How to Handle Warped PCBs During Rigid Flex
Handle Warped PCBs During Rigid Flex
One of the biggest challenges in pcb rigid flex is dealing with warped boards. This issue has many causes, but can cause significant production delays and lower quality. It can also lead to rework, retesting and scrapping. In addition, if the PCBs are not flat, they cannot be fed into a pick-and-place machine or drilled on. This is a major pain for designers, fabricators and manufacturers.
The main reason for warpage is that the PCB’s layers do not expand at the same rate when heated up. The different expansion rates can cause the board to curve inward. It can also create uneven stresses between layers. This stress concentration can cause the conductive copper to crack through the ENIG coating that encapsulates it.
Several other factors can contribute to the warping of a PCB, including asymmetric material layup, CTE/Tg mismatches, and excessive reflow and wave cycles. These issues can be prevented by using a gradual heating profile, by minimizing the number of reflow and wave cycles, by avoiding clamping, and by limiting the number of layers that go through rework or reflow.
Rigid flex design is a complicated process that requires technical knowledge and an observant eye for potential DFM mistakes. These include asymmetric layer stack-up, inadequate copper distribution, excessive bend angles, neglecting the bend radius, pad supports and stiffeners, and insufficient clearances. It is possible to avoid these errors by performing a trace mapping procedure, which predicts warpage by analyzing the copper constituent.
How to Handle Warped PCBs During Rigid Flex
To prevent warping, designers should use a symmetrical and balanced material layup with no gaps in the layer stack. They should also keep flex layers in the center of the structure. They should also avoid using a high-resistance dielectric material for the flex sections and instead opt for low or no-flow prepreg.
They should also try to minimize reflow and wave cycles by only putting the boards through two reflows and one or two wave cycles. This will help prevent stress buildup between layers and ensure the integrity of the solder mask and conductive copper.
It is also important to avoid the use of acrylic adhesives in flex coverlayers and bondplies because they have high Z axis CTE expansion rates. This can put huge stresses on plated through holes and can cause them to fail. Finally, the PCBs should be laminated with a no-flow core or no adhesive in the flex section to avoid air gaps and high-resistance shorts.
Recognize signs of warping such as unevenness or bending in the PCB. Check for warping both before and after assembly processes. Determine the cause of warping which could include improper storage conditions, uneven heat distribution during manufacturing, or design flaws. Store PCBs in a controlled environment with stable temperature and humidity to prevent moisture absorption and subsequent warping. Apply corrective techniques such as flattening the PCB using a flat surface and applying gentle pressure. Heat can also be used cautiously to reshape the PCB, but ensure temperatures are within safe limits to avoid damage.