|Ask the Experts|
July 6, 2020
Blow Holes and Disturbed Joints
We are finding many blow holes and disturbed joints after reflow. Our PCB is 1.7 mm thick and baked prior to reflow at 120 C for 6 hours. The process is non RoHS. Peak temp for reflow is 220 C. What could be going wrong?
|Expert Panel Responses|
It is hard to diagnose a reflow problem without seeing a profile, but I would suspect that the heating rate during the early part of the profile is too high.
Manager, Process Technology
In the case of multilayer PCBs it is unlikely that moisture trapped deep within the boards will be fully released. Baking schemes such as yours are generally effective for latent moisture only around the PCB edges.
Factors regarding moisture mitigation and elimination have been considered through committee at the IPC and include:
Circuit Connect, Inc.
It is not identified where and when the boards were baked, be it either prior to solder paste deposition and component installation or after the paste deposition and component placement.
Also not included in the question is the paste type used,although it mentioned that it is a no RoHS paste, the thermal profile used and the type of reflow system used. Hence, with this lack of information I offer three concerns to be reviewed.
Vice President, Technical Director
Have you verified the correct copper plating thickness by cross section?
S T and S Testing and Analysis
This issue can be caused by a lot of parameters. I do not recommend to change them all but you can look at:
Engineering and Operations Management
The root cause is the violation of the Time/Temperature ratio. Age old. There is not enough time at preheating temperature to drive out the solvents. Everything else branches out from there. Once the joint is at or past solidus the vapors are still outgassing hence the blow holes and joint distortion.Ike Sedberry, ISEDS
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