Solder paste is one of the great ironies of electronics manufacturing. On one hand, solder paste printing is a key process that controls production yields. On the other hand, because it is continually reacting, solder paste can be very difficult to work with.
More than 50 percent of all surface mount manufacturing defects are caused by errors in the screen printing process. Those faults are a major source of board failure. Unfortunately, understanding the causes of those defects can be challenging because there are numerous factors to consider. For instance, paste formulation, viscosity, temperature and humidity affect print quality, in addition to printer alignment, pressure and speed.
"A disturbingly long list of variables influence this seemingly simple process," warns John Vivari, a process engineer in the Solder Paste Group at EFD Inc. (East Providence, RI). "It only takes one element out of tolerance for the process to fail."
Solder paste consists of finely granulated solder powder suspended in a viscous flux medium. Paste can be deposited in virtually any required shape or thickness by a number of different dispensing methods.
"Solder paste printing is universally accepted as the practical way to put solder paste on the surface mount device pads on the printed circuit board (PCB)," says William Coleman, vice president of technology at Photo Stencil (Colorado Springs, CO). Printing was developed concurrently with surface mount technology in the early 1980s, and made the surface mount assembly process economically competitive with through-hole assembly processes.
"Surface mount technology has been around since the late 1960s for the ceramic hybrid industries, but components were manually assembled," explains Douglas Dixon, product manager for Multicore soldering products at Henkel Loctite Corp. (Rocky Hill, CT). "The move into surface mount assembly with solder paste was driven by cost, size and performance requirements. Surface mount technology has benefits over through-hole technology in all areas."
No other solder material is as well-suited to high-quality, high-volume surface mount electronics assembly as solder paste. In …
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