Solder Joint Explosions

We have seen joint voids that look like "explosions" after wave soldering. Some members of the team believe the problem is caused by humidity brought to the process by the bare board. The problem was solved by changing the bare board lot. Have you seen this problem before? Do you agree that the cause is likely due to moisture absorption in the bare boards? Can the suspicious lot of boards be salvaged?

G. V.

Expert Panel Responses

It is difficult to diagnose accurately without seeing an example of the failure and doing a cross section on it. However, this problem could be caused by either circumferential or just large voids in the plated through holes of the bare board. This would allow the fibers of the glass in the laminate to absorb moisture, not just water but possibly acids or other process chemicals. This moisture boils when the board is soldered causing the "explosion" like occurrence you describe. Some fabricators do a final bake to avoid this problem and in fact that is something that you can do to reduce the incidence of these failures. Be aware however that baking might improve the apparent solder joint quality (the symptom) it will not fix the underlying issue of poor quality bare boards with voids or possible very thin or under spec hole wall plating, if, in fact that is the cause. The fact that it is batch related makes me very suspicious of the underlying quality of the batch in question and I would certainly inspect the holes for voids and I would cross section a sampling of any suspicious areas for voids as well as for minimum hole wall plating thickness.

Daniel (Baer) Feinberg
President and Founder
Fein-Line Associates

Mr. Feinberg is a 52 year industry veteran and President and founder of Fein-Line Assoc, a consulting group serving the Global Interconnect and EMS industries.

First make sure the temperature that the board's solder joints are being subjected to are within reasonable temperature limits by running a thermal profile of the board with thermocouples located on the solder joints specifically where the "explosions" are happening.

This will assure that the temperatures are not too hot and are of a reasonable time duration. Not to hot is: no more then the component's maximum temperature rating as specified by the manufacture, and yet well above the solder's liquidus temperature, and the correct duration is typically 2 to 4 seconds.

Besides moisture, one possible cause of these explosions (AKA: blow holes) is air escaping from between the layers of the PCB through pin holes in the barrel of the plated through hole. This will happen no matter how much you pre-bake the board because it's air and not moisture being forced out as the board heats.

If pre-baking the bare boards to drive off moisture does not solve the problem, then the only solution is to look to your board vendor to make sure the through-hole plating is 100% with no pin holes.

Paul Austen
Senior Project Engineer
Electronic Controls Design Inc

Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 39 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes and well as moisture sensitive component storage solutions.

It sounds like you have found out the problem, humid PCB storage. To salvage the remaining boards you should pre-bake them just like you would a PBGA component.

Edward Zamborsky
Regional Sales Manager
OK International Inc.

Ed Zamborsky is a Regional Sales & Technical Support Manager for Thermaltronics, located in New York. His position requires frequent customer visits throughout North America and the Caribbean and his position encompasses not only sales but the role of trainer and master applications engineer for all of Thermaltronics products. His expertise includes such specialties as hand soldering, convection and conduction reflow techniques, array rework, fluid dispensing equipment, and fume extraction. Ed has authored many articles and has presented many papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead-Free Hand Soldering, High Thermal Demand Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.

Most likely this would be what is called blow holes. They will have the appearance of a volcano top shape and edge, They will be predominately appear on the bottomside solder joints after a wave soldering process. As you indicated, these defects are caused by moisture in the board. The moisture accelerates into vapor from the heat or the wave process and escapes after exiting the wave. At times there is enough moisture that the barrel can actually be cracked by the escaping vapor. Depending on the level of moisture entrapped you maybe be able to evaporate the moisture by using a pre-bake in a heat chamber.

John Norton
Eastern Manager
Vitronics Soltec

John Norton started his soldering career in 1983 for Hollis Engineering. He has also worked with Electrovert as a technical training manager and Vitronics Soltec for the last ten years. He has held various technical development and sales positions.

If the PCB's are HASL finished, the problem may be due to improper rinsing of the HASL fluids/fluxes used. They absorb moisture like mad and can cause long term reliability issue's.

Greg York
Technical Sales Manager
BLT Circuit Services Ltd

Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

Reader Comment

We had experienced the same problem not long ago. As we bake our boards before reflowing them, we send one board for a cross section inspection. The problem was small hole inside the plating and some chemicals was trap inside these holes and "explodes" during wave soldering. I suggest you to bake your boards before soldering them and if you already do that, ask for a cross section inspection.

Julien Chollet, Meggitt SA, Suisse

Reader Comment

Bob Willis has an excellent video clip to illustratemoisture entrapment turning into blow holes. A drop of light machine oil is applied to the barrel of a PTH. The board isplaced under magnification and then a fine tip soldering iron is applied to thepad of the PTH in question. If there is moisture in the PCB, it will turn intosteam from the heat of the solder iron and escape through the barrel platingbut be captured in the oil as a bubble. As long as the heat source remains, thebubbling will continue proving this is a board fab and non machine relatedissue.

Ray Chartrand

Usually this type of defect is associated with moisture entrapped in the bare board's structure. The fact that changing the lot made the defect disappears gives you a place to start:

is this a lot problem from the fabrication process? The boards' supplier will have to answer this question and prove the quality of the boards with their coupon
is this a storage issue? Improper storage can cause the moisture to get inside the bare boards and during the reflow process (SMT oven, wave soldering, selective soldering, etc.) this moisture will try to escape causing the "explosions" The improper storage is something that can happen at the supplier and/or at your facility A common practice to salvage the bare boards is a baking process. My recommendation is to try a vacuum bake which will allow you to get the moisture out faster and using lower temperatures. Excessive baking temperature or duration can cause other issues to your process.

Please e-mail me for more details and a plan to solve the problem. georgiansimion@yahoo.com

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

As I have already had a name check on the outgassing problem in this page (Ray Chartrand) if readers want to see the two outgassing videos from my defect of the month series they can visit https://www.youtube.com/user/MrBobwillis

Bob Willis, Bobwillisonline.com

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