The most common embedded component in a printed circuit board (PCB) is a plated through hole (PTH), which serves as a conductive conduit from one layer of the board to another. They are created by drilling a hole in the board and plating the inside with a conductive material, usually copper. There are many drill diameters, plating thicknesses and materials that can be used. Figure 1 shows a cross section of an intact PTH.
Figure 1: Anatomy of a plated through hole.
Thermal fatigue of PTH barrels is a significant issue in electronics reliability. The difference in coefficients of thermal expansion between the metal plating of the through hole and the laminate, typically a fiberglass/epoxy resin composite, causes a large enough strain in the embedded copper structures to induce a fatigue failure mode during thermal cycling. One of the most difficult aspects of detecting a PTH crack is that the electrical failure first occurs when the board is at high temperature, since this induces tensile strain on the barrel. When the board is then tested at room temperature the fault is not found because the board has shrunk and the two sides of the crack are touching.
Fortunately, sound design decisions regarding laminate type and copper plating material, along with modeling and an understanding of product field environment and desired lifetime, allow manufacturers to ensure their reliability goals are met.
Plated Through Hole Fatigue
In temperature cycling, PCB expansion and contraction in the out-of-plane (z) direction is much higher than that of the in-plane (x-y) direction. The glass fibers constrain the board in the x-y plane but not through the thickness. As a result, stress can build up in the PTH barrel during thermal cycling and eventually cracking occurs, typically near the center of the barrel. Several cross sections of cracks in PTH structures are shown in Figure 2.
(Top row left to right: Image 2, Image 3; Bottom row left to right: Image 4, Image 5)
Figure 2: Cross sections showing cracks in plated through holes.
Managing PTH Fatigue
It is impossible to eliminate PTH fatigue completely, but managing it using these tips can improve PTH reliability in several ways:
1. Match Coefficients of Thermal Expansion (CTEs)
PTH reliability can be improved if the board laminate and PTH plating material have a similar CTE for the out-of-plane direction. The board glass style and resin can be modified to some degree to help match CTE values. For example, increasing the glass content can help reduce the CTE mismatch, though it will be harder to drill the holes into the board, which may increase cost.
The predicted numbers of cycles to failure are plotted in Figure 3 for different glass styles using the same plating material, plating thickness, temperature profile and drill diameters. Moving from left to right in the figure, glass content increases, lowering CTE and increasing the elastic modulus of the laminate. There is a clear trend in improved PTH fatigue performance for low CTE and high modulus glass styles. However, the use of underfills, potting compounds and thick conformal coatings can greatly influence the failure behavior under thermal cycling and must be considered. A further complication arises if the laminate goes through its glass transition temperature within the thermal cycle.
Figure 3: Effect of glass style on PTH fatigue performance.
2. Reduce Aspect Ratio
PTHs with a small diameter for a given board thickness have a high aspect ratio, meaning PTHs tend to have less plating in the middle of the barrel compared to each end. Therefore, the chance of cracked barrels increases due to z-axis expansion during thermal cycling. Board designers should keep the aspect ratio as small as possible for a given design.
3. Plating Parameters
Adjustment of the copper plating parameters, such as reducing the current density or using reverse pulse plating, helps to ensure uniform thickness of the barrels and improves reliability. Plating parameters can also affect the plating microstructure. If current density is too low, coarse grains in the copper can result that will reduce ductility and thermal fatigue life.
4. Life Prediction
Designers may further manage reliability by using the industry accepted failure model outlined in IPC-TR-579 to predict the appearance of cracks in PTH barrels. DfR Solutions’ Sherlock Automated Design Analysis™ software includes a calculator to facilitate computations using the IPC model.
While thermal fatigue of PTH barrels is a significant issue in electronics reliability, managing it with sound design decisions and life prediction modeling helps manufacturers ensure that their reliability goals will be met.
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Image 5: http://www.semlab.com/blog/?p=93