Capacitor pin breakage failure mechanism and solution - Database & Sql Blog Articles

SMD aluminum electrolytic capacitor

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The environmental stress screening test (ESS test) is a common means of assessing the quality of a product. In the ESS test, the random vibration stress is intended to assess the defects of the product in terms of structure, assembly, and stress. Larger capacitors, after soldering, if a separate treatment is not applied, the problem of pin breakage is likely to occur during vibration testing. This experiment simulates the stress conditions of transport vibration, running vibration, and impact collision.

The mechanism of the fracture is stress concentration, which generally occurs at the junction of the capacitor lead or the pad. When the vibration environment, the capacitor pin and the pad connection point will bear the impact of the transverse shear and longitudinal stretching of the entire capacitor. Especially when the capacitance is large, such as a large electrolytic capacitor.

The occurrence mechanism of this phenomenon is simple, and the solution is not complicated. The conventional experience is to apply a ring of silicone rubber GD414 at the bottom of the capacitor to bond and fix, but this treatment is not feasible.

The tensile strength of silicone rubber is 4-5 MPa, the elongation is 100%-200%, the intermolecular force is weak, the adhesion is poor, and the bonding strength is low; when used for bonding capacitors, the surface is fixed. However, when the impact stress is large, the silicone rubber is stretched to a large extent, and the capacitor itself is still subjected to large tensile stress and shear stress; therefore, the preferred material for fixing is E-4X epoxy resin. The rubber has a tensile strength of more than 83 MPa, an elongation of less than 9%, good adhesion, high bonding strength, low shrinkage and dimensional stability. It is obvious from the performance that the E-4X epoxy resin can achieve a true fixing effect.

The gluing process must also be refined, requiring the epoxy adhesive to have a height of 1/3 of the capacitor body and forming a ridge-like support on both ribs, so that the capacitor is integrated with the E-4X glue, and the vibration is no longer fluttered. The pins are protected.

In addition, in addition to the glue fixing, the process of circuit board assembly production will also be introduced, first assembling the capacitor, and then assembling other components, so that the vertical capacitor is the highest point, which is susceptible to bumping or external force when it is turned or placed; In the process, first install other components and bond the column and then assemble the high capacitance, so that when the coil is turned or placed, the capacitor is protected by the force of the column with a slightly higher capacitance.

Prior to the improvement process, the board was vacuum coated (approximately 15 μm thick Peelin film material was formed on the surface of the capacitor ceramic) and then fixed with silicone rubber. After the improvement, the epoxy is coated on the capacitor and then vacuum coated on the entire circuit board, so that the film and the outer surface of the rubber are integrally formed into a film. Since the surface roughness of the Peelin film is smaller than that of the ceramic surface, the contact angle of the rubber on the Peelin film surface is larger than that of the ceramic surface (the smaller the contact angle is, the better the wetting effect is), and the improved fixing effect is better.

A summary of the above problems and solutions has three conclusions: 1. The fracture property of the capacitor pin is fatigue fracture; 2. The design of the assembly method is unreasonable, the bonding strength of the fixing glue is insufficient and the process is imperfect is the cause of the pin breakage; This problem is solved engineeringly by switching to epoxy resin and adjusting the production process.