What Causes PCB Warpage?
Warpage is one defect that can result during PCB manufacturing, both in a bare circuit board and in a PCBA. The thermal stresses experienced during manufacturing can cause a bare board to bend and twist during production, and this could become “locked in” if the conditions are right. To prevent warpage, there are a few things a board designer can do and that a manufacturer can do.
In high-volume production, a QA team should work to ensure processing parameters are consistent as deviations can lead to multiple defects including warpage. Some of these important factors are outlined below, and there are some practices that designers or QA engineers should not do to try and fix PCB warpage.
Causes of Warpage in a PCB
There is one thing you must note about PCB warpage: every board will be very slightly warped. This is unavoidable and it results from the curing process during the lamination step in bare board fabrication. Other fabrication steps involving temperature cycling will cause the board to warp as well. Overall, the board should be below some limit on warpage as measured in terms of flatness, bend, and twist/bow.
With this in mind, we can look a bit deeper at the major causes that contribute to larger amounts of warpage in a bare board and a PCBA.
Materials With Mismatched Tg and Stackup Asymmetry
During lamination, the board is heated up until the resin in the board flows, which is generally above the Tg value of even high-Tg PCB materials. The same level of heating occurs during reflow. If your board contains materials with mismatched Tg and CTE values, excessive stress can occur between layers and the board can warp. The same problem can occur when there is excessive asymmetry in the PCB layer arrangement.
CTE mismatches between board materials can stress layers in the PCB stackup as well as components in the PCBA.
If you design your own board stackup, make sure the CTE values and Tg values are compatible for your FR4-grade materials. A good strategy to ensure you don’t make a mistake is to take your fabrication house’s standard stackup, and use their material set to build your own stackup. This way, you can have some assurance that the requested board stackup will be compatible with your fabricator’s process without high chances of warpage defects.
Clamping During Wave/Reflow
While it may be tempting to clamp a bare board to a fixture during assembly, this should not be done. The board will be heated during either process and it will expand significantly; tight clamping to a fixture during these soldering processes can permanently deform the PCBA, especially if populated with a lot of heavy components that require soldering. Instead, allow the board to expand and contract on its own; it will slightly do this in the x-y plane, and the components will shift along with the rest of the PCB.
Notice how this PCB is going into a reflow oven without being clamped.
Uneven Copper Distribution
This is usually used as a justification for copper pour in all signal layers, but this is not a strict requirement for manufacturability. In multilayer PCBs, the board could have some layers that use copper pour on signal layers, but the complementary balancing layer does not have the same level of copper coverage. This can create an uneven temperature distribution in the unfilled layer that can contribute to warpage.
The solution is to ensure that complementary layers have the same coverage of copper. This could require that one of the layers use copper pour in all un-routed areas. Note that you need to set clearances in your design rules, especially if the filled layer requires controlled impedance.
Can a Warped Board be Fixed?
Unfortunately, if a bare board or a finished PCBA is warped, there is little change of fixing the problem. If the warpage occurs during the bare board fabrication phase, the board should be scrapped and the processing parameters should be evaluated to determine if something caused warpage at one or more of the fabrication processing steps. According to IPC-A-600, the allowance on PCB warpage is limited to 0.75% maximum; this provides a threshold above which a board can be scrapped.
The same ideas apply to a PCBA that has made it through reflow/wave soldering and is ready to be shipped. If the PCBA leaves the SMT/inspection line warped, then it is possible the board was already warped as-supplied, or the board warped during soldering. There are some things you definitely should not do to try and fix a bare board or PCBA:
- Don’t try to press or bend a warped PCBA with a clamp or fixture, the applied stress will crack solder joints.
- Don’t try to heat and press a bare board, basically mimicking the lamination process; this could make the warpage worse or cause delamination if not done properly.
- Don’t try to desolder and rework a warped PCBA as the warpage will be made worse and some board materials will further degrade.
Instead of trying to fix a batch of warped PCBs, it’s best to investigate the root cause and prevent the condition from arising again.
When it’s time to specify your qualification and performance requirements for your bare boards and your PCBA, use OrCAD from Cadence to create your fabrication artwork and drawings with your design requirements. OrCAD includes the industry’s best PCB design and analysis tools for design, layout, and simulation. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.
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