Warpage in PCB and PCBAs
Have you noticed that a particular board is no longer flat after assembly? Or maybe that a bare board that you received isn’t exactly flush? Board warpage is a general term used to describe an irregular PCB shape, regardless of that shape itself (bent, bow, twist etc.). One major struggle is that a board may transition from flat to warped as part of the soldering cycle so it cannot always be caught before hand . Therefore, the original shape may not directly correlate to the shape of the PCB post-soldering. Warpage in PCB and PCBAs can pose a problem in assembly or final implementation. We will dive into the reasons for both and how to prevent this problem.
When a bare PCB is populated, it goes into a pick and place machine that places components on top of it in preparation for the soldering process. With a warped board, this unevenness may lead to improper placements or parts falling off. Flatness is also important when the board goes through the reflow oven. These machines heat up the boards to high temperatures which could alter the materials and make the board warp. This could also lead to parts sliding off, not being placed right, creating solder bridges, or other soldering issues.
Warped PCBs leads to a poor final product and low acceptability. Ensuring the flatness of a board is crucial in preventing issues related to assembly, including bridged or open joints, which ultimately causes product failure.
Causes and Prevention of PCB Warping
PCBs are built from layers of different substrates that allow voltage and signals to pass through. Fabrication shops utilize many materials suitable for our industry. These materials have good thermal stability, resistance to ion migration, low dielectric constants, good processing performance, and no substrate wandering. The current circuit board manufacturing technology ensures that warpage in a PCB remains less than 0.5% in all states of the PCB assembly process.
The soldering process may be the largest catalyst for board warping. While in the reflow oven or wave solder machine, a PCB is exposed to high temperatures resulting in the materials in the board expanding and contracting. Due to the differences between expansion coefficients of the copper and the substrate materials, there may be unequal expansion and contraction between the two, resulting in the creation of internal stresses. As a result, the board may become warped as it cools and settles into its resting state. Improper storage and handling may also play a role in warping. If a board absorbs moisture this can cause areas to heat and cool at different rates causing warpage.
Other things that could cause boards to warp could be the actual design. Balance between the circuit area and the conductor pattern and symmetry of the circuit board stacking are things an engineer must consider during the design process. Operating temperatures of the PCB could also play a role if they exceed ratings. Lastly, during the – PCB fabrication process, PCB products undergo several thermal excursions and heat treatments. As the processing temperature exceeds the Tg of the copper clad laminate, the fabricator must heat both sides of the substrate equally and uniformly, while keeping the processing time as short as possible to reduce warpage of the substrate.
Conclusion on Warped PCB
A warped PCB poses a problem for pick and place assembly. SMT components will not land on the intended pads, components will not enter the intended hole in the board, components may slide off, or solder could bridge and short. An irregular PCBA is more difficult to install into a chassis and could pose reliability issues in the field. All of this makes it very important for all stakeholders to do their due diligence to not allow warpage in the PCB. To eliminate this problem, make sure you are getting your boards fabricated from quality shops and that your warehousing pays attention to board handling and storage especially in humid areas. Additionally, engineers can pay attention to design to ensure a balance board or they could spec higher Tg materials which would better allow the board to withstand different stresses. Vinatronic has a network of vetted fabrication shops across the globe and is happy to help in bare board procurement or in house assembly! Please Contact us if you have further questions.