|Ask the Experts|
November 13, 2020
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
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.
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
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
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