Every electronic device has a rated life, but due to mechanical shock, thermal shock and vibration, premature failure may still occur. Thermal cycling is just a thermal simulation of vibration-repeated mechanical stress acts on the structure in the PCB, causing fatigue and failure. After repeated thermal cycles, the later stage of temperature increase and volume expansion will cause mechanical failure.

Thermal cycling resistance is not a specific physical property that can be measured. Instead, it simply refers to the ability of electronic devices to withstand thermal cycling. Components must comply with their absolute maximum temperature ratings, but repeated thermal cycling can cause failure of both PCB structures (solder and vias). Let's look at how each of these structures produces thermal failures and how to prevent thermal failures.

The through hole is a structure that is prone to fatigue failure and fracture under repeated thermal cycles. Although these structures are prone to fracture under extreme temperatures, how to prevent thermal cycle failure is the mechanical design of the through-hole. The choice of the correct substrate material also helps to improve thermal reliability. These points are suitable for high aspect ratio vias and blind/buried vias in HDI boards.

Just like the reliability of solder balls, the mismatch of the CTE value of the copper and the substrate can also cause thermal cycle failure. FR4 is an anisotropic material with a CTE value perpendicular to the surface of the board ~70ppm/°C; please note that this is different from the CTE value along the surface (~13ppm/°C). For comparison, the CTE value of copper is ~16ppm/°C. This means that when the board is heated to a high temperature, the stress is mainly borne along the axis of the through hole, as shown in the figure below.

Force is applied to the through hole due to the thermal expansion of the substrate

The figure above shows the force acting on plated through holes, but a similar schematic diagram can also be drawn for blind holes or buried micro holes. The influence of this stress depends on the geometry of the through hole, especially the aspect ratio and structure of the through hole. The location of the stress concentration also depends on the through-hole deposition and electroplating methods, which involve deposition from solution.

Blind and buried vias

The plating of the blind hole near the neck of the through hole is slightly thinner. This is the main breaking point of the blind hole on the surface layer, where the expanded substrate is up against the neck of the through hole. The stacked blind vias and buried microvias are easily broken at the interface of the stack. In other words, the neck area of ​​the buried via can be separated from the bottom of the via above, resulting in a high resistance connection or open circuit.

IPC recently issued a reliability warning that will be included in the IPC6012E standard, and the reliability of blind and buried vias must be evaluated during the manufacturing process. In order to ensure the reliability of the manufacturing process, the standard test method in IPC-TM-650 (Method 2.6.27) requires that the test sample should be subjected to normal solder paste reflow soldering to reach 230°C or 260°C Peak temperature. When C is connected to a 4-wire resistance probe, six complete return curves can be obtained. As long as the resistance increase does not exceed 5% during this repeated reflow curve, it can be considered that the board has sufficient thermal cycling resistance before it can be put into use.

High aspect ratio through hole

When a typical through hole or buried plated through hole is deposited during the manufacturing process, the electroplating solution will have capillary action. When the board is immersed in the plating solution and copper begins to deposit along the walls of the vias, the surface tension affects how the plating solution is drawn into the vias. If the electroplating solution is not fully stirred and agitated during the deposition process, the copper precursor will be consumed faster near the center of the through-hole barrel due to the formation of a meniscus.

This results in the plating layer in the central area of ​​the via barrel being thinner than the plating layer in the neck portion of the via. If the aspect ratio of the via is large, the copper coating inside the via will be thinner. Once the substrate expands at high temperatures, the center of the through hole will withstand greater stress concentration and will be easier to crack.

There are three possible solutions, involving design and manufacturing:

Use through holes with a smaller width and height. For longer through holes, this only means increasing the through hole diameter.

Use PCB substrate with CTE value close to copper. For some designs, such as high-speed designs that require low-loss laminates, you may need to compromise the performance of other substrate materials to lower the CTE value.

Make sure that your manufacturer uses a lower viscosity plating solution under agitation to more evenly deposit the copper coating in the through-hole barrel and neck.

Welding reliability

Just as vibration fatigue can cause mechanical failure in solder balls, so can thermal cycling. When the temperature of the solder joint rises to a very high level, the expansion rate of the solder is lower than that of the base material, but the forces involved are difficult to predict. The stress concentration in the solder ball is easier to predict, and the finite element method (FEM) model shows that the stress is concentrated near the top and bottom of the solder ball, causing fracture.

As long as the solder joint is sufficiently strong and ductile, the solder joint can withstand repeated thermal cycles. This means that manufacturers need to address any factors that may affect the strength and ductility of solder joints. These include corrosion, insufficient wetting, insufficient solder, and other factors that can only be addressed by the manufacturer. When preparing for manufacturing design, make sure that the manufacturer understands the thermal environment on the circuit board to prevent solder joint failure under repeated thermal cycles.