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The Printed Circuit Assembler’s Guide to... Encapsulating Sustainability for Electronics, by Beth Turner, MacDermid Alpha Electronics Solutions
Chapter 3: Industry Sectors and Relevant Test Methods
General Purpose
General purpose encapsulation resins are suited to a wide range of applications where protection and electrical insulation are required. Typically, they show good flow and self-levelling properties to completely encapsulate all components. Good flow properties are important to prevent air entrapment that can occur when the viscosity is so high that air is unable to escape. The pot life, working life, and gel time can vary greatly from product to product. Pot life measurements are useful to understand how a mixed system is likely to flow when dispensed into a unit. Both the general-purpose epoxy (GPE) and polyurethane typically show good initial flow properties and are suited to most potting applications. The general-purpose polyurethane (GPP) shows a lower mixed system viscosity, so might be more suited to applications with tight spacing between components or leads. The pot life of the GPE is longer, meaning it will retain its workable and flow properties for more time.
Good adhesion is essential to completely seal a module. Adhesion to wires, leads, terminals, and housing materials is required for complete physical protection against deleterious fluids and other contaminants. As the popularity of portable and wearable electronics continues to rise, everyday devices must be able to tolerate exposure to dust and moisture, thus ruggedized performance is essential for both indoor and outdoor environments. Ingress protection (IP) ratings describe protection against solids and liquids. To achieve any decent level of protection at a system level, an encapsulation resin must adhere appropriately to the substrates to which it is applied.
Lap shear strength testing is a useful method to compare adhesive strength on different substrates. Test methods such as ASTM D1002 (metals) and ASTM D3163 (plastics) provide a useful framework to compare the adhesive performance of encapsulation resins, evaluate effectiveness of surface preparation techniques, and assess durability of bonded substrates in different environments. The test procedure involves bonding two identical surfaces into a single lap joint and applying shear stress until rupture occurs. The load at failure and type of failure is recorded and the shear strength at failure can then be calculated. Both the GPE and GPP typically show good adhesion to most substrates including those commonly used as housing materials. They do show some limitations when it comes to difficult substrates, including untreated PET. Specialist grade encapsulants are required for certain low surface energy substrates.
