Problems and promises of BTCs: from QFNs to DFNs to MLPs, these packages are cheap, but tricky.

Author:Prasad, Ray
Position:BETTER MANUFACTURING
 
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WHAT ARE BOTTOM termination components? You may not know the term "BTC," but because IPC is focusing on these packages, expect to hear more about them.

IPC has coined the term BTCs for descriptive package names such as QFN, DFN, SON, LGA, MLP, etc. They are in some ways similar to BGAs, which also have hidden terminations, but they are also very different. They do not have balls, but rather, metalized terminations or pads underneath the package.

Most, but not all, BTC packages have a large ground or power termination surrounded by smaller signal terminations. When it comes to inspection, they pose even more challenges than BGAs, which permit inspection by endoscopes. You may not be able to see side solder fillets, and even when you can, they may look non-wetted or dewetted. But, dewetted and non-wetted side fillets in BTCs are acceptable.

BTCs provide good electrical and thermal performance, and they are the cheapest package on the market. However, they do require perfection - not only in the assembly processes, but perfection on the part of the PCB and component suppliers as well, which need to supply flat PCBs and packages.

Guess the chances of getting perfect PCBs and packages: not likely. Through-hole components have been in use for more than half a century, but are still far from perfect, so don't expect to achieve nirvana when it comes to this latest package trend.

The basic driver for BTCs is cost, which to a cost-competitive industry should come as no surprise. It is a package with the lowest per-pin cost, as low as half a cent per pin. To put this in perspective, if a package costs less than one cent per pin, it is considered a very low cost package. Thus, it is easy to imagine the attraction of this package, especially if the application is high volume such as cellphones or other mobile products.

Designers love this package because there are no leads, making BTCs an excellent choice as a result of their very low resistance and capacitance - also known as parasitic loss. Heat transfer from the package to the PCBs is also excellent, due to their large thermal pad in direct contact with the PCBs.

But there is no room for error. With no leads or balls, they must be perfectly flat, and the PCB must also be flat. How often does that happen?

The only variable the user has is solder thickness paste. However, if too much solder paste is used to compensate for package and PCB warpage, the package will float and may be misaligned. Also...

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