Dr. Ronald C. Lasky: Indium Corporation's Ed Briggs had the great idea to write a paper on the many applications of solder preforms. He asked me to edit and review the paper; the title was The Versatile Preform.
A preform is a piece of solder in a wide variety of shapes. Often, the preforms are coated with solder flux or have flux in the center of the metal. Preforms formed with a flux in the center like this are typically called flux-cored. However, preforms come in six solder families: gallium, bismuth, indium, tin, lead and gold-based. There are more than 200 different alloys that can be formed into preforms.
I should be clear that solder paste are profoundly important as they are the fundamental material in electronic surface mount assembly. However, their very nature creates some challenges. The solder paste flux must be active enough to remove oxides during the reflow process and yet not affect the solder powder while the paste is in storage. Since solder paste is about 50% by volume flux, the flux coats all of the solder powder particles making long-term paste storage a challenge.
This paste shelf life is measured in months. Since the preforms are solid, there is much less surface area of solder alloy to interact with the small amount of preform flux during storage. Preform shelf life is almost infinite. There is a wide variety of temperature range for preforms. Tin-based solder paste such as tin-lead and lead-free paste have a melting range in the 183 to 229°C range. With the 6 alloy families for preforms, the range goes from 8°C to 1,064°C.
Some of the higher temperature applications include attaching silicon-integrated circuit die using J-STD-020, which requires a solder alloy that melts from 260 to 265°C. Some of the higher temperature applications include attaching silicon-integrated circuit die using J-STD-020 which requires a solder alloy that melts at 260 to 265°C. There are several lead-based preforms that melt in this temperature range. Similarly, there are several gold-based alloys that are used in this temperature range for radiofrequeny and hermetic sealing.
There are several preform applications in SMT assembly. Many electronic circuit boards have both surface mount and through-hole components. Typically, one would expect to require surface mount and wave soldering in the assembly line to assemble such a circuit board. However, printing solder paste over the through-holes can often enable just surface mount assembly. This process is often called pin-in-paste as the through-holes are literally placed in and through the through-hole solder paste deposit, but as we recall, solder paste is about 50% by volume flux. Therefore, when the solder paste is reflowed, the through-hole solder joints are often solder starved and have an excess of flux.
This is a cross-section of a pin-in-paste assembled circuit board. Note the lack of solder and excess of flux. Solder preforms are the solution. By placing a preform on the solder deposit, the solder starved situation is eliminated. The preforms are typically the shape of a passive component like an 0603 resistor. They are packaged in tape and reel and placed by a component placement machine on the solder deposit. The resulting solder joints are, typically, nearly perfect.
Many of the new highly dense mobile phone designs often make it difficult to print enough solder paste to form an effective solder joint to connect the electrical shields to the printed wiring board pad. In such cases, the shield will not perform its electromagnetic and electrostatic discharge protection tasks.
Solder preforms are often used in such solder starved conditions. This process is called Solder Fortification®. The preforms are placed by a component placement machine. More information on Solder Fortification® can be found in a paper by Carol Gowans and colleagues entitled Applications of Solder Fortification® with Preforms. This topic could be discussed all day. There are many other applications in laser, induction, and robotic soldering.
For more information about using preforms for soldering process challenges, visit our website at www.indium.com.
Reading Ed's paper for SMTAI 2016 would help answer questions you may have, but most importantly call Indium Corporation's technical services group to have your preform questions answered. And, as always, you may email me at rlasky@indium.com.