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54 DESIGN007 MAGAZINE I NOVEMBER 2022 Solder Movement Under Large Pads It is important to consider the unpredictabil- ity of solder movement during reflow. If the board is warped or there is not enough solder under the slug, capillary action can pull the solder to one side of the chip. Attempt to cor- rect this by applying more solder, and the chip might float right off the signal pins. Excess sol- der can overflow the pad, sending solder balls out to short or bridge other areas of the board. e varying amount of solder pulled through the vias only exacerbates the problem and hin- ders attempts to correct for it by adjusting the amount of paste used. Finding the Solution A proper solution requires fixing two differ- ent problems, but both are related to solder. First, solder must be prevented from wicking through vias and ending up on the wrong lay- ers of a PCB. Second, solder must be prevented from moving past its area of application. e solution can be approached with two different methods: • First, apply solder mask over the landing pad and open circular "islands" for paste application. If the solder will not behave in a large area, break that area up into an array of smaller areas. Since solder mask restricts the paste to its area of application, this reduces the amount of solder connecting the chip to the board, increasing the consistency. e circular solder paste apertures release the solder more reliably than those with sharp corners, which helps prevent loose solder balls. • Second, surround the "islands" with small (~12 mil or smaller) vias that are tented and covered with solder mask. Removing the vias from the immediate area being soldered and tenting them prevents any stray solder from wicking down to the other side of the board while still provid- ing good thermal transfer to the pads underneath. Add these vias as close as possible to the islands. e solder mask tenting will block any solder that wicks onto an exposed via due to manufacturing tolerances. When implementing this solution, keep in mind a few key rules: • Make sure the pad under the chip is a solid copper plane, so it will spread out the heat. • To give maximum coverage to the heat slug, use a hexagonal packing pattern for the solder islands. • e solder mask tents should be the same size as the resist mask opening on these islands. is ensures that adequate solder is present to bond to the chip. • ermal spokes will reduce thermal conductivity, so don't use them on any layer of the vias. • Vias should be solder mask tented, not plugged or filled. e epoxy in solder mask plugged vias may not cure fully and will tend to expand and erupt if subject to enough heat.

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