Portable electronics, miniaturization, and cost reduction have been key drivers in the dramatic increase in use of bottom termination components (BTC’s) over the past several years. Similar to when the Ball Grid Array (BGA) was introduced into the market, BTC’s have brought with them new challenges and adjustments in design and assembly processes. For high reliability applications, these adjustments must be made with caution to avoid creating the potential for long term reliability issues and latent field defects.While low cost is an attractive feature of most BTC component packages, cost is typically not the driver for the use of BTC’s in low volume, high reliability electronics. Because BTC packages have the die very close to the PCB and because there are no leads extending from the sides of the package, they typically exhibit low parasitic losses due to low resistance and capacitance. Another benefit of using BTC’s on high reliability products is excellent thermal dissipation due to a relatively large component thermal pad that is attached directly to the PCB. This explains why we are seeing, and will continue to see, an increased use of bottom termination components in high reliability Aerospace and Defense applications.
With the rapid implementation of BTC’s in high reliability applications two key issues have evolved. The first issue is that of improper land pattern design and the second is improper solder stencil design. Unfortunately, these two issues are at opposite extremes of the product cycle. When issues arise, this can make it difficult to answer the question, “Is it a design problem or is it a manufacturing problem?” In many cases the answer has been found to be a combination of both.
While there are numerous component packages that make up the BTC family of components, those discussed in this paper will be limited to the following industry packages: Quad Flat No-lead (QFN), Dual Flat No-lead (DFN), Small Outline No-lead (SON), and Micro Lead Frame (MLF). All of these packages are similar in construction, related by common fabrication processes, and therefore share similar design and assembly criteria.
The target audience for this paper are those that are involved in printed circuit board (PCB) design and assembly, specific but not limited to high reliability electronic circuit card assemblies (CCA’s). The intent of this paper is to provide specific BTC guidance to companies that design and/or assemble high performance electronics for applications such as aerospace and defense.