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Highly reliable solderless contact with press-fit technology for power modules
Keywords: press-fit, solderless contact, power modules
Lead free has been required due to RoHS directive, the application of solderless press-fit technology has been featured1-2). Press-fit is a technology of connecting pin and through-hole of printed circuit board (PCB) and it is used instead of soldering pins. Press-fit or soldering pins for power modules are required to pass high current which is tens or hundreds of amperes. A press-fit pin consists of body part on the modules and press-fit part. Press-fit part is composed of a spring having an opening portion and is inserted into the plated through-hole of a printed circuit board. The mechanical contact force of the pin acts on a through-hole, and a contact with low ohmic resistance can be obtained. There are mainly four advantages of applying press-fit technology. 1) solderless process, 2) less damage of PCB by no heat process, 3) saving time since the pins can be inserted all at once into the through-hole of the PCB, 4) saving space in a component by no heating process. Because of those advantages, press-fit technology has been used rapidly for in car devices or industrial equipment in recent years. Press-fit contact is very easy process and is starting to be applied under harsh environment such as higher current and higher temperature, but it is still unknown what will happen to the press-fit contact under more harsh environment. Taking the power modules as an example, a mismatch of coefficient of thermal expansion of power modules and PCB is more critical due to the increase in capability of power modules by variety of usage and high current density by SiC application3-7). We investigated to apply press-fit technology to the power modules which are required to use in harsh environment. First of all, to ensure contact force between press-fit pin and through-hole are important to maintain the long term reliability. It was clarified that the press-fit part plastically deforms and the contact force decreases due to the influence of relative displacement of the pin and through-hole. A displacement of the press-fit contact due to the temperature rise also influences decreasing of the contact force. As a countermeasure, we designed brand new concept of structure which consists of a constricted portion in the middle of a body part of press-fit pin which the body part is between the insert case of power modules and press-fit part. This structure was applied to the power modules and the effect was confirmed by evaluating the temperature cycle test. The results obtained are as follows. 1) By changing a through-hole diameter, an influence of a contact force on a contact resistance is examined. As a result, it was clarified that the contact resistance increases when the contact force is low. It was also clarified that the contact resistance changes rapidly when the contact force is lower than 50N. In order to maintain long term reliability, it is important to ensure contact force. 2) Because of a plastic deformation of the press-fit part due to a relative positional displacement between the pin and the through-hole, a contact force to the through-hole decreases to 0N by using structural analysis. To ensure the contact resistance, we designed brand new structure which consists of the constricted portion in the middle of the body part of the press-fit pin which is between the insert case of power modules and press-fit part. By providing constricted portion, plastic deformation of the press-fit part is reduced and the contact force is ensured. In particular, by providing the constricted portion of remaining width of 1.2 mm or less in the body part of width of 2.8 mm, the contact force drops only 4 % compared with no positional displacement. 3) Although the resistance of the body of the pin rises by providing the constricted portion of remaining width of 0.8 mm or more, it is clarified that the influence is very small as compared to the resistance of the whole constituent member and contributes little to the temperature rise of the PCB. 4) An effect of providing constricted portion is verified by the temperature cycle test(-40/85 degrees) of the power modules using the PCB with some through-hole positions offset by 0.25mm. In the case without the constricted portion, the contact resistance rises in order from the farthest contact from the center of the modules, and it rise up to 0.5 m ohm at 600 cycles, whereas if the constricted portion of 1.0 mm was provided, the contact resistance was maintained at 0.1 m ohm or less even after the temperature cycle test, which is equivalent to the initial contact resistance. The contact resistance is also equivalent to the experiment with single pin and through-hole which is conducted without power modules. It was demonstrated that by applying the constricted portion in the body part of the pin to the press-fit pin of the power modules, the tolerance to the relative position displacement and the temperature cycle load can be increased and high reliable press-fit contact has been realized. 5) A cross sections of the press-fit contact after thermal cycle test are analyzed. It is clarified that an area of the press-fit contact is larger with the constricted portion compared to the structure without constricted portion. We also clarified a mechanism of the contact resistance increase and further clarified an interface state required to maintain long term reliability.
Minoru EGUSA,
Mitsubishi Electric Corporation, Components Production Engineering Center
Amagasaki City, Hyogo

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