Dispelling the Black Magic of Solder Paste

Tony Lentz
FCT Assembly
Greeley, CO USA

Solder paste has long been viewed as "black magic". This "black magic" can easily be dispelled through a solder paste evaluation. Unfortunately, solder paste evaluation can be a challenge for electronic assemblers. Interrupting the production schedule to perform an evaluation is usually the first hurdle. Choosing the solder paste properties to test is simple, but testing for these properties can be difficult. Special equipment or materials may be required depending upon the tests that are chosen. Once the testing is complete, how does one make the decision to choose a solder paste? Is the decision based on gut feel or hard data?

This paper presents a process for evaluating solder pastes using a variety of methods. These methods are quick to run and are challenging, revealing the strengths and weaknesses of solder pastes. Methods detailed in this paper include: print volume, stencil life, response to pause, open time, tack force over time, wetting, solder balling, graping, voiding, accelerated aging, and others. Hard data is gathered and used in the evaluation process. Also presented in this paper are a set of methods that do not require expensive equipment or materials but still generate useful data. The goal is to help the electronics assembler choose the best solder paste for their process.

It is possible to dispel the "black magic" of solder paste through the use of challenging test methods. The methods presented in this paper allow the user to differentiate between solder pastes. The methods are quick to run and give quantitative data about solder paste performance. Performance metrics can be calculated to give an overall rating for each solder paste. This enables the user to choose the best solder paste for her or his process based on quantitative data rather than gut feel.