When providing through hole PCB assembly services, reliability is one of the key requirements that PCB manufacturers need to meet. At its essence, reliability refers to the probability of a printed circuit board functioning in the desired manner for a long period of time. To achieve this, it is important to ensure that there are no defects, which will lead to PTH failure.
Understanding Different Types of PTH Failures
If you wish to create a reliable PCB, you first need to understand the different types of possible failure scenarios. These can include:
- Foil Cracks
- Corner Cracks
- Barrel Cracking
- Thermal Excursion Stress
- Interconnection Defect (ICD)
ICD is one of the biggest problems faced by PCB manufacturers. This is caused by stress created by differences in the PTH system and the Coefficient of Thermal Expansion (CTE). The resin material’s glass reinforcement prevents the resin from expanding in the x and y planes. This results in resin expansion at a faster rate in the direction of the z-axis. This further leads to stress on the adhesion between the interconnections and the plated copper, causing them to separate.
How Stress can affect PTH PCBs
Stress is one of the biggest factors that lead to PCB failure. It can lead to foil, barrel, and corner cracks. There is an order of failures that take place due to excess stress on the PCB. You can understand the order by reading the points below.
- Resin substrates are anisotropic, i.e., they have different measurement values in different directions.
- When stress is applied onto a substrate, its CTE rises above the specified glass transition temperature. This results in strain on the z-axis, which in turn creates stress on the plated through holes.
- The through holes resist the expansion, but as a consequence, this resistance leads to barrel cracking.
These failures can take place either together in one operations cycle, or slowly over a period of multiple cycles.
Possible Solutions to these Problems
There are certain solutions that can be suggested to customers who are availing through hole PCB assembly services. These include:
- During the prototyping phase, subject high aspect ratio through hole PCBs to repeated thermal shocks. This will help identify the temperature range in which failure takes place. This will allow PCB designers to make changes to the design, and use more appropriate materials.
- Choose a resin material that has a high temperature of decomposition (Td). This will help the PCB not lose resin material weight, and will be able to handle thermal strain.
- Maintain the electroless copper thickness on the plated copper interconnections. Also, ensure that the grain deposit of the copper structure is strong. Both these factors can help reduce the possibility of ICD.
- Ensure good copper plating practices.
All of these points can help manufacturers who provide through hole PCB assembly services to create high reliability PCBs.