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February 14, 2017

Solder Caking to Bottom of PCBs After Wave

We recently drained and refilled our wave solder pot with SN100 due to contamination. Flux coverage looks good. Preheat temps are good. During soldering the PCB is 1/4 submerged and pot is level. Angle set at approximately 7 degrees. We getting solder adhering and caking on bottom of the PCBs. What is the cause?

D.H.

Experts Comments

Solder sticking to the bottom-side of the board during wave soldering can be due to several items. These are listed below:
  1. Insufficient flux
  2. Flux activity not sufficient for the heat excursion it sees
  3. Excessive preheat times and temperature as to de-activate the flux
  4. Too long a dwell time in the solder
  5. Solder temperature may be to high
  6. Solder mask under cured
Insure that the flux being used is designed for lead-free wave soldering. These fluxes have activators made for lead-free higher pot temperatures and the longer dwell times in solder. If a flux decomposes completely during the wave process, solder will tend to stick or web on the board.

Applying more flux especially if a low solids no-clean flux is used helps reduce solder stick. Reducing preheat temps and at times increasing the conveyor speeds helps also. This avoids flux decomposition and de-activation.
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Peter Biocca
Senior Market Development Engineer
Kester
Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.
Hard to say exactly, but if the problem appeared immediately after changing out the pot, I would bet on differences in contact time and flow due to nozzle adjustment. If solder is sticking to the PWB, it's a sure bet that the flux is being burned off during wave contact. So if the application is good (same rate of application as before, good uniformity) then look at the configuration/adjustment of the nozzles. If you have contact time data, compare the before/after data to ensure that you don't have longer contact now.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
It is not mentioned as to what type of solder was drained from the solder pot. Was it the same alloy composition? Was it a leaded solder? Was the solder alloy which was chosen, evaluated according to the requirements of J-STD-001 and J-STD-006 for compatibility to the product? For example was the new alloy checked for compatibility to the solder mask? All these questions also apply to the selection of the flux, especially how compatible is the flux to the thermal profile?  

From the information provided, I would lean towards an incompatibility between the flux, solder temperature and the solder mask. The elevated temperature of the new alloy is softening the solder mask and the solder is not peeling off the bottom side of the board. When rosin flux was used on a dry wave the rosin flux solids percentage was at 35%, which created a wet surface for the solder to peel off the bottom side of the board. With the use of low solid content flux and elevated temperature there is nothing between the board and the solder and the solder sticks to the bottom side of the boards.  

Recommend trying different types of solder mask, different fluxes and modifications to the thermal profile.
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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.
You do not specify what type of contamination you've experienced. After cleaning and re-filling the solder pot, did you perform an analysis to make sure the contamination was eliminated? Did you have the same problem prior to pot cleaning? It can be a consistent issues across all your PCBs.

Having the incorrect solder mask applied in the board manufacturing process can cause serious issues on elevated temperatures (lead free) including contamination of the solder. I do not recommend changing the flux dispensing rates and/or thermal profiles until you get the PCBs investigated. Changing too many things at one time will make you lose track of the variables and finding the root cause impossible.
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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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