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66 SMT007 MAGAZINE I JANUARY 2021 For PCBs with larger thermal mass—such as found in high layer count boards or boards with higher weight copper layers—proper and consistent hole fill can be a challenge. It is critical to make sure that these non-visible defects do not become quality escapes while also making sure the proper rework techniques are applied; to get these plated holes properly filled is important. There are some common board layout or design challenges which can lead to improper or inconsistent hole fill for those PCBs with high thermal mass. When plated through-holes are connected to ground planes or higher- weight inner layer copper, the thermal mass of that component lead is higher than surround- ing ones. This means that to get the board up Fixing Vertical Hole Fill in Plated Holes to the correct reflow temperature requires a longer period of time. If the lead/hole is not brought up to the proper temperature, the wettability of the solder in the hole is lower, thereby not allowing the solder to flow all the way up the barrel. In other layouts there may be an uneven dis- tribution of thermal mass to each of the com- ponent leads which may then have the same impact on hole fill. Another design challenge comes into play when more temperature-sen- sitive devices are in close proximity to the through-hole devices thereby causing the pro- cess engineer to "dial back" the temperature during wave soldering preheat in order to not damage the component. This lower temper- ature can cause the same insufficient lower Knocking Down the Bone Pile by Bob Wettermann, BEST INC.

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