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SMT electronics assembly manufacturing forum.

Solder Paste Reflow Profiling

Kenny Tan


Solder Paste Reflow Profiling | 23 July, 2001

Good day! As we all know the normal recommended solder paste(Sn62/Pb37) reflow profile time is around 40~60sec above the temperature of melting point. Now I'm setting my peak temperature at the high site of solder paste recommended spec. due to metalization defect(Poor solder finishing)at the terminal which look like a micro crack line.In able to overcome this defect I'm need to set our profile reflow peak temperature at 218~220�C and the reflow dwell time at 65~75 sec.

(1) Will this cause any defect or impact to the solder joints strength ? (2) Any impact on other component for a double side reflow PCB design ? (3) Any other site effect other that what been mention above ?

Can anybody kindly comment on this issue based on your valueble experience.Thanks in advance !

Best regard, Kenny Tan

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Solder Paste Reflow Profiling | 23 July, 2001

Responding to your questions: (1) The greater the temperature the greater intermetallic layer growth, the thicker the intermetallic layer, the weaker the solder connection. (2) If your components are affected by higher temperatures, you should limit their exposure to high temperatures. Further, the more temperature cycles through Tg, the more stress on PTH and via of a board. (3) Yupper!!!

(3a) Consider that you probably are not repairing [overcoming] your �micro crack line� defect by reflowing solder at a higher peak or longer time at liquidous temperature. Possibly, you are merely doing a better job of filling the break in the base metal with solder. We agree that you see a crack in the solder overcoat, but solder is very soft, does not fracture easily, and very unlikely to crack without the underlying base material cracking. If this is the case, it demands more attention that it�s apparently getting, considering product reliability requirements, bla bla bla.

(3b) Various 63/37 solder paste formulations allow much higher peak and longer time at liquidous temperature than those you mention. Other aspects about formulations are: * Long stencil life paste requires higher temperatures or slower conveyor. * More active flux in paste allows lower peak temperatures.

(3c) As you tip-toe along the boundaries of your paste supplier�s recommended profile, you need to be aware of when the process is no longer in control. For instance: * Cool-down ramp too slow => Flux charring * Cool-down too long => Coarse grain

ps: Some would argue that potentially you'd get greater response in the process forum rather than the design forum

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