Design007 Magazine

Design007-Nov2022

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52 DESIGN007 MAGAZINE I NOVEMBER 2022 Heat sink failures can be difficult to detect, especially when the failure rates are low. But even if the volume of failures is low, those costs quickly run into thousands of dollars. One of the primary causes of heat sink failure is incon- sistent soldering of thermal pads. Given the cost of reliability problems, finding a path to improvement is crucial. Uncontrollable, Unpredictable Solder e standard, manufacturer-recommended approach for heat sinking is to place large cop- per pads on both sides of the board under the chip. ese pads are then connected with vias to conduct heat, and full solder paste coverage is used to get the best thermal contact. Unfor- tunately, this approach can cause problems. Managing Solder for Fewer Heat Sink Failures e issue is that solder is unpredictable once melted, which puts every board at risk of a poor heat sink connection. is problem is so pervasive that you might be tempted to accept it as the unavoidable cost of doing business. However, there are ways to solve this chal- lenge. ere are two main mechanisms that cause these failures: solder wicking through vias and solder movement under large pads. Solder Wicking Through Vias If the landing pad is covered in tiny holes, molten solder can run through them to the back of the board. is results in less solder connecting the pad, with the remaining solder distributed unevenly. e wicking will vary from board to board, causing unpredictable consistency and poor reliability. Co m m o n v ia s a r e not precision compo- nents, and the amount of copper plated onto t h e m v a r i e s . S o m e m a y b e w i d e r t h a n intended and others may be plated partially or fully shut. Identi- cally designed boards that went through the same manufactur ing process may now have significantly different thermal responses. Connect the Dots by Matt Stevenson, SUNSTONE CIRCUITS

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