I just read an article published on an SMT-focused website by Paul Socha. Mr. Socha was the Manager of Applications Engineering for many years and was instrumental in creating the group that Indium Corporation deploys today. The information in the article is timeless and, in anticipation of Product Manager Carol Gowans' soon-to-be-released technical paper related to this article, I wanted to share Paul's '10 Basic Steps to Choosing Preforms'.
- Make sure you have a thorough understanding of what is required for your application.
- Consult with your solder supplier (preferably Indium Corporation, shameless plug) for help with form, alloy, and metallization compatibility. The metallizations involved in the solder joint, as well as the operational temperature of the device once in service, are crucial when determining the best alloy for your application.
- Review the application to determine what form of solder is best. If a solder preform is warranted, you will need to consider the shape, dimensions, and tolerances.
- Don’t over-specify the preform with regard to solder purity or size tolerances as this will have an effect on its pricing and lead time.
- If possible, prepare a drawing for your solder supplier to review.
- Is a flux needed? If so, the solder preform can be coated with flux to save a step during manufacturing. A tacky flux can also be used to provide the fluxing action and to hold the preform in place during reflow. You need to determine if the flux residue will be removed and, if so, what cleaning solvent will ensure a clean joint.
- The method of placement is important to determine the type of packaging needed — rigid vs. fragile and low-volume vs. high-volume production.
- Consider the method of reflow. The peak reflow temperature should be 20°–50°C above the liquidus temperature of the alloy. Excessive temperature may char the flux or damage temperature-sensitive devices in the assembly.
- Determine the correct volume of solder. If you are using solder preforms with solder paste, identify the ratio of solder paste to solder preforms. Plan on 10–20% extra solder to ensure a good fillet.
- Solder preforms can be used in many applications not related to surface mount. These include: mechanical attach, vacuum seals, cryogenic seals, hard-to-reach areas requiring solder, clad preforms for reinforcement, and die-attach. Solder preforms are used in aerospace, medical, military, energy, automotive, communications, security, and many more industries. Discuss with your solder supplier if solder preforms are right for your application.
You can learn more on solder preforms at http://www.indium.com/solders/preforms.