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64 SMT007 MAGAZINE I MARCH 2020 Disadvantages of BTCs Of course, there are downsides to any pack- age type, and BTCs are no exception. The most significant issue is the lack of good solderabil- ity of BTC terminations. It is easy to see why. BTCs are essentially built on a sheet of cop- per in a waffle pack like a sheet. After attach- ing the silicon and molding, the sheet is either chopped up or punched into individual pack- ages. This results in an exposed copper surface at the end of termination. Exposed copper is very susceptible to oxidation; hence, the poten- tial for getting a side fillet is very low even with an aggressive flux. This is why the IPC- 610 standard does not require side fillets in BTC solder joints. Speaking of using aggressive flux, it is gen- erally a no-no for BTCs because there is prac- tically no gap between the bottom of the BTC and the top of PCB surfaces. Flux trapped under the tight space is almost impossible to remove. Essentially, you have no choice other than no- clean flux when using BTCs, even if there is only one BTC on the board. The other major issue with BTCs is flatness or coplanarity of the package. These pack- ages require perfection. The package and PCB have to be perfectly flat. The amount of sol- der that is applied also has to be perfect. If all these conditions are not met, either there will be opens due to a lack of enough solder paste or too many voids and bridges due to too much solder paste. Overall, you need per- fection in the package, PCB flatness, printing, and reflow. How often is that achievable in a high-volume manufacturing environment? Rarely. In addition, because the terminal features do not protrude beyond the package body, visual inspection and verification of the solder inter- face are difficult. What this means is that the low package cost may not immediately trans- late into overall low assembly cost since this package presents many challenges in assem- bly, inspection, and rework. We will discuss the technical details as to how to address these concerns when I discuss design and assembly issues in the next two future columns. another way to think of BTCs. We are all very familiar with ball-grid arrays (BGAs). BTCs are like BGAs without the balls. Think of BTCs as a poor man's BGAs because BGA is the most expensive package, and BTC is the least expen- sive package. This minor difference in physical construction makes all the difference in cost, design, assembly, and rework between BTCs and BGAs. BTC Drivers Why is there such a big interest in this pack- age, and what are the main drivers for BTCs? These days, billions of BTCs are being assem- bled worldwide, primarily driven by mobile products in general and mobile phones in par- ticular. Some smartphones have as many as eight BTCs in each phone. BTCs are also used as voltage and power regulators and in many other automotive and industrial applications. Designers find the small body footprint and minimal PCB area requirements very attractive. BTCs have excellent electrical and thermal properties, and they are a very robust package for handling without any concern for lead dam- age. One of the most attractive features of BTCs is the cost. In general, if a package costs one cent per I/O (e.g., $1 for a 100-I/O package), it is considered a very good price for the pack- age. BTCs can cost as low as 0.5 cents per I/O, or even less. That is an attractive price and why they are so widely used in high-volume appli- cations, such as cellphones and other mobile applications. These days, billions of BTCs are being assembled worldwide, primarily driven by mobile products in general and mobile phones in particular.

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