From QFNs to DFNs to MLPs, these packages are cheap, but tricky.
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, expect excessive voids with too much paste.
If too little solder paste is used, and if there is any warpage in the package or PCB, expect many opens. There are difficult choices to make in the process selection, and inspection doesn’t help, since the end of the visible terminations are bare copper; hence, do not expect them to solder to achieve side fillets.
This is the reason why IPC has stepped in to help the industry develop a design and assembly guideline for BTCs. This author co-chaired IPC-7093, Design and Assembly Process Implementation for Bottom Termination SMT Components. The document describes the design and assembly challenges for implementing bottom termination surface mount components whose external connections consist of metalized terminations that are an integral part of the component body. The focus is on critical design, assembly, inspection, repair, and reliability issues associated with BTCs.
The target audiences are managers, design and process engineers, and operators and technicians who deal with the electronic design, assembly, inspection, and repair processes. The intent is to provide useful and practical information to those companies using or considering SnPb, Pb-free or other forms of interconnection processes for assembly of BTC-type components.
This document, although not a complete recipe, identifies many of the characteristics that influence the successful implementation of robust and reliable assembly processes, and provides guidance to component suppliers regarding issues being faced in the assembly process.
The exchange of information between the component supplier, product design, and assembly personnel about those parameters that influence good assembly practices is more critical with BTC implementation than with any other surface mount part.
During IPC Apex this year, committee members voted to accept the document pending minor revisions and final editing at a meeting to be held Aug. 31. We hope to complete deliberations Sept. 2, and plan to publish the document by the end of September.