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August 22, 2018

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?

P.P.

Experts Comments

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.
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Fred Dimock
Manager, Process Technology
BTU International
Mr. Dimock is the manager of Process Technology at BTU International. His extensive experience in thermal processing includes positions at Corning, GE, and Sylvania. He has authored numerous articles on lead free processing and process control, taught classes at SMTAI, and participated in the IPC Reflow Oven Process Control Standard committee.
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:
  • The moisture content of the printed board
  • The desired degree of dryness after the bake
  • The moisture desorption characteristics of the laminate
  • The overall printed board thickness
  • The locations and structures of copper plane layers
  • Design features such as plated edges
To those points and many more I recommend a review of the IPC-1601: Printed Board Handling and Storage Guidelines.
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Robert "Bob" Lazzara
President
Circuit Connect, Inc.
Bob has been in PCB design and fabrication since 1976. He has held elected positions with the SMTA, is a member of the MSD Council, has served as a committee member for various IPC standards and is a Certified IPC Trainer.
Interesting question.  

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.  

  1. Solder paste metal content vs flux content.
  2. Thermal profile, temp rise, dwell, and reflow spike temp.
  3. Conveyor vibration.
Each one of these must be evaluated as potential sources of your problems.
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Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
Have you verified the correct copper plating thickness by cross section?
Gerard O'Brien
President
S T and S Testing and Analysis
Gerald O'Brien is Chairman of ANSI J-STD 003, and Co Chairman of IPC 4-14 Surface Finish Plating Committee. He is a key member of ANSI J-STD 002 and 311 G Committees Expert in Surface finish, Solderability issues and Failure analysis in the PWA, PWB and component fields.
This issue can be caused by a lot of parameters. I do not recommend to change them all but you can look at:
  • The speed of the conveyor - high speed combined with wrong temperature settings (pre-heaters and/or solder pot temperature) results in non-conforming solder joints.

  • The profile temperature settings - an aggressive ramp will burn the flux instead of activating it before reflow.

  • Is the flux spraying system functioning correctly? A glass pallet can answer that questions really quick.

  • Is the flux within the expiration date?

  • Mechanical issues with the conveyor (vibrations).

  • Solder pot contamination or excess dross formation.

  • Through hole component leads contamination or bare board contamination - foreign materials: flux, finger prints, dust, oils, oxidation, etc. Oxidation is a common effect of the baking process for long periods of time and 6 hours at 120C it is long and hot!
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Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.
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