Lead-free Solders and Their Properties


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This is the second column in a three-part series on lead-free solders.

Ray P. Prasad

Along with thermal, mechanical, creep and fatigue, melting point is one of the most important solder properties. Table 1 provides a list of lead-free solders that have been available for some time.1

It should be noted that lead-free solder compositions are still being optimized to achieve the desired properties. The compositions of the solders shown in Table 1 may vary slightly from commercially available solders at different points in time. For example, Table 2 shows some of the commercially available solders from different suppliers under their trade names.

The lead-free alloys containing high amounts of indium (e.g., the first alloy in Table 2) have the potential incompatibility of indium with lead if it is present either on the board surface or on component leads. To have a truly lead-free process, it may be necessary, if using alloys containing indium, to use a lead-free surface finish on the PCB. The industry is focusing on developing alternate plating. Examples include Alpha Metals` AlphaLevel flash silver plating and Motorola`s tin/bismuth plating for board and component lead finishes.

From Table 1, we can see that lead-free solders either have much lower melting points or much higher melting points than tin/lead eutectic solder. Table 2 shows mostly higher temperature lead-free solders.

Special flux is necessary when low-temperature solders are used, because the standard flux may not be active at lower temperatures. Another problem associated with low-temperature solders is the reduction in wetting properties caused by the lower fluidity at sub-eutectic temperatures.

For low-temperature applications, solders containing indium are gaining acceptance. One indium alloy being used by some companies because it provides better rework/ repair characteristics contains 52 percent In and 48 percent Sn. Because the alloy melts at 244°F (118°C), rework can be performed at lower temperatures - many times without causing thermal damage. If the printed circuit board (PCB) must be plated with gold as an antioxidant, indium solder can be used to prevent gold leaching.2

Another low-melting-point lead-free solder is 58Bi/42Sn. If we look at the phase diagram of the Sn/Bi alloy, we will find the melting point to be 138°C. Bismuth is used in soldering alloys to achieve low soldering temperatures, but its alloys generally exhibit poor wetting characteristics.

Many other alloys shown in Table 1 have much higher melting points than tin/lead eutectic, which has a melting point of 183°C. For example, zinc/tin, a high-temperature lead-free solder, has a melting point of 198°C.

The high-melting-point solders will be incompatible with widely used board materials such as FR-4. In addition, higher temperatures necessary for rework can significantly increase the potential for board damage.

There are no drop-in replacement lead-free solders at this time, although some suppliers describe their solder as "near drop-in."3 Even these require a soldering iron temperature of 750°F (400°C) for rework. This may be too high a temperature in some applications and can cause potential thermal damage.

Also, one of the key problems when using high-melting-point solders in wave soldering (Tables 1 and 2) is that they increase the potential for capacitor cracking. The wavesoldering temperature needs to be kept at approximately 230° to 245°C, which is about 45° to 65°C above the melting point of tin/lead solder. A lead-free solder with a melting point of 220°C will require 265° to 280°C wave temperature. This increases the delta temperature between the preheat and the wave, and enhances the potential for capacitor cracking.

In general, almost all lead-free solders exhibit less wetting (spreading) than tin/lead eutectic, causing an inferior solder fillet. To improve the wetting properties, special flux formulations are required. The fatigue properties of lead-free solders are also not as good, although in one study no degradation of solder joint integrity was observed after thermal cycling with high-temperature 96.5Sn/3.5Ag (the last alloy in Table 1).4

Ideally, the melting point of the selected solder should be around 180°C so that reflow temperatures of 210° to 230°C, wave pot temperatures of 235° to 245°C, and hand-soldering temperatures of 345° to 400°C (650° to 700°F) can be used. Only very experienced operators can handle higher hand-soldering temperatures to avoid thermal damage.

J-STD-006, available from the IPC - Association Connecting Electronics Industries, provides a detailed list of tin/lead and lead-free solders. None of the lead-free solders is considered a drop-in replacement for tin/lead eutectic, however. The industry is still looking for the right lead-free solder that can truly be a substitute for tin/lead eutectic. It is a challenge that the industry must address. In my next column, I will discuss who is using lead-free solder in their products and whether you should consider lead-free solder in your products.

REFERENCES

1 Norbert Socolowski, "Lead-free Alloys and Limitations for Surface Mount Assembly," Proceedings of Surface Mount International, 1995, p. 477-80.

2 R. Keeler, "Specialty Solders Outshine Tin/lead in Problem Areas," EP & P, July 1987, p. 45-47.

3 Karl Seelig, "A Study of Lead-free Solder Alloys," Circuit Assembly, October 1995, p. 46-48.

4 Cindy Melton, "How Good Are Lead-free Solders?" SMT, June 1995, p. 32-36.

RAY P. PRASAD is an SMT Editorial Advisory Board member and author of the text book Surface Mount Technology: Principles and Practice. He is also founder of the Ray Prasad Consultancy Group which specializes in helping companies establish strong internal SMT infrastructure. Contact him at P.O. Box 219179, Portland, OR 97225; (503) 297-5898; Fax: (503) 297-0330; Web site: www.rayprasad.com.

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