New-Tech Europe | September 2016 | Digital Edition

Adapting Non-Sealed Surface Mount Parts for Hi-Rel Customer Assembly Processes

Jonid Xhakollari, Manufacturing Process Engineer, Mini-Circuits

components for customer assembly processes, preserving high reliability after aqueous wash and conformal coating. Reliability Risks of Open and Semi-Sealed Surface-Mount Parts Mini-Circuits’ surface-mount parts may be categorized into three basic package types: Plastic encapsulated MMIC devices. Examples include QFN and SOT-89 case styles. A semiconductor die is wirebonded to bonding pads and sealed in a plastic enclosure. Figure 1: plastic encapsulated MMIC devices. Ceramic, hermetically sealed cavity packages for hi-rel requirements. MAC- series mixers, for example incorporate a semiconductor die on GaAs wirebonded to baluns embedded in an LTCC multi- layer substrate. The die is hermetically sealed in a controlled nitrogen atmosphere. Open or semi-sealed PCB component assemblies such as VCOs, mixers, filters and diplexers. Circuitry is either fully

exposed or shielded in a non-sealed case or “can”. Plastic encapsulated and hermetically sealed components are generally well- suited to hi-rel assembly processes without any special adaptations. Open and semi-sealed components, however, are susceptible to performance shifts and even catastrophic failure when exposed to the conditions of aqueous wash and conformal coating processes. During aqueous wash, components are fully immersed in de-ionized water at high temperature which can ingress into the unit package and become trapped. After aqueous wash, moisture along with any contaminants trapped inside the case of semi-sealed units can lead to dendritic growth, shorting, corrosion and a variety of other known reliability risks. Open component assemblies are less problematic than semi-sealed assemblies with respect to trapped moisture, but for some more sensitive open structures such as filters and diplexers, the customer’s conformal coating can change

The perfect integration of a product within the customer’s assembly process can be just as critical to the success of that product as the electrical performance. Mini-Circuits and many RF/microwave component manufacturers build surface- mount components that are soldered onto customers’ PC boards using a reflow process. Following reflow, the board must be cleaned to remove solder balls, flux, salt deposits and other debris. While a number of cleaning methods exist, the industry – and especially manufacturers of equipment for military and other hi-rel applications – have gravitated towards aqueous wash. A conformal coating is then applied to protect the circuit from moisture and other adverse environmental conditions. This process poses unique challenges for the integration of surface mount parts into customer assemblies, particularly components with open and semi-sealed case styles. This article presents a proprietary technique Mini-Circuits has developed to adapt non-sealed surface mount

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