Choose The Right Solder Alloy

Selecting the proper solder alloy is critical to meeting quality standards in electronic assembly. There are three main considerations when determining alloy content:

1. Desired melting temperature
2. Physical properties of the alloy
3. Specific requirements of the application

The most common tin-lead alloy is Sn63Pb37. Upon melting at 361°F, this alloy exhibits no plastic range. In other words, the solder melts and becomes instantly liquid. This particular alloy is commonly referred to as being "eutectic" because of this characteristic and also because when the alloy cools below 361°F, the solder instantly becomes solid again. This provides for rapid and defect-free soldering. Sn63Pb37 is the most popular, tin-lead eutectic alloy used in PCB assembly applications today.

Another alloy often used is Sn62Pb36Ag02 and is used primarily on silver coated ceramics and palladium/silver substrates. The silver content in this alloy impedes the rate in which the silver on the part leaches into the molten tin/lead solder alloy. This helps to keep the silver-bearing surface metallizations on the part during typical reflow operations. This allow has a melting range of 354°F to 372°F. The narrow plastic range of this alloy makes it compatible with most soldering applications.

Lead-free alloys are used when high joint strength is needed and/or when a high melting point is required. Also, it is used where lead-free solder is mandated. Sn96.5Ag3.5 is a high temperature, eutectic alloy that provides excellent joint strength along with rapid melting and hardening of the alloy. Sn95Sb05 with five-percent antimony has a high melting range of 450°F to 464°F. This alloy is used where connections see ambient temperatures nearing 400°F. Although not an alloy, Sn100 or pure tin is often used for making tin additions to solder pots.

Alloys for very special applications are also available. These include Sn05Pb92.5Ag2.5 - a eutectic alloy that melts at 536°F. This alloy is used where connections are exposed to extreme heat. Sn43Pb43Bi14 with fourteen-percent bismuth has a melting range of 291°F - 325°F and is used in low temperature electronic assembly applications. All of these alloys are available in wire, bare and standard solder forms. Sn63Pb37, Sn62Pb36Ag02, Sn96.5Ag3.5 and Sn43Pb43Bi14 are also available in solder paste from Kester Solder. Ask one of our representatives for more information on these alloys or to help you find the right solder alloy for your application.