U.S. patent application number 11/178551 was filed with the patent office on 2006-01-19 for lead-free solder pastes with increased reliability.
This patent application is currently assigned to W.C. Heraeus GmbH. Invention is credited to Alexander Brand, Steffen Grebner, Wolfgang Schmitt, Jorg Trodler.
Application Number | 20060013722 11/178551 |
Document ID | / |
Family ID | 35124644 |
Filed Date | 2006-01-19 |
United States Patent
Application |
20060013722 |
Kind Code |
A1 |
Brand; Alexander ; et
al. |
January 19, 2006 |
Lead-free solder pastes with increased reliability
Abstract
The introduction of undesired intermetallic phases (IMP) is
reduced in lead-free solder joints. For this purpose, solder pastes
are prepared, which have a metal powder mixture that generates a
melting range of at least about 5.degree. Kelvin and preferably
less than about 30.degree. Kelvin. Suitable for this purpose are
metal powder mixtures, in which the melting points of the metal
powder components lie apart from each other more than about
5.degree. Kelvin, particularly more than about 10.degree. Kelvin,
preferably more than about 15.degree. K, and optionally less than
about 30.degree. Kelvin, particularly up to about 25.degree.
Kelvin. However, metal powder mixtures are also possible, in which
the melting point spread can equal about 100.degree. Kelvin and
more, if the higher melting component dissolves in the melt below
the melting point of this component, so that the melting range can
be held under about 30.degree. Kelvin. With these solder pastes
undesired intermetallic phases can be held nearly arbitrarily
low.
Inventors: |
Brand; Alexander;
(Elsenfeld, DE) ; Grebner; Steffen; (Mombris,
DE) ; Schmitt; Wolfgang; (Rodgau, DE) ;
Trodler; Jorg; (Erlensee, DE) |
Correspondence
Address: |
AKIN GUMP STRAUSS HAUER & FELD L.L.P.
ONE COMMERCE SQUARE
2005 MARKET STREET, SUITE 2200
PHILADELPHIA
PA
19103
US
|
Assignee: |
W.C. Heraeus GmbH
|
Family ID: |
35124644 |
Appl. No.: |
11/178551 |
Filed: |
July 11, 2005 |
Current U.S.
Class: |
420/560 |
Current CPC
Class: |
H05K 3/3463 20130101;
H05K 2201/0272 20130101; B23K 35/025 20130101; B23K 35/262
20130101; H05K 3/3485 20200801 |
Class at
Publication: |
420/560 |
International
Class: |
C22C 13/00 20060101
C22C013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 13, 2004 |
DE |
10 2004 034 035.8 |
Claims
1. A lead-free solder paste, comprising at least two metal powders,
whose melting points or melting ranges are at least about 5.degree.
Kelvin apart from each other.
2. The lead-free solder paste according to claim 1, wherein the
melting points or melting ranges are at least about 10.degree.
Kelvin apart.
3. The lead-free solder paste according to claim 1, wherein the sum
of the metal powders is at least a ternary metal system.
4. The lead-free solder paste according to claim 3, wherein the
solder paste is a tin-silver-copper solder paste.
5. The tin-silver-copper solder paste according to claim 4, wherein
the powder mixture in total comprises about 2 to 5% silver and
about 0.1 to 1% copper.
6. The tin-silver-copper solder paste according to claim 4, wherein
the powder mixture in total comprises about 3 to 4% silver and
about 0.3 to 0.8% copper.
7. The lead-free solder paste according to claim 1, wherein the
melting points or melting ranges lie less than about 30.degree.
Kelvin apart from each other.
8. The lead-free solder paste according to claim 7, wherein the
melting points or melting ranges lie less than about 25.degree.
Kelvin apart.
9. A method for producing a stable solder joint with reduced
introduction of intermetallic phases into the solder joint, the
method comprising providing components for alloying, and first
generating a final alloy from the components during a soldering
process for forming the solder joint, wherein the components
provided for alloying contain fewer intermetallic phases than the
final alloy of the older joint.
10. A method for reducing intermetallic phases in a solder joint,
the method comprising subjecting metal powder to a re-melting
process to form a solder joint, wherein the metal powder is a
mixture of at least two powders with a melting point or melting
range differing by at least about 5.degree. K.
11. A method for manufacturing a solder paste, the method
comprising mixing at least two metal powders having a melting point
or melting range differing by at least about 5.degree. K with a
fluxing agent into a paste.
12. The method according to claim 9, wherein the solder joint
comprises at least two different metal powders for wafer bumping or
printed circuit board assembly.
13. The method according to claim 9, wherein the solder joint
comprises at least two different metal powders on a printed circuit
board assembly
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to solder pastes, particularly
to lead-free solder pastes based on SAC.
[0002] Solder pastes, for example SnAg4Cu0.5, tend toward void
formation and consequently to reduced stability relative to
lead-containing solder and thus reduced reliability of the solder
joints.
[0003] According to U.S. Pat. No. 5,527,628, in addition to an
eutectic composition Sn93.6Ag4.7Cu1.7, other non-eutectic
compositions of this three material tin-silver-copper (SAC) system
or compositions of two material tin-silver and tin-copper systems
are used in an alloy in order to generate a melting range for the
soft solder. Such technical solder compositions, particularly SAC
solders, exhibit intermetallic phases (IMP). In general IMP with
stoichiometric compositions (e.g., Cu3Sn, Cu6Sn5) have low crystal
symmetry and are usually very brittle. Compounds that are stable
over a large range (e.g., Ag3Sn) in general do have a higher
symmetry and are usually also more ductile, but nevertheless can
reduce the stability of a solder structure.
[0004] Thus, IMP are undesirable in soft solders that have several
compositions. These so-called critical intermetallic phases are
therefore undesirable, because they are preserved during the
soldering process, grow there, and thus impart brittleness and
break-off points in the final solder joint, which negatively affect
its mechanical stability. Intermetallic phases are also formed
during soldering at the contact surface, but other intermetallic
phases, which are present anyway, are imported by the solder. In
turn, these phases increase during the soldering process and then
continue to enlarge over time and therefore can lead to
intercrystalline fracture or grain deterioration.
BRIEF SUMMARY OF THE INVENTION
[0005] The problem of the present invention is to further improve
the quality of solder pastes and in particular to reduce the
tendency toward void formation.
[0006] To solve this problem, the introduction of undesired
intermetallic phases (IMP) is reduced. For this purpose, solder
pastes are prepared comprising a metal powder mixture that
generates a melting range of at least about 5.degree. Kelvin and
preferably less than about 30.degree. Kelvin. A suitable mixture
here is a metal powder mixture, for which the melting points of the
metal powder components are separated from each other more than
about 5.degree. Kelvin, particularly more than about 10.degree.
Kelvin, preferably more than about 15.degree. K, and optionally
less than about 30.degree. Kelvin, particularly up to about
25.degree. Kelvin. However, metal powder mixtures are also
possible, for which the melting point spread can reach about
100.degree. Kelvin and more, if the higher melting component
dissolves in the melt below the melting point, so that the melting
range can be held under about 30.degree. Kelvin.
[0007] With the solder pastes according to the invention, undesired
intermetallic phases can be held nearly arbitrarily low. In
addition, the void formation according to the void test can be
reduced considerably. In particular, the void formation of tin,
silver, copper (SAC) alloys can be reduced by up to an order of
magnitude. Consequently, these pastes enable more stable and
therefore, in turn, more reliable solder joints, which are of great
interest for electronics assembly. Particularly important
applications result for printed circuit board construction and
wafer bumping. The final alloy of the solder joint is first
realized at least partially during soldering. Therefore, with the
powder mixture, fewer critical intermetallic phases (IMP) are
introduced during the soldering process. If IMP were to be fed to
the soldering process with a final alloy (target alloy) of 100%,
then IMP would be reduced by the powder mixture according to the
portion that first forms during the soldering as the final
alloy.
[0008] In preferred embodiments:
[0009] the powder mixture is meltable in a temperature range, which
exceeds the temperature of an eutectic of this system by less than
about 30.degree. Kelvin;
[0010] the sum of the metal powders corresponds to a ternary,
quaternary, or higher metal system;
[0011] the metal powders differ in terms of their particle size
distribution, wherein it is particularly preferred that the finest
powder exhibit the highest melting point or have the lowest mass
fraction;
[0012] the lead-free solder paste has powders of differing metal
composition, wherein at least two metal powders are produced in
terms of one alloy component apart from the main component, such
that one powder has a higher content of this component and the
other metal powder contains less to none of this component;
[0013] the solder paste is a tin-silver-copper solder paste;
[0014] the main component is an SAC alloy, particularly with an
alloy percentage of 2 to 5% silver and 0.2 to 1% copper, preferably
3 to 4% silver and 0.3 to 0.8% copper; and/or
[0015] the SAC powder mixture has a total of 2 to 5 wt. % silver
and 0.2 to 1 wt. % copper, preferably 3 to 4 wt. % silver and 0.4
to 0.8 wt. % copper.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0016] The foregoing summary, as well as the following detailed
description of the invention, will be better understood when read
in conjunction with the appended drawings. For the purpose of
illustrating the invention, there are shown in the drawings
embodiments which are presently preferred. It should be understood,
however, that the invention is not limited to the precise
arrangements and instrumentalities shown. In the drawings:
[0017] FIGS. 1a and 1b are micrographs of SnAg4Cu0.5 showing the
IMP in the alloy.
[0018] FIG. 2a is a micrograph of Sn99Ag1 wherein hardly any IMP
can be detected.
[0019] FIG. 2b is a micrograph of Sn99Cu1 wherein hardly any IMP
can be detected.
[0020] FIG. 3 is two micrographs showing the low IMP of
Sn97Cu3.
[0021] FIG. 4 is two micrographs of Sn95Ag5 wherein no IMP can be
detected.
[0022] FIG. 5 is a graph showing the melting range of the powder
mixture of Example 1.
[0023] FIG. 6 is a graph showing the melting point of the alloy
produced from the powder mixture of Example 1.
[0024] FIG. 7a is a bar graph comparing Examples 1 and 2 with a
standard, wherein the left side shows the portion of small voids
and the right side shows the portion of large voids.
[0025] FIG. 7b is a bar graph comparing a quality analysis of
Examples 1 and 2 with a standard.
DETAILED DESCRIPTION OF THE INVENTION
[0026] For manufacturing pastes according to the invention, at
least two metal powders with different melting points are mixed
with a fluxing agent. For this purpose, the known manufacturing
methods are suitable, e.g., for preparing, stirring, kneading, and
homogenizing pastes.
[0027] According to the invention, a soldering process can be
designed, in which a metal powder is subjected to a re-melting
process to form a solder joint, with reduction of undesired IMP.
The decisive factor for this process is the coating with a paste,
which has powders with different alloy compositions, optionally
also pure metal, which, in turn, have in general a different
melting range or melting point. The composition of the solder joint
is here predetermined by the mixture ratios of the metal powders,
but first realized during the soldering process, i.e., upon melting
of the paste. Here, IMP can be essentially controlled in terms of
quality and quantity.
[0028] Furthermore, it is possible to combine technical standard
alloys as metal powder mixtures, such that the composition of the
solder paste corresponds to an individual target alloy.
[0029] Preferably, the solder pastes of the present invention
contain as the main component one of the elements of tin, copper,
silver, gold, or platinum. The other components are selected so
that the powder composition generates a melting range in the
vicinity of an eutectic. The metal powders mixed in the solder
paste have, besides different compositions, preferably also
different particle sizes. Here, it has proven effective when the
powder with the lowest melting composition, for example of an
eutectic composition, is the main component of the metallic powder
and preferably has the largest powder particles on average. The
particle sizes should differ at least by the factor 0.7, preferably
up to an order of magnitude, wherein, however, even two orders of
magnitude can be advantageous.
[0030] This is particularly the case with a relatively high melting
alloy and particularly when the high melting alloy must be
dissolved by the melt. Therefore, the melting range need not
absolutely agree with the melting range of a phase diagram.
Essential for the melting range is that the metal powders form a
melt in this range and all powders melt, at least to a significant
degree, within a melting range of less than about 30.degree.
Kelvin, particularly up to about 25.degree. Kelvin, or dissolve in
the melt. In terms of the melting range, both the lower and the
upper melting range limit can differ from the respective lower or
upper value of another alloy or the melting point of a pure metal.
The individual metal powders can be pure metals or alloys, but must
be in the condition, as a powder mixture, to form an alloy,
preferably a ternary or higher alloy, within a melting range of
less than about 30.degree. Kelvin, particularly a maximum of about
25.degree. Kelvin. Thus, the powders must be in condition to form a
melt, which contains at least two elements, preferably at least
three.
[0031] With the lead-free pastes, the new legal regulations in many
countries should be taken into consideration. As a limit for the
lead content, 1% is provided (Elektronik Praxis Marktreport
Bleifrei [Electronic Practice Market Report Lead-free]--June 2004,
page 6).
[0032] With the powder compositions the fraction of critical
intermetallic phases (IMP) can be reduced by more than an order of
magnitude. This is associated with significantly increased
stability of the solder joint.
EXAMPLES
[0033] Solder pastes with the target alloys SnAg4Cu0.5 and
Sn96.5Ag3Cu0.5 were manufactured according to a conventional
manufacturing process for solder pastes, with the difference that
here various alloys are mixed into one powder mixture. The powders
correspond to powder grain size 3 according to EN 61190-1-2:2002.
Clearly recognizable IMP were detected only in the SnAg4Cu0.5
powder and the Sn96.5Ag3Cu0.5 comparison powder.
[0034] SnAg4Cu0.5 (melting point 217.degree. C.)
[0035] Sn96Ag4 (melting point 222.degree. C.)
[0036] Sn99Cu1 (melting point approximately 227.degree. C.)
[0037] Sn95Ag5 (melting range 221 to 240.degree. C.)
[0038] Sn99.15Cu0.85 (melting point approximately 229.degree.
C.)
[0039] Target alloy SnAg4Cu0.5
Example 1
[0040] 80% SnAg4Cu0.5 and 16% Sn95Ag5 and 3% Sn97Cu3 and 1%
Sn99Cu1
Example 2
[0041] 60% SnAg4Cu0.5 and 32% Sn95Ag5 and 6% Sn97Cu3 and 2%
Sn99Cu1
Example 3
[0042] 50% SnAg4Cu0.5 and 15% Sn100 and 10% Sn80Cu20 and 25%
Sn99Cu1
[0043] Target alloy Sn96.5Ag3Cu0.5
Example 4
[0044] 50 wt. % SnAg4Cu0.5, 25 wt. % Sn96Ag4, and 25 wt. %
Sn99Cu1
Example 5
[0045] 50 wt. % SnAg4Cu0.5, 20 wt. % Sn95Ag5, and 30 wt. %
Sn99.15Cu0.85
Example 6
[0046] 50 wt. % Sn95.5Ag4Cu0.5 and 20 wt. % Sn95Ag5 and 30 wt. %
Sn99.15Cu0.85
[0047] The solder pastes are each soldered to a copper joint on a
printed circuit board and the solder joints are examined.
[0048] The solder joints produced from the powders according to the
invention contain significantly fewer IMP than those from the
alloys SnAg4Cu0.5 (Comparison example 1) and Sn96.5Ag3Cu0.5
(Comparison example 2). The mixture with 80% SnAg4Cu0.5 lay
approximately between the comparison examples and the other
examples according to the invention, which exhibited only
approximately half as many IMP as the comparison examples.
[0049] The mechanical loading capacity in terms of sensitivity to
shock and vibration is improved in the solder joints manufactured
according to the invention, as well as the resistance to
temperature fluctuations.
[0050] The melting range of the powder of Example 1 was determined
according to FIG. 5 by differential thermal analysis (DTA). The
subsequent DTA (FIG. 6) shows that the alloy produced from the
powder mixture is nearly eutectic in view of the sharp melting
point.
[0051] According to the void test described in the article
"Avoiding the Solder Void," by Richard Lathrop, Heraeus Circuit
Materials Division, Philadelphia, Pa., the pastes according to the
invention are improved relative to the standard alloys. In FIG. 7a,
the measured voids are shown. According to the invention, the large
voids could be considerably reduced. From these results, a quality
analysis according to FIG. 7b was produced, in which the
compositions according to the invention from Examples 1 and 2 with
68 and 64 points achieve a clear increase in quality relative to
the prior art ("Standard")(Comparison example 1).
[0052] It will be appreciated by those skilled in the art that
changes could be made to the embodiments described above without
departing from the broad inventive concept thereof. It is
understood, therefore, that this invention is not limited to the
particular embodiments disclosed, but it is intended to cover
modifications within the spirit and scope of the present invention
as defined by the appended claims.
* * * * *