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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.

Expert Panel Responses

It is hard to diagnose a reflow problem without seeing aprofile, but I would suspect that the heating rate during the early part of theprofile 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 ofmultilayer PCBs it is unlikely that moisture trapped deep within the boardswill be fully released. Baking schemes such as yours are generally effectivefor latent moisture only around the PCB edges. Factors regarding moisturemitigation and elimination have been considered through committee at the IPCand 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 areview 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 iteither prior to solder paste deposition and component installation or after thepaste 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 andthe type of reflow system used. Hence, with this lack ofinformation 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 thesemust 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 thicknessby cross section?

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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 notrecommend to change them all but you can look at:
  • The speed of the conveyor - high speed combined with wrongtemperature settings (pre-heaters and/or solder pot temperature) results innon-conforming solder joints.
  • The profile temperature settings - an aggressive ramp will burnthe flux instead of activating it before reflow.
  • Is the flux spraying system functioning correctly? A glasspallet 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. Oxidationis a common effect of the baking process for long periods of time and 6 hoursat 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.

Reader Comment
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