|Ask the Experts|
July 17, 2018
Improve DPPM for Wave Soldering Using Nitrogen
We are wave soldering complex boards with 3000+ components using OSP finish and SAC305 alloy. Our current Defective Parts Per Million (DPPM) level for soldering is 400-500 and for solder voids is around 1000. To improve our DPPM level we plan to install a nitrogen environment around the wave soldering tank to reduce oxidation. Is this likely to improve our DPPM level?
|Expert Panel Responses|
In all probability, yes. Nitrogen will significantly reduce the amount of "air" contacting your PCB, dependent on the quality & efficiency of the equipment utilized to effect the "blanket." The question then becomes "How much of a savings will you achieve, rework Vs cost of the equipment + cost of labor + ongoing cost of nitrogen?"
Highly unlikely in fact no one to my knowledgehere in the UK use Nitrogen inerting anymore. I would also not use SAC305 andOSP as the Cu dissolution would be massive. Make sure your dwell times are nomore than 2-3 seconds importantly use a good flux and ensure the alloy does nothave a gold color on the surface. Lastly make sure the solder continuallyflows over the back of the nozzle and actually flows away and doesn't gatherreducing the flow.If you want to send me some images of the defect then I maybe able to helpfurther
Technical Sales Manager
BLT Circuit Services Ltd
By using an inert atmosphere in the wave soldering process will reduce several defects including bridging, icicles and will improve the voiding issue. Through hole fill will also be improved. Reduction will be dependent on the O2 ppm levels and the type of inerting system that is used. Our work in this area has seen overall reduction of total defects of over 50%. Dross reduction is also a benefit that we have experienced over the years with our technology. Here is a chart that one of our customer provided on defect reduction using an inert atmosphere for SAC 305 in wave soldering.
Global Segment Manager
Air Products & Chemicals, Inc.
AddingN2 to your system will help with increasing your process window but may not beworth the cost as your defect levels are pretty low. There have been numerouspapers presented on the subject of voiding with SAC in wave that requiredchanges in device lead to hole ratio changes to promote better hole fill andthus reduce the void defects - have you modified designs from when they werebuilt in SnPb or are these new designs? Secondly, how consistent is the OSPdeposit you are receiving from your board suppliers? Consistency of deposit iskey to reducing these defects - I would suggest looking at depositthickness Cpk for each of your suppliers using a non destructive measuringsystem, such as the one offered by Filmetrics. Are your supplierscontrolling the OSP process by destructive UV vis method or non destructivemicroscope methods? Are they applying the coating in a horizontal coatingsystem or in a vertical dip tank?
S T and S Testing and Analysis
Itwill improve but not eliminate.
SME - PWB Technologies
It's possible that a nitrogen environment will improveperformance. It's also possible that for some defect types it will increasedefects. The nitrogen blanket will reduce the surface tension of the soldersurface, changing how it "snaps off" from component terminations and/or leads. Without specificknowledge of the design of the boards, the configuration of the wave equipment,the flux being used, etc. it's hard to say, but I think that overall a nitrogenblanket has the potential to be a benefit. Realize that you will have tooptimize the process differently once you implement. If possible, arrange tosolder some of your more difficult assemblies on a machine so equipped prior toinstalling on your machine.
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