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30 SMT007 MAGAZINE I FEBRUARY 2020 If the pitches get below 10 mils, most people have a hard time printing paste because the pads are so close that you may have bridging. one of the bottlenecks in the performance of overall electronics systems. Dan Feinberg: Size and weight are huge, which is a big factor right now, especially as we demand the capability of various devices to do more and more things, yet they need to be smaller. Prasad: Yes, but at the end of the day, it all depends on the application. In consumer applications, of course, weight and cost will be the key drivers. Then, there are cases that the processor is a 25-watt device, in which case, the thermal performance would be the main driver. Of course, cost always plays a role in any application. The point is that the same thing doesn't drive everything in every appli- cation. If you are using a device in a satellite and going to space, they don't care as much about the cost. Even in consumer applications, like laptops, if you have a big wattage device, and don't want a fan to cool it, the thermal performance would become important. It all depends on the application. Matties: If we bring it into the assembly pro- cess, what are the challenges that you think our industry is facing with the finer pitch? Prasad: What are the processes on the assem- bly side? You print it, place it, reflow solder it, clean it, and test it and rework it if necessary. All these process steps are affected by the pitch somewhat. Printing is affected the most. If the pitches get below 10 mils, most people have a hard time printing paste because the pads are so close that you may have bridging. For a company with good infrastructure, they have no problem paste printing up to roughly 16-mil pitch. But when you get below 16 mil pitch, printing capability is a key constraint for most companies. I don't think the real problem is in the placement and reflow. Matties: Are you talking about stencils and jets? Prasad: Yes. The biggest problem is there are only two kinds of defects, even though IPC- 610 is a thick book with a couple of hundred pages. IPC-610 has a list of all kinds of defects, defects that are important are shorts (bridges) and opens. If you really think about it, shorts and bridges are the real defects that determine whether something passes or fails. An interesting fact is that most companies have more opens than shorts. Ask somebody at any company to collect the data for the last six months, and say, "Break down all your defects into three categories: opens, shorts, and everything else." The number of opens in any company is about four or five times more than the shorts. What I like to advocate is you should shoot for the number of shorts to be way more, even six times more, than the opens. But why? Shorts can never escape the test because they will flunk in-circuit, functional, visual, etc. They will be caught, and the customer will never know, but the opens or insufficient defects will escape. They will even pass in-cir- cuit test, and that's why there is lot of confu- sion and disagreement between suppliers and users when there is a failure either at the cus- tomer site or in the field. Resolving the root cause can be very expensive. However, if I apply maximum paste and have a bit more bridges, they won't escape since I can catch them before shipping. I also want to emphasize that I don't mean you should have lots of bridges. All I am saying is that there should be more bridges and fewer opens in your process; the ratio of bridges divided by opens should be closer to six. When we keep reduc- ing the pitches, the defect rate will be higher.

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