Ask the Experts
March 12, 2019
Solution for Warped PCBAs
From a large lot we have 20 populated boards with components on both sides that are warped. Components vary in height. What method can we use to flatten them?
Expert Panel Responses
Sorry but you will not be able to flatten these assemblies. You would have to take the PWB above its glass transition temperature and under pressure, hold it there and then allow it to cool down while still under pressure. Apart from the damage while applying the pressure, the temperature needed to be used will compromise the life of the active components on the board and if a high Tg material possibly also the passive devices. Negotiate with the board supplier to cover costs of the BOM.
S T and S Testing and Analysis
Gerald O'Brien is Chairman of ANSI J-STD 003, and Co Chairman of IPC 4-14 Surface Finish Plating Committee. He is a key member of ANSI J-STD 002 and 311 G Committees Expert in Surface finish, Solderability issues and Failure analysis in the PWA, PWB and component fields.
Flattening may jeopardize solder joint integrity inducing cracks immediately or with time. This is always a worry when mounting a severely warped PCB to a chassis. Those cracks may not be readily apparent, especially under area array devices such as BGAs, CSPs etc. Generally trying to flatten an assembled board is not recommended. An additional thermal excursion will increase solder joint intermetallic thickness and decrease the assembly's reliability.
IF the assembled board is entirely surface mount and IF the board is high value, then try clamping the board in a flattening fixture and run it through a minimized reflow cycle. The fixture should clamp down on the board itself and not on any components. Before running product boards with any clamping fixture run a thermal profile first with a thermocoupled profile board to understand the impact of any flattening fixture on the thermal profile and the temperature experienced by the PCA.
As stated previously, be aware that the added reflow cycle may impact reliability. That being said, most boards and components are specified to survive multiple reflow cycles as needed for original assembly and repair.
Best to get to the root-cause of the warpage. Discuss with your board supplier to see if anything can be done to minimize warpage or try boards from a different supplier to see if the warpage persists. Is the warpage resulting from a repair cycle(s)? Sometimes the differential heating (or overheating) during hot-air repair or selective solder removal/repair will induce warpage.
A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.
Once the PCB is populated there is really not much you can do to remove the warp. Unwarping a PCBA might cause stress on the solder joints formed where the warpage is most noticeable creating other issues. I'd use some type of weight to press the warp down and place the PCBA on bake at 250 degrees F for 1-2 hours.
Senior Manufacturing Engineer
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.
This might be a problem depending on how serious the warp is - there are IPC acceptability criteria for this that you can follow.
Forcing these PCBs to get to a level of flatness that you are comfortable with can cause component damage on one side or both sides. A PCB rack or chassis with card locks might be a solution.
Engineering and Operations Management
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at firstname.lastname@example.org.