Technique advancements in immersion treatment have led to the emergence of the ImAg surface finish. With its supreme electrical performance, reliable solder joints and competitively low cost, this surface finish has gained considerable attention from PCB manufacturers and suppliers. However, some potential reliability risks with this surface finish also prevent it from gaining wider market adoption. Among these risks, dendrite formation and surface corrosion are the two failure modes with the most serious controversy. A comprehensive analysis of these two failure modes is the main objective of the present study....[ Read more ]

Technique advancements in immersion treatment have led to the emergence of the ImAg surface finish. With its supreme electrical performance, reliable solder joints and competitively low cost, this surface finish has gained considerable attention from PCB manufacturers and suppliers. However, some potential reliability risks with this surface finish also prevent it from gaining wider market adoption. Among these risks, dendrite formation and surface corrosion are the two failure modes with the most serious controversy. A comprehensive analysis of these two failure modes is the main objective of the present study.

First of all, the dendrite formation was evaluated using a simple water drop test method. With the presence of liquid water between conductors, the dendrite formation will increase in both speed and severity of several orders of magnitude. The substantial test time reduction makes the water drop test an effective method to check the migration propensity. A comparative study was carried out on the solder pads with ImAg and OSP surface finishes. Experimental conditions included various DC voltage biases, media and gaps between test pads. The time-to-failure (TTF) was defined as the period for the leakage current to reach a certain level. The test results indicate that, when at a higher voltage bias, the TTF of dendrite formation is significantly shorter on the ImAg surface finish than that on the OSP surface finish. Dominant silver in dendrite products reveals its highly susceptible migration propensity. Therefore, a conservative usage of the ImAg surface finish in high-voltage applications is recommended.

Then, the tarnish mechanism of the ImAg surface finish was elucidated. A high sulfur-based clay was used to generate a test environment containing sulfide gases. A galvanic corrosion mechanism is proposed to explain the preferred copper corrosion during the clay test. It is more accurate to use the copper content on the ImAg surface as the criterion for both tarnish detection and quantification. In addition, the solder wettability on the tarnished ImAg surface was also assessed through the solder spreading test method. A decrease of solder spreading on the tarnished surface is concluded. This impact turns out to be significant on the test patterns of strip line with smaller widths. Great attention needs to be paid when assembling the fine-pitch surface mount components on a PCB with a tarnished ImAg surface.

Finally, the solder joint reliability on the tarnished ImAg surface was studied by means of X-ray, mechanical cross-section, high speed ball pull and fractographic examination. The test conditions included isothermal aging of up to 1000 hours at temperatures of 100 °C, 125 °C and 150 °C respectively. The extra voids in solder joints are concluded to be the only detrimental effect of the tarnished ImAg surface. It is possible to apply the existing industry standard on the solder joint void requirement as the acceptance criterion for solder joints on a tarnished ImAg surface.