Many issues can occur while manufacturing printed circuit boards (PCBs), most of which are either preventable or correctable. It’s crucial to pay attention to them and take appropriate measures to prevent further problems down the line.
One of the biggest issues is warpage, which typically occurs during PCB assembly. Left unattended, the warpage can result in issues like stretched joints and unfit boards. Today we will look at what warpage is, what causes it, and how you can prevent it from ruining a fresh batch of PCBs.
What Is Warpage?
PCBs are flat by design, offering a solid surface on which to mount components. Traces run along their surface, connecting the individual components into a single circuit. Whether the design is single or multi-layered, one thing must always be true – the board must be flat.
A PCB suffering warpage was twisted, bent, or warped somehow. The surface is no longer flat but will bend lengthwise, widthwise, or in combination. The warpage is known as bow or twist, depending on where the curve is; along a single axis (bow) or both axes (twist).
In the case of bows, the board will form a single arch that will bow upward, known as positive, or downward, known as negative. With twists, the board will have warped to have both positive and negative curves together. This curvature will drastically disrupt the nature of the traces, components, and holes on the board’s surface, causing many problems with the assembly.
Warpage primarily will cause issues during the assembly process, leading to:
- Incorrect component placement – Pick-and-place assembly machines rely on a flat surface to properly place components. As the machines cannot compensate for the curvature, components cannot slot into their designated spaces.
- Stretched or open joints – Joints for components can become stretched along the bend in the board, affecting their shape and preventing them from correctly holding components. This will also cause excesses or inadequate amounts of solder for each hole, as their sizes will no longer be uniform.
- Solder bridges – Cracks or warpage in the traces and vias can lead to solder spilling. If this occurs too close to other solder points, bridges can form, leading to short circuits in the final assembly.
- Solder flooding – Wave solder machines melt solder balls set in component pins to hold components in place. If the board is warped, the solder will not be held in place and will run into or off the board, pooling and solidifying in the wrong place.
- Boards unfit for the final build – Warped boards will no longer have the same dimensions as their flat counterparts. As such, they will not fit correctly into the final assembly intended for the board.
What Causes Warpage?
There are two main causes of warpage in PCB production – an over-extension of physical stress upon the board or coefficient thermal expansion (CTE).
Much like any other pliable substance, the board will become bent if a piece of the substrate is stacked incorrectly or held in place wrong. If left in this position for longer periods, or allowed to weaken due to temperature changes, then the bend may become permanent. This can happen due to the boards needing to be stored flat or against another surface, particularly in tight areas. It may also occur if you secure the board too tightly in a frame during the assembly process. In the first case, if you put the board under undue physical stress during storage or assembly, the stress can express itself in warpage.
In cases of thermal expansion, the differences between the copper percentages in the traces and the substrate will lead to different expansion rates. The board will expand at different rates as the board gets heated up, either during storage or the reflow stage. The expansion can cause the board to warp around without accounting for it.
Regardless of the cause, once a board begins to warp, it will only get worse. This is because further heating during the reflow/solder stage will amplify the warpage that has already occurred. Fortunately, it is possible to predict the potential for warpage and plan accordingly to prevent it.
How to Prevent Warpage
Unfortunately, there are no industry specifications for PCB warpage, for neither its causes nor how it will affect the final assembly. However, one standard known as IPC-A-610E specifies that the maximum bow or twist must not exceed 0.75% from the board’s original axis. In these cases, the warped PCB will be denied use, classed as a failure.
When it comes to preventing warpage, there are several considerations that the designer should make that will affect the chances of it occurring. These are:
The balance of copper content through all layers
The best way to reduce warp is to ensure that all layers of the board have an equal balance of copper content. By ensuring that the copper levels are within small variations, the range of thermal expansion will remain similar. This way, with no parts of the board expanding more than others, the board itself will remain stable and unlikely to warp.
The balance of substrate in all layers
The same is true of the substrate used in the board’s construction. The expansion rate will be maintained and limited by keeping equal substrate levels. Using more substrate material will help compensate for the copper expansion when copper cannot be balanced. This can add further rigidity to the board, especially in multi-layered boards.
The design of the pallet used in production
The pallet is the part of the machine in which the board is secured during component application and reflow. You can avoid putting excess pressure on the board’s edges by ensuring that the PCB assembly fits snugly in the pallet. Plus, consider the clearance of the board and its components above the edges of the pallet. Consider the springs used to hold the board in place where possible.
Pre-treatment of the board
Planning the reflow oven’s temperatures around the board’s treatment will allow for a more even spread of heat throughout. A slower heat will allow the layers and laminates to soften equally and slowly, preventing warpage.
Warpage is an issue that will have far-reaching effects on a printed circuit board during its assembly. You can easily plan for and prevent it—all it takes is careful consideration and pre-planning.