Ask the Experts
September 24, 2018
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?
Expert Panel Responses
Soldersticking to the bottom-side of the board during wave soldering can be due toseveral items.Theseare listed below:
Insurethat the flux being used is designed for lead-free wave soldering.Thesefluxes have activators made for lead-free higher pot temperatures and thelonger dwell times in solder. Ifa flux decomposes completely during the wave process, solder will tend to stickor web on the board.
Applyingmore flux especially if a low solids no-clean flux is used helps reduce solderstick. Reducing preheat temps and at times increasing the conveyor speeds helpsalso. This avoids flux decomposition and de-activation.
- Insufficient flux
- Flux activity notsufficient for the heat excursion it sees
- Excessive preheattimes and temperature as to de-activate the flux
- Too long a dwell timein the solder
- Solder temperature maybe to high
- Solder mask undercured
Senior Market Development Engineer
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 appearedimmediately after changing out the pot, I would bet on differences in contacttime and flow due to nozzle adjustment. If solder is sticking to the PWB, it'sa sure bet that the flux is being burned off during wave contact. So if theapplication is good (same rate of application as before, good uniformity) thenlook at the configuration/adjustment of the nozzles. If you have contact timedata, compare the before/after data to ensure that you don't have longercontact now.
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 fromthe solder pot. Was it the same alloy composition? Was it a leaded solder? Wasthe solder alloy which was chosen, evaluated according to the requirements ofJ-STD-001 and J-STD-006 for compatibility to the product? For example wasthe new alloy checked for compatibility to the solder mask? All these questionsalso apply to the selection of the flux, especially how compatible is the fluxto the thermal profile?
From the information provided, I would lean towards anincompatibility between the flux, solder temperature and the solder mask. Theelevated temperature of the new alloy is softening the solder mask and thesolder is not peeling off the bottom side of the board. When rosin flux wasused on a dry wave the rosin flux solids percentage was at 35%, which created awet surface for the solder to peel off the bottom side of the board. With theuse of low solid content flux and elevated temperature there is nothing betweenthe board and the solder and the solder sticks to the bottom side of theboards.
Recommend tryingdifferent types of solder mask, different fluxes and modifications to thethermal profile.
Vice President, Technical Director
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 whattype of contamination you've experienced. After cleaning and re-filling thesolder pot, did you perform an analysis to make sure the contamination waseliminated? Did you have the same problem prior to pot cleaning? It can be aconsistent issues across all your PCBs.
Having the incorrect solder maskapplied in the board manufacturing process can cause serious issues on elevatedtemperatures (lead free) including contamination of the solder. I do notrecommend changing the flux dispensing rates and/or thermal profiles until youget the PCBs investigated. Changing too many things at one time will make youlose track of the variables and finding the root cause impossible.
Engineering and Operations Management
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
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