Transcript - IPC Training Home Page

DVD-67C ScriptLead Free Surface Mount ReworkThere are two basic types of rework. The first involves removing the component, preparing thelands, then repositioning and resoldering the component. The second involves simply touchingup a solder connection that doesn’t meet quality and reliability standards.We’ll discuss component removal and replacement first. There are a number of ways to remove acomponent, prepare the lands and to resolder the connections. The method you’ll choose has todo with the type of component being reworked and your company’s policy.For a comprehensive demonstration of the various techniques available, please refer to IPC’sSurface Mount Rework Series, as well as our training DVD on Rework Stations. This video willconcentrate on the differences between tin-lead and lead free rework processes – and we won’tget into each of the soldering processes for removing and replacing components.Component removal is pretty much the same whether you’re reworking a tin-lead solder joint, ora connection made with a lead free alloy. The main difference is the amount of heat you’ll needto melt the solder joints. The lead free alloys require a higher temperature.Let’s watch a common technique for removing an SOIC component using a soldering iron fittedwith a special tip. Before beginning, it’s important to clean and tin the tip of the iron. Now,place the tip over the component. Once all the solder joints have melted, lift the iron and removethe component. Then set the hot component on a heat resistant surface until it cools down. Let’swatch another component removal procedure using hot air.At this point, the solder that remains on the lands needs to be removed so that the component canbe properly repositioned and resoldered. Land preparation is typically done with a solder braid.Again, higher soldering temperatures are used to “wick” the lead free solder onto the braid.You’ll use a fine point soldering iron tip to accomplish this operation. Using a compatibleexternal flux will improve the wetting characteristics and allow the solder to wick onto the braidquicker. This decreases the possibility of thermal damage to the circuit board.Another method is using continuous vacuum extraction. After the tip heats up, we'll apply a lightcoating of solder right on the tip. Again, you may also want to apply an external flux over all ofthe lands. Both of these steps will help speed up the heat transfer process. Now we position theflat end of the tip over the first few lands.The vacuum should now be activated, and allowed to run on a continuous basis. The surge of airthrough the orifice will tend to cool the tip. One advantage of the larger tips is they can maintaintheir operating temperature better than the thinner tips - since there is a larger mass through whichthe heat is transferred. Notice how we move the tip along the length of the lands - to remove all ofthe solder. It's very important not to put any downward pressure on the lands. Once the solderhas been removed from the lands, any flux residue should be cleaned.Now, we’ll reposition and solder the component. The easiest and safest way to pick up andposition this SOIC component is with a vacuum pickup tool. You can usually position an SOICcomponent onto the lands without the aid of a microscope. Just make sure that all of the leadsare centered evenly - from side to side. You also need to make sure that the heel and toe of eachlead is centered evenly in the lengthwise direction.3