A sometimes forgotten fact about reflow and wave soldering is that anywhere from 25% to 50% of the time a solder joint spends above its melting temperature, aka: time above liquidus (TAL), takes place in the "cooling zone". Much time is spent getting the heating portion of the oven recipe finely tuned to produce a robust thermal profile, only to toss the product, covered with liquefied solder into a cooling zone where the solder joints must return to a solid state. The rate at which this occurs (cooling slope) is even more critical using lead- free solders. Giving the cooling zone some well deserved attention when defining the requirements of the thermal profile is essential to a good Thermal Quality Management program for your soldering process.
The cooling zone is where the quality of the solder joint is defined, with the cooling slope influencing the joint strength, and overall longevity. These two qualities are often at odds with each other because strength often comes from slower cooling rates, while longevity results from faster rates. Different cooling slopes have been tested to try to find what rate produces the best combination of strength and long life when subjected to accelerated thermal-cycling. These studies have concluded that slow cooling rates (1 to 2 °C/sec) allow too much time for intermetallic alloy growth, a strong but often brittle alloy prone to cracking when stressed. Faster cooling (5 to 7 ºC/sec) can form a softer solder joint with less overall strength, not to mention possible component damage. Cooling slopes between 3 and 4 °C/sec were found to be the best at producing a solder joint with both good strength and overall longevity.
So… don’t forget the cooling zone when developing the best thermal profile for your solder process.
"Cooling Rates in Lead-free and Tin/lead Reflow"
SMT Magazine by Denis Barbini, PhD.; Ursula Marquez
"Accelerated Thermal Fatigue of Lead-Free Solder Joints as a Function of Reflow Cooling Rate"
Journal of Electronic Materials by Qi Y; Zbrzezny A R; Agia M; Lam R; Et al
"Proceedings of 2005 International Conference on…"
Asian Green Electronics by Qiang Hu; Zhong-suo Lee; Zhi-li Zhao; Da-le Lee