SMT007 Magazine

SMT007-Apr2021

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APRIL 2021 I SMT007 MAGAZINE 73 material is controlled by the vendors who sup- ply them. Who is responsible for vendor selec- tion? e purchasing guy who used low price as the criteria for placing the order? When it comes to making the decision about vendor selection, how much focus is given on price vs. quality and the total cost? "Garbage in, garbage out" could not be truer than in the assembly of SMT compo- nents where pitches are shrinking and pro- cess windows tightening. As a result, there is no way to improve manufacturing yield if the boards and components have poor solderabil- ity or unacceptable co-planarity. Referenc- ing industry standards such as J-STD-002/003 is a good idea. Take another example: BTC is now a very widely used component. It is a very good component with many good fea- tures, such as excellent electrical and thermal performance, size, and weight, etc. But the one important feature of this package is that it is the cheapest package you can buy. As a rule of thumb, if you can buy a component pack- age that costs one penny per lead (i.e., a 100- pin packaging costing a dollar), you have an inexpensive package. A BTC costs less than half a penny per lead—no wonder it is a very popular package. But when it comes to man- ufacturing, you need a perfectly flat package, and a totally perfect flat board. Since there is no lead or ball in this package, packages and boards must be flat to ensure a good connec- tion. To compensate for flatness, you cannot print excess solder which will cause floating and excessive voids. And you cannot print very thin paste thickness since the poten- tial for opens will go up if either the package or the board is warped. Plus, the ends of the BTC terminations are not solderable since they are exposed copper. So, if you consider these manufacturing challenges, the package is not as cheap as one might think. I suggest you refer to the just-released IPC 7093B (I chair the committee) for a deep understand- ing of the complexities involved in design and manufacturing with BTCs. Manufacturing Processes e key responsibility of manufacturing is to use process control for all the manufactur- ing processes. However, no matter how good the process controls are, or how well they pro- gram their machines to dispense or print paste or use the right profile, they cannot eliminate design- and material-related defects. Yes, they can compensate for some of the issues with some process changes but there is a limit. How should one identify key manufacturing process issues? For manufacturing there are a lot of challenges. For example, the equipment must be characterized thoroughly. is can be defined as understanding all parameters that affect the equipment's performance. A good understanding of all the key parameters that affect quality will take a lot of time and effort. Large companies can afford to assign many engineers to characterize the process. In small companies, "learn as you go" is the common motto since they don't have the resources. Vendors may say it is easy; it is not. For exam- ple, if you look at the data sheets of various sol- der pastes from different suppliers, what you will see is that just about any profile will work with their paste. at is simply not so. While you do consider their recommendations, there is no substitute for developing a unique profile for each product. How should you proceed? First, characterize the process, then document the details of equip- ment- and non-equipment-dependent variables that control yield. ere are some misconcep- tions that if you use a certain paste all your In small companies, "learn as you go" is the common motto since they don't have the resources.

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