LEAD-FREE
FREQUENTLY ASKED QUESTIONS

SOLDER ALLOY SELECTION

Is there a standard lead-free solder alloy?

Just as with the lead-based solder alloys, there are several alloys that seem to be the most commonly used. The Solder Products Value Council (SPVC), a group of global solder manufacturers, standardized on SnAg3.0Cu0.5 as the alloy of choice. There are many variations of SnAgCu from 3-4% silver and 0.5-0.7% copper, but the melting temperature only varies in the range 217-220°C.

Does the substantially lower density of lead-free solder compared to tin-lead alloys have any advantages?

The specific gravity, or density in g/cc, of Sn63Pb37 or Sn63Pb36Ag02, the most standard lead-based alloys, is about 8.4. The specific gravity of the lead-free alloys, being mostly tin, is about 7.3, so the same volume of solder would be about 15% less weight for the lead-free solder.

What are some lower temperature lead-free alloys and where would they be used?

Lead-free solder alloys that melt lower than the tin-silver-copper alloys are composed of tin or tin alloys with the addition of bismuth, indium, or zinc, and all have advantages and disadvantages.

•  Bismuth alloys can contain a small amount or a large amount of bismuth. Generally, solder alloys with more than about 8% bismuth will be brittle.

• SnAg3.0Bi3.0 melts 213°C
• SnAg3.4Bi4.8 melts 202-215°C
• SnAg2.0Cu0.5Bi7.5 melts 211°C
• SnBi58 melts 138°C

•  Indium alloys can contain a small amount or a large amount of indium. Generally, solder alloys containing around 50% indium have little strength, but ductility is good. The high cost of indium is a consideration.

• SnAg3.3In4.8 melts 212-214°C
• SnAg3.0Cu0.5In10 melts 194-200°C
• SnIn52 melts 117°C

•  Zinc alloys usually contain less than 10% zinc, but the ease of corrosion minimizes the use of zinc alloys for most applications.

• SnAg3.0Zn15.0 melts 200-202°C
• SnZn09 melts 199°C

Of course there can be alloy compositions that contain bismuth, indium, and zinc.

• SnBi3.0Zn9.0 melts 187-195°C and wets better than SnZn.
• SnBi50.0In2.0 melts 135-137°C with better ductility than SnBi
• SnBi15.0Zn5.0 melts 170-193°C

What are some higher temperature lead-free alloys and where would they be used?

Currently, the high lead-content (>85%) are exempt from banning because there are no viable alternatives. However, there are some possibilities.

• Sn95Sb05 melts 232-240°C, but is not much higher melting than SnAgCu (217°C)
• Sn20Au80 melts 280°C but may be cost prohibitive
• BiAg2.5 melts 262°C
• BiZn2.7 melts 255°C
• BiZn15 melts 255-313
• SnAl05 melts 382
• Sn65Ag25Sb10 melts 230-235°C
  

What are the patent issues with using lead-free solder?

With more than 150 patents globally, selection of a lead-free solder alloy must be done carefully so proper license fees are paid for using patented alloys. Some alloys are not patented because of a long history of use and can be used without licensing.

• SnAg, SnCu, SnSb, SnIn, SnBi and SnZn alloys
• SnAg3.0Cu0.5 and SnAg4.0Cu0.5 (depending on the application)

Most other metal combinations for lead-free solder have been patented. The most prevalent patents for SnAgCu alloys include:

• Iowa State University Research Foundation (ISURF) USA patent 5,527,628 covering alloys SnAg(3.5-7.7)Cu(0.9-4.0)
• Senju/Matsushita Japanese patnet JP302744 covering alloys SnAg(3.0-5.0)Cu(0.5-3.0)

Kester is licensed globally to manufacture and sell these patented SnAgCu solder alloys, and the licenses pass through to Kester customers to use the alloys.

What is the specification of lead impurity composition in the lead-free solder?

The specification for lead impurity content in lead-free solder content is under 500ppm.

There will always be a small amount of lead contamination from the refining process of tin. This value of o.05% is lower than the 0.1% RoHS limit but care must be taken to avoid lead contamination especially during wave soldering.

How is lead-free solder analyzed?

Lead-free solders can be analyzed with spark analyzer and atomic absorption spectrometers. Both spark analyzer and atomic absorption spectrometer provide results equivalent to ICP-AES, however spark analysis is the preferred method.

Is there a lead-free solder of high melting point like Sn10Pb90?

There is no lead-free solder substitute for Sn10Pb90. In addition, there is no problem in using solders with high melting points because they are RoHS exempt if they contain a lead content of greater than 85%.

Is there a lead-free solder with a low melting point?

There are lead-free alloys for solder paste with 35-58% Bi content and others containing a variety of elements such as zinc with a lower bismuth contents, example Sn89Zn8Bi3.

Bi is effective to lower the melting point but it also makes solders weak and brittle. Bismuth does lower the melting point and improves wetting but has limitations. Zn oxidizes very quickly and needs extreme caution when used due to its electrolytic corrosion potential. Primarily consumer electronic assemblers use these alloys.