Selecting the optimal heavy copper thickness to apply to the plated through hole (PTH) plays a critical factor towards the overall reliability of the printed circuit board. There are two key elements to consider when determining optimal PCB copper thickness. The first is the current capacity of the barrel for acceptable heat rise. The second is the mechanical strength determined by the copper thickness, hole-size and whether or not there are any support vias.
Choosing Your PCB Materials
There are a variety of dielectric materials that circuit board manufacturers and designers can choose from such as the standard FR4 (operating temp. 130°C) to high-temperature polyimide (operating temp. 250°C). If your application will be prone to extreme environments or high-temperature situations, consider using a more exotic material, but if the circuit traces and plated vias are standard 1 oz/ft2; will they survive the extreme conditions?
A test method has been designed specifically for the printed circuit board industry to determine the thermal integrity of a finished circuit product. Thermal strains come from various board fabrications, assembly and repair processes, where the differences between the coefficient of thermal expansion (CTE) of Cu and the PWB laminate provide the driving force for crack nucleation and growth to failure of the circuit. Thermal cycle testing (TCT) checks for an increase in resistance of a circuit as it undergoes air-to-air thermal cycling from 25°C to 260°C.
Any increase in resistance is an indication of a breakdown in electrical integrity via cracks in the copper circuit. A standard coupon design for this test utilizes a chain of 32 plated through holes, which is considered to be the weakest point in a circuit when subjected to thermal stress.
Printed Circuit Board with Heavy Copper Plated Through Holes
The TCT results clearly show that the failure rate, no matter what the board material, can become unacceptable. Thermal cycle studies done on standard FR4 boards with 0.8-mil to 1.2-mil copper plating have shown that 32% of circuits fail after eight cycles (a 20% increase in resistance is considered a failure). Boards with exotic materials show significant improvements to this failure rate (3% after eight cycles for Cyanate Ester), but are prohibitively expensive (five to 10 times material cost) and difficult to process. An average surface-mount technology assembly sees a minimum of four thermal cycles before shipment, and could see an additional two thermal cycles for each component repair.
Regardless of what material is being used the TCT results will clearly distinguish failure rate resulting in an unacceptable board. Thermal cycle studies done on standard FR4 boards with 0.8-mil to 1.2-mil copper plating have shown that 32% of circuits fail after eight cycles (a 20% increase in resistance is considered a failure). Boards with exotic materials show significant improvements to this failure rate (3% after eight cycles for Cyanate Ester), but are prohibitively expensive (five to 10 times material cost) and difficult to process. An average surface-mount technology assembly sees a minimum of four thermal cycles before shipment, and could see an additional two thermal cycles for each component repair.
Summary
Effectively and efficiently utilizing heavy copper circuit boards would reduce or eliminate most failures altogether. Plating of 2 oz/ft2 of copper to a hole wall reduces the failure rate to almost zero (TCT results show a 0.57% failure rate after eight cycles for standard FR4 with a minimum of 2.5-mil copper plating). In effect, the copper circuit becomes impervious to the mechanical stresses placed on it by the thermal cycling.
