Common Printed Circuit Board Failures & How to Avoid Them. Pt. 2
In our last blog post, we looked at the phenomena of failure in Printed Circuit Board Assemblies – discussing that due to their complex designs and the intricate manufacturing process required to construct them, that failure in some PCBs is not altogether surprising.
We then examined three of the common reasons PCBs fail: plating voids, acid traps, and missing solder mask between pads. We discussed each reason in detail, stressing the fact that each failure could have been avoided by design or process modifications – something that an experienced PCB manufacturer is more likely to catch than a company that is assembling PCBs on an ‘occasional basis’.
In this post, we’ll take a look at three additional reasons that PCB Assemblies fail – and how these board failures can be avoided.
- Copper-to-Edge Clearance on the PCB is Inadequate
While being an optimally conducive metal, copper is also a soft metal that corrodes easily. PCB manufacturers typically cover the copper in PCBs with other materials to reduce the opportunity for corrosion. When the PCB is trimmed, part of the coating can be trimmed as well, which exposes the copper layer underneath. Corrosion then occurs at these points, causing the board to fail.
This problem can be avoided by inspecting the finished PCB and making sure the space between the edge of the board and the edge of the copper follows acceptable standards for the type of board being manufactured. If space is insufficient, the PCB will need to be scrapped or modified to reduce the space.
- Starved Thermals
Thermals are small traces around the pads on a PCB, that connect the pads to the board. Thermals primarily function to help pads dissipate heat. On occasion, voids between the thermal and the plane, or between the thermal and the pad, resulting in an incomplete connection, which minimizes the effectiveness of the thermal, resulting in functional problems. These are called starved thermals.
Starved thermals can affect a PCB in many ways. For one, since they take longer to transfer heat from pads to the rest of the board, problems during soldering can occur. The pad may solder incorrectly and will slow down the assembly process by taking an abnormally long time to reflow. Finally, PCBs with starved thermals may not transfer heat well, causing overheating and heat damage. This can cause PCB failure.
As with insufficient copper-to-edge spaces, the best way to avoid producing PCBs with starved thermals is to alter the manufacturing process and to inspect the final product, replacing starved thermals with properly functioning ones.
- Slivers Forming During the Etching Process
During the manufacture of the bare PCB, narrow wedges of copper can be produced, causing serious problems in the PCB. These small narrow wedges are called slivers.
Slivers are usually produced during the etching process when a long thin part of the copper is etched away. These slivers can then float around in the chemical bath, attaching themselves to the board and by so doing, adding an unintended connection. This will ultimately result in failure or partial failure of the PCB.
Slivers can be avoided by utilizing a PCB design that optimizes line and width spacing, thus reducing the amount of material that needs to be etched. Additionally, inspection of the PCB after manufacturing can reveal the presence of slivers – thus eliminating the board from use.
As many problems related to PCB Assemblies occur during the fabrication and assembly stages, it is critical that processes be established for inspecting a circuit board to minimize failures that occur during the manufacturing process. Design considerations are very important in avoiding plating voids, acid traps, inadequate copper-to-edge clearance, the formation of slivers, and other causes of failure in PCBs.