|Ask the Experts|
November 13, 2017
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?
|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.
President and Founder
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.
Senior Project Engineer
Electronic Controls Design Inc
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.
Regional Sales Manager
OK International Inc.
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.
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.
Technical Sales Manager
BLT Circuit Services Ltd
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
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:
Engineering and Operations Management
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/MrBobwillisBob Willis, Bobwillisonline.com
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