The Effect of Coating and Potting on the Reliability of QFN Devices

The fastest growing package types in the electronics industry today are Bottom Termination Components (BTCs). While the advantages of BTCs are well documented, they pose significant reliability challenges to users. One of the most common drivers for reliability failures is the inappropriate adoption of new technologies. This is especially true for new component packaging like BTCs. Obtaining relevant information can be difficult since information is often segmented and the focus is on design opportunities not on reliability risks. Most users have little influence over component packaging and most devices offer only one or two packaging styles. And, when faced with challenges, users typically fall back on tried and true solutions that may not work for new packaging. This has already occurred with BTC components. Commonly used conformal coating and potting processes have resulted in shortened fatigue life under thermal cycling conditions. Why do conformal coating and potting reduce fatigue life? This paper details work undertaken to understand the mechanisms underlying this reduction. Verification and determination of mechanical properties of some common materials are performed and highlighted. Recommendations for material selection and housing design are also given. Basically, the lack of a compliant lead structure makes BTC devices more susceptible to PCB warpage related failures so proper precautions must be taken to ensure adequate fatigue life for high reliability applications. Introduction Since the advent of Surface Mount Technology (SMT) in the mid 1970’s the electronics industry has continued to evolve from a packaging perspective by continually reducing the size of the component packages. First there were leadless chip carriers, later Ball Grid Arrays, Chip Scale Packages, and more recently Bottom Terminated Components. These package types are primarily designed for low cost applications or for products where a short lifetime is acceptable (e.g. cell phones). However, designers have been incorporating BTCs into products that expected to survive environmental stresses for prolonged periods of time. This paper will address the effects that conformal coating and potting have on these BTC packages and offer a new technology that may provide additional reliability benefit. What is a QFN? A QFN (Quad Flat Pack No Lead) or Quad Flat Non-Leaded has been called the poor man’s ball grid array (BGA). It has also been identified as a Leadframe Chip Scale Package (LF-CSP), MicroLeadFrame (MLF), MLP = molded lead package, LPCC – Leadless Plastic Chip Carrier, QLP = Quad Leadless Package, HVQFN = heatsink very thin QFN. This package type utilizes an overmolded leadframe with bond pads exposed on the bottom and arranged along the periphery of the package as shown in Figure 1. Figure 1 – Typical QFN Cross Section The package was developed in the early to mid-1990’s by Motorola, Toshiba, Amkor, etc. and was standardized by JEDEC/EIAJ in the late-1990’s. It is the fastest growing package type in the electronics industry today.