
[0030] An image sensor package according to the invention is shown
in pictorial view in FIG. 2A and bottom view in FIG. 2B. The
plastic frame or body 30 defines a cavity area in which the image
sensor is mounted on the copper surface 32 provided by the
leadframe. Copper leads 34 provided about the periphery of the
cavity extend through the molded body to positions 36 on the bottom
of the leadframe as shown in FIG. 2B.
[0031] A leadframe is fabricated of copper or copper alloy using
conventional methods (e.g., etching or plating). For reasons of
cost and manufacturing ease, the leadframe is usually fabricated in
a reel. The leadframe is exposed to a treatment, described below,
to improve adhesion for molding with the plastic material. This
treatment can be performed in either reel-reel or strip format.
[0032] A portion of a leadframe strip is shown in FIG. 3 in which
six leadframe units 40 are disposed across the width of the strip
42. In one embodiment, six package bodies are molded simultaneously
across the width of the leadframe strip. The leadframe strip is
then advanced, and an additional six bodies are molded to the array
of leadframes disposed across the width of the strip. In similar
manner, the leadframe strip is populated with package bodies and
the strip is wound onto a continuous reel. Alternatively, the reel
can be separated into strips of intended length and width to suit
user requirements. In a further alternative, the package bodies
molded to each leadframe unit are separated or singulated into
individual units which are then supplied to a user for use in
packaging the sensor.
[0033] The lid assembly is shown in FIGS. 4A-4C. The lid assembly
comprises a high temperature plastic frame 50, preferably an LCP
material. A glass lid 52 is chemically bonded to the plastic frame
50 preferably by thermal bonding to provide a hermetic seal between
the glass lid and plastic frame. The glass lid is preferably a
boro-silicate glass of optical quality to provide appropriate light
transmission to the image sensor housed within the package. The
frame 50 has a recessed shelf area as shown in which the lid 52 is
disposed. The lid is bonded to the shelf 53. The plastic frame has
a peripheral configuration 54 which is complementary to a
configuration provided about the periphery of the plastic frame of
the package, as seen in FIG. 5. The lid assembly is ultrasonically
welded at these mating configurations of the lid frame and package
frame to provide an ultrasonic seal. The sealed package with the
lid assembly bonded to the package frame or body is shown in FIG.
5.
[0034] The leads are preferably made of an alloy of copper
including iron in a range between about 2.1% and about 2.6%,
phosphorus in a range between about 0.015% and about 0.15%, zinc in
a range between about 0.05% and about 0.2%, with the balance being
copper. Other combinations of these materials are, however,
acceptable. The leads are more preferably made of about 97.5%
copper, about 2.35% iron, about 0.3% phosphorus and about 0.12%
zinc. Such an alloy is available from Olin Corporation under the
UNS designation C19400.
[0035] Alternatively, the leads can be made of a ferrous alloy such
as alloy 42 which is a nickel-iron alloy. Copper is plated onto the
surfaces of the leadframe prior to molding of the plastic body or
frame to the leadframe.
[0036] The package body or frame is a thermoplastic material which
preferably is a high temperature liquid crystal polymer (LCP)
material. The plastic frame of the lid assembly is also preferably
an LCP material. For many applications, the package body is
composed of a Type I LCP material which has a relatively high
melting temperature in the range of 300-350.degree. C. This high
temperature material is beneficial to withstand the temperatures
employed for gold-tin die-attach which is often used for attachment
of the image sensor chip to the copper substrate of the package.
For other applications, such as for lead free solder die-attach, a
Type II LCP material can be used which has a melting temperature in
the range of about 280-320.degree. C.
[0037] The LCP composition includes filler particles which are
added for dimensional stability, adjustment of coefficient of
thermal expansion (CTE), adjustment of anisotrophy, lower
permeability, attaining a small particle size to reduce
contamination by dust, and optimizing their hermetic sealing. The
filler particles can include talc, glass, graphite, titanium
dioxide, calcium carbonate, mica, boron nitride, quartz, and fused
silica. The particles may be in the form of nanospheres, or platy
structures which are of flat plate-like configuration. A blend of
such particles forms may also be employed. In one composition, the
filler particles are talc in the form of a 50% blend of platy
structures and nanospheres. The talc particles may be less than 1
micron in diameter or width or may be in a larger size range of
about 1-3 microns. The platy structure of the particles interacts
with the LCP molecules to provide some "bending" in the molecule
which helps decrease anisotrophy. Talc in the range of about 30-50%
by weight in the LCP composition is one preferred range to optimize
the CTE of the material. Preferably the CTE of the LCP material
should be in the range of about 6 ppm/.degree. C.-25 ppm/.degree.
C. to be compatible with the CTE of the leadframe. The filler
particles of nano size are useful in enhancing the hermetic
sealability of the material to the leadframe. In another
composition, glass is used in various combinations of geometrical
sizes, including fibers, milled glass, and flakes.
[0038] The frame of the lid assembly is also preferably an LCP
material having filler particles preferably in the range of about
10-30% by weight.
[0039] Examples of LCP material compositions are shown in Table
1.
TABLE-US-00001 TABLE 1 Glass Milled Flake Sample Polymer Talc %
Fiber % Glass % Glass % 1 Polymer 10-40% 2 Polymer 30 10 3 Polymer
45 4 Polymer 10 35 5 Polymer 15 15 6 Polymer 20 20 7 Polymer 25 25
Polymer Composition Thermotropic LCP Containing the following
repeat units hydroquinine (HQ) teraphalic acid isophalic acid 2,6
naphalene dicarboxyclic acid 4-hydrobenzoic acid (HBA) bisphenol or
biphenol (BP) hydroxynaphthoic acid Talc = magnesium silicate
hydroxide, pophyllosilicate Glass Fibers = sio2, CaO, Al203, B2O3,
length 4000-13,000 microns Milled glass (same composition) 50-350
microns Flaked glass (same composition)
[0040] In order to enhance the adhesion of the leadframe to the LCP
package frame and to provide better hemeticity, an interfacial or
intermediate layer is provided on the surfaces of the leadframe at
least in the areas of the leadframe to which the plastic material
is to be molded. This interfacial layer is composed of two
sub-layers; namely, a base layer of cuprous oxide formed on the
leadframe or intended portions thereof, and a cupric oxide layer
formed on the cuprous oxide layer. The cupric oxide outer layer has
an acicular, dendritic or needle like structure which provides an
interlocking mechanism for adhesion to the plastic material molded
thereto in forming the package. Typically, the entire leadframe is
coated prior to molding with the interfacial material. After
molding of the frame to the leadframe, the exposed portions of the
intermediate layer material outside the areas of the molded package
body are removed.
[0041] The interfacial layer is provided by a chemical conversion
process by which the copper surfaces of the leadframe are oxidized
under process conditions which allow the cuprous oxide base layer
and cupric oxide outer layer to form. Under the appropriate
conditions, the cuprous oxide and cupric oxide layers grow or form
substantially simultaneously. An oxidizing material is employed
such as a chemical oxidizer which has been modified by the addition
of an alkaline solution of, for example, sodium chloride in an
amount to provide a reaction temperature which is greater than
about 125.degree. F. The reaction time and temperature after
application of the oxidizing material to the leadframe determines
the growth of the two interfacial layers. The reaction time is
greater than about ten minutes. In one embodiment, the reaction
time after exposure of the leadframe to the oxidizing solution is
about twenty minutes at a temperature of about 212-216.degree. F.
The leadframe is typically immersed in a bath of oxidizing solution
to provide the interfacial layer.
[0042] The interfacial layer has a thickness in the range of about
1-10 microns. The base layer is thinner than the cupric oxide layer
in a ration of about 1:5, although this range can vary under
various process conditions. The oxide material is of high density
and has high adhesion strength to the copper on which it is formed.
This dual oxide layer serves to relieve stress in the interfacial
layer and can withstand temperatures of at least 400.degree. C. to
avoid cracking or other degradation of the oxide which could
otherwise occur during molding of the package body to the leadframe
or by die-attach temperatures which may be employed in attaching
the image sensor or other chip to the package mounting surface. The
acicular structure of the cupric oxide coating typically has a
random pattern. The cupric oxide also provides protection for the
underlying cuprous oxide layer which is in contact with the copper.
Such protection is by shielding the underlying layer from damage by
hot temperatures which are present in molding the package and
during die attach, and which can cause degradation of the
copper/oxide interface.
[0043] The plastic package body molded to the leadframe which has
been provided with the interfacial layer achieves a moisture
resistant and hermetically sealed bond between the plastic body and
the leadframe. The subsequent bonding of the lid assembly to the
package body, after installation of the image sensor in the package
cavity, also provides a moisture resistant and hermetically sealed
bond between the package body and lid assembly thus resulting in a
hermetically sealed package.
[0044] The interfacial layer 60 is seen in the photomicrographs of
FIG. 6 which shows cross-sections of the LCP package body material
bonded to the copper leadframe. The interfacial layer, composed of
the cuprous oxide layer formed on the copper surface and the cupric
oxide layer formed on the cuprous oxide base layer, has the
following properties and benefits. The cuprous oxide forms a strong
bond to the copper leadframe and is resistant to delamination. The
acicular topography of the cupric oxide reduces surface tension to
promote wetting of the LCP for adhesion and enhanced surface
roughness for increasing the surface area for bonding of the LCP
material. The cupric oxide also provides an inert top surface which
impedes contamination and an inorganic low permeability material
for moisture resistance.
[0045] The interfacial layer 60 is illustrated in the diagrammatic
view of FIG. 7 and shows a cuprous oxide base layer (Cu.sub.2O) 62
for strong adhesion to copper leadframe 64, and cupric oxide layer
(CuO) 66 having an acicular or dendritic structure 48 and which is
molded to the LCP package material.
[0046] As stated above, the layer can be provided by a chemical
conversion process, which can be implemented in a standard in-line
bath. The leadframe can typically be in rack or reel-reel
format.
[0047] This interfacial layer can be used with ferrous metals and
alloys, as well as the copper described above. Such an interfacial
layer can also be used as an interface between other metal and
thermoplastic materials. By use of this interfacial layer, a metal
element can be hermetically sealed to a thermoplastic element for a
variety of purposes in which a hermetic seal is required between
the metal and plastic components. One such purpose is for
hermetically sealed electronic packages as described herein, but is
more broadly useful for a variety of electrical, electronic,
mechanical and other structures where a hermetic or near hermetic
seal is desired between metal and plastic materials.
[0048] FIG. 8A shows the steps of making the image sensor package.
The leadframe is made by stamping or chemical etching in step 70.
An interfacial layer is formed on the leadframe in step 72, at
least in the portions to be molded to the LCP material. The LCP
material is molded to the leadframe in step 74 to create the
package cavity. The leadframe is plated in intended portions, such
as the leads thereof, in step 76. In step 78, the image sensor
package may be singulated into individual pieces or packed in a
strip or reel form.
[0049] The fabrication of the lid assembly is shown in FIG. 8B. A
piece of glass is sawed or otherwise cut to a desired size in step
80. A ringframe of LCP material is made by injection molding in
step 82. The glass lid is attached to the ringframe by a process of
thermal attachment, such as thermal bonding in step 84. The lid may
be attached to the lid frame by other techniques, such as insert
molding.
[0050] FIG. 9 shows the steps a customer, or an end-user, may take
for mounting an image sensor device in the image sensor package. In
step 90, the device is bonded to the copper mounting surface of the
package cavity. The device contacts are wirebonded to potential
contacts of the package, in step 92. The lid assembly is attached
to the package frame such as by ultrasonic welding in step 94. The
completed package is singulated in step 96, if fabrication was in
strip form.
[0051] The package constructed in accordance with the invention
achieves a hermeticity which is comparable to conventional and more
expensive ceramic packages. The performance of the present
invention in meeting stringent hermeticity requirements is
described below.
[0052] The traditional method of evaluating the hermeticity of a
cavity package is by performing a helium leak test (MIL-STD-883).
In this test, a sealed package is placed in a helium pressurized
vessel (termed "bomb"). Some helium will enter the cavity package
though one of the "leak" channels. After removal of the cavity
package from the bomb, the cavity package is connected to a helium
leak tester, and the leak rate of the cavity package is detected.
The amount of helium released depends upon the size of the "leak"
channel and the helium pressure within the cavity package. The
helium pressure in the cavity package depends upon the amount of
helium and the internal volume of the cavity package. The levels of
hermeticity are governed by MIL-STD-883 test condition 1014. The
following are the hermetic rating and test methods:
[0053] (i) Test Condition A: Fine Leak using helium tracer gas:
[0054] A1: Fixed Method [0055] A2: Flexible Method [0056] A4: Open
Can Leak for Unsealed Packages
[0057] (ii) Test Condition B: Fine Leak using Radioactive Tracer
Gas
[0058] (iii) Test Condition C: Gross Leak and Fine Leak Test
Techniques [0059] C1: Gross Leak Bubble Test [0060] C3: Gross Leak
Vapor test [0061] C4/C5: OLT Optical Leak Detection (Gross and Fine
Leak)
[0062] (iv) Test Condition D: Gross Leak using a Dye Penetrant
(Destructive)
[0063] (v) Test Condition E: Gross Leak by Weight Gain
Measurements
[0064] To be designated as a hermetic package, the helium leak
rates of a cavity package must meet the following criteria shown in
Table 2:
TABLE-US-00002 TABLE 2 Helium Leak Rates and Hermetic Ratings
Maximum Leak Rate, (atm- Package Volume, (cc) cc/sec) <=.01 5
.times. 10.sup.-8 0.01 < V <= 0.4 1 .times. 10.sup.-7 >0.4
1 .times. 10.sup.-6
[0065] The flow of helium and other gases through a fine leak is
molecular because of the fine leak channel. The number of molecules
striking a unit area of surface is proportional to the pressure of
the gas and inversely proportional to the square root of its
molecular weight. Below in Table 3 are the properties of several
molecular species (gasses) of interest for the image sensor
package, as it is desirable to protect the image sensor package
from such molecular species:
TABLE-US-00003 TABLE 3 Properties of Gases Molecular Viscosity
Molecular Molecular Weight Diameter Micro-Poise @ Mass Species
(Gram) (.times.10.sup.-8 cm) 20 Celsius (.times.10.sup.-24 Gram)
Helium 4.0 2.2 194 6.64 Neon 20.2 2.6 311 33.5 Argon 40.0 3.7 222
66.2 Nitrogen 28.0 3.8 177 46.5 Oxygen 32.0 3.6 202 53.1 Air 28.7
3.7 184 47.6 Water 18.0 3.2 125 @ 100 29.9 Celsius Carbon 44.0 4.6
148 73.0 Dioxide
[0066] For an image sensor package to be compliant with
MIL-STD-883D testing, leakage due to openings at an interface
should be small.
For the image sensor package to pass the Gross Leak Test Condition
C1: MIL-STD-883, the following applies: [0067] Helium Leak Rate
<=1.times.10.sup.-5 atm-cc/sec and [0068] Leak channels with a
cross sectional dimension greater than 1.times.10.sup.-4 cm will
cause the image sensor package to fail this test.
[0069] FIG. 10 shows the interfacial problems associated with a
plastic molded LCP image sensor package using a standard LCP
material. Adhesion of this material to the leadframe is
non-optimum, and dye inserted in the cavity will leak through the
LCP/Leadframe interface. This package with conventional plastic
material will not meet the applicable industry standard.
(MIL-STD-883, Test Condition D: Gross Leak using a Dye
Penetrant).
[0070] For comparison, FIG. 11 shows the construction of the
present invention after the dye leak test. The dye penetrant did
not leak through the LCP/leadframe interface, allowing this package
to meet the MIL-STD-883 standard.
[0071] FIG. 12 shows several materials which are used in
semiconductor packaging. In non-hermetic packages, polymers are
used which have a permeability such that fluids and moisture can
penetrate easily. In hermetic packaging, materials are used which
are relatively impervious to fluids and moisture. As can be seen,
the permeability of the "non-hermetic" materials is high, such that
fluids and moisture can relatively easily penetrate the package.
Glass, ceramic, and metal have relatively low permeability,
allowing for them to be suited for hermetic packaging. Also shown
is the measured permeability of the LCP formulation used for the
image sensor package of the present invention. The permeability of
the LCP formulation is equivalent to glass, making it most suitable
for "hermetic" packaging.
[0072] As the package constructed in accordance with the invention
is hermetically sealed, the package can be employed to house
components such as LEDs which may include a liquid or gel filled
interior. For such purposes, the present package avoids leakage of
the gel which can occur in conventional packages which are not
adequately sealed.
[0073] While the invention has been described for use in an image
sensor package, the invention is not limited to image sensor
packages or optical packages but is useful for providing a plastic
package for containing other semiconductor, electric or electrical
components, devices or circuits. For applications where a glass lid
is not needed, the lid assembly can be molded in one piece which
includes the lid or cover portion and surrounding frame which is
bondable to the package frame. In addition, the interfacial layer
of the present invention is not limited to use between a leadframe
and a plastic frame but is more generally useful as an interfacial
layer between a metal and a plastic in other than circuit or device
packages. Accordingly, the invention is not to be limited by the
embodiments shown and described but is to embrace the full spirit
and scope of the accompanying claims.
* * * * *