Conductive ink for through hole application

Silver Through Hole (STH) technology describes a method of creating an electrical interconnect between the top and bottom sides of a printed circuit board. STH production has gained and continues to rapidly gain acceptance worldwide due to its low-cost, reliable and environmental friendly process. The detailed process for through hole connection is described in this paper. Even though the technology necessary to produce STH boards is mature, the performance of current through hole ink relies on different printed circuit board (PCB). The stability of electrical conductivity on exposure to solder baths or to thermal cycling is a predominant issue. This problem is associated with different metal fillers, resin systems and PCB types. For example, the electrical resistance of the ink on FR2 substrate gradually increases after each solder bath dip. Fundamental study has been conducted on the root cause of the hole resistance drifts after solder bath. TMA and DMA have been used to characterize CTE and curing performance of the ink and PCB substrates. The failure mechanism has been proposed and confirmed by designed experiments. Experimental results demonstrated that a hole resistance drift on FR2 substrate is caused by several factors. First, the higher thermal expansion of the substrate itself stretches the coating layer. Secondly, the continued curing makes the resin matrix fixed at higher temperature. The final factor is the CTE mismatch between the conductive ink and PCB substrate.