The PCB Design Magazine

PCBD-Aug2017

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14 The PCB Design Magazine • August 2017 WHOSE FAULT IS THAT BAD BOARD? The CAD program itself had introduced thermal bridges where the pads were linked to large copper areas, but the designer's eye did not like the way they looked. This was a hap- py case because the designer had presented the project before sending the order for fabrication. But other times, matters were much more se- rious. When a board came in for assembly, it was necessary to manually heat the pad and the component with two soldering irons. Some designers understood this aspect (especially af- ter they were walked through the factory to see the whole technological process), while others even got angry, yelling, "I will send the proj- ect to China and they will do it!" Yes, they will, but they will fabricate exactly what was sent, including the design errors. This was the case once when a designer forgot to send an Excel- lon file; the printed circuit board was manufac- tured without the holes for the 40-pin DIL pack- age of a microcontroller. Some designers will gladly fit the schemat- ic on the entire sheet. One designer learned that, with the right modifications, the area of the printed circuit board could be reduced, and thus the cost of the board could be reduced. He replied, "Oh, it is for the Army, and they have enough money not to worry for the size of the board!" I think such an army has lost the battle even from the design stage: More materials mean heavier weapons, more fuel consumption for transportation, larger pack sizes, less weapons, less money for further development, and so on. So, in order to be a professional designer of to- day's electronics, it is not enough to be an elec- tronic engineer; it is also necessary to have the proper technological knowledge, or to team up with a technologist. This is one reason that we decided to include a DFM course in the elec- tronics curriculum at my college. Nowadays, printed circuit boards are more complex. Many PCBs now have controlled im- pedance requirements. Flex-rigid is more pop- ular than ever, and we're seeing glass circuit boards and many metal-backed PCBs. They have become dedicated passive components; some are even active, such as the embedding technology from Würth Elektronik, which can embed flip-chips within the multilayer struc- ture. Moreover, an assembled electronic module may have several thousand solder joints, which are also components. As my colleague Ioan Plotog used to say, "Solder joints are living things." They are not immutable; due to the environmental condi- tions, different kind of stress (mechanical, ther- mal) and aging, they are changing outside (tin whiskers), and they are changing inside (micro- structure), so their electrical, mechanical and thermal functionalities are affected. It is known that ESD issues may hit a long time after the product is launched into the market. With circuit boards, one hand doesn't al- ways know what the other hand is doing. For example, did the component supplier or the electronic assembly provider respect all the preventive procedures against electrostatic dis- charge? I heard about a board that functioned on a rack, but stopped functioning when it was removed and inserted into another rack several meters away. That was the day when the op- erators from the electro-mechanical telephone exchange discovered that walking on carpet charges you with several thousand volts and the circuit layers of the electronic module must not to be touched. The cleaning of the boards is an- other problem which may have a delayed effect due to electrochemical migration. Figure 1: V-cut panelized 10" x 10" circuit board.

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