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Several variables interact to create mlcrovoids: flux chemistry, plating thickness, surface roughness, and reflow temperature.
Microvoids are numerous small voids at the interface of a solder joint. Also known as champagne voids, microvoids are less than 40 microns in diameter. They are typically found just above the intermetallic layers between the copper from the circuit board and the tin from the solder paste.
Like all solder voids, microvoids are troublesome when they exist in sufficient number to reduce the cross-sectional area of the joint. In extreme cases, a solder joint can have thousands of microvoids, causing the joint to fail physically and electrically.
Engineers have known about microvoids for years. However, the widespread use of high-resolution X-ray inspection equipment has raised awareness of the problem. Indeed, it's possible that microvoids have always been present in solder joints, and inspection methods were simply insufficient to detect them.
Another reason engineers are detecting more microvoids is related to today's high-density circuitry. Complex electronic assemblies are more difficult to reflow due to wide temperature variations across the PCB. Even with great care, temperature variation may exceed 20 C. To decrease that variation, engineers often reduce overall reflow temperatures, but prolong reflow time. However, in the effort to avoid overheating sensitive components, some areas of the PCB may not receive enough heat. The flux will volatilize, but not all the gas can escape the joint before the solder hardens.
The increase in microvoids is also related to the growing popularity of lead-free solder. The high temperatures needed to reflow lead-free solder exacerbate the problem of temperature variation across the PCB. Moreover, lead-free solder itself may encourage microvoid formation due to differences in surface tension and flux chemistry.
The transition to lead-free solder has brought with it new surface finishes for PCBs. One finish that is growing more popular is immersion silver. Could this finish be contributing to the problem of microvoids? We conducted a study to find out.
Initial Investigation
Our investigation of microvoids and immersion silver was prompted by the high-visibility failure of a ball grid array (BGA) on a PCB during postassembly power cycling. Microvoids were the culprit. But, what caused the microvoids? Several theories were put forth, including:
* insufficient temperature during reflow.
* outgassing...