The most popular lead-free alloy now being used on PC boards is Sn, Ag, Cu Tin(96.5%) Silver(3.0%) Copper (0.5%), with a higher melting temperature of 218°C (424°F), resulting in a melting temperature increase of 35°C and (63°F). This new Lead-Free alloy has different wetting characteristics, resulting in a longer dwell time (the amount of time that the reflow heat is applied to create the solder joint). Also the cooling rate to solidify takes longer. Finally the completed lead-free solder joint looks different from what technicians would consider a perfect solder joint. The new appearance has a grainy dull look that most would consider a defective solder joint. Visual inspection of this new solder joint will take some getting use to. With this new higher melting temperature, thermal safety margins are being pushed to the limit, and the potential for circuit board damage increases. As technicians we must become familiar with the new characteristics of this new lead-free alloy. We must always abide by the old theory of reduce heat, reduce time, reduce damage.

REDUCE HEAT
REDUCE TIME
REDUCE DAMAGE

The lead-free alloy's higher temperatures and slower wetting times have challenged circuit board manufacturers. The traditional rework methods of Thermal Conduction, Thermal Convection, and Hand Soldering will require some changes in procedure. Hand soldering is still the most popular method used for low volume rework by the Technician. It is affordable, easy to learn, and in many cases the only way to repair and save a circuit board. The choice of a good soldering iron is now more important than ever. The solder iron tip should always maintain the set temperature under varying thermal loads. This means that when the tip is placed onto the solder joint, the temperature does not drop and slowly recover. Good temperature stability will effectively allow you to operate at a safe lower temperature with minimum dwell time. When a solder joint is completed, it is important that the soldering iron be removed quickly because of the different cooling characteristics. It is very important to select a well kept clean and tinned solder tip with a size and shape that will allow maximum heat transfer to the solder joint. Use a good active rework paste flux that is formulated for lead-free soldering. All of these individual items will contribute to the quality of your lead-free rework. You will immediately notice the slower wetting as you watch each solder joint being formed. To avoid contamination of solder joints, solder tips used with lead-free alloys must be kept separate from those used with tin/lead. There is evidence that the contaminated joints may not be as reliable. Also the appearance of the new solder joint will take time to become familiar with.

REMOVING COMPONENTS FROM LEAD-FREE CIRCUIT BOARDS WITH Chip Quik®

For lead-free SMD desoldering, the Chip Quik® SMD Removal Kit has been tested and evaluated on PC boards that were manufactured with no-lead solder. The results were excellent. Also for those that are required to work in a lead-free environment, the new Chip Quik® No Lead Formula is now available. Many solder braid and solder supply manufacturers specify that their products are lead-free compliant. When using convection hot air and conduction equipment for rework, extreme caution must be taken to prevent inflicted damage. For thru-hole rework the same basic principles used for working at higher temperatures and slower wetting still apply. The Chip Quik SMD Removal Kit is still safe, and will not be affected in any way by the new lead-free solder joints.

MY OWN LEAD-FREE REWORK EXPERIENCE by Marv Cohen

My own initial experience with lead-free solder and desoldering has shown positive results. I have already worked with lead-free test boards that have SMT components. Taking into consideration all of the lead-free recommended rework requirements, I proceeded to remove a 208 pin quad flat pack with Chip Quik®. I used an iron set at 600°F. After generously applying the paste flux to all the leads, I slowly applied the Chip Quik® removal alloy with a chisel tip according to the instructions. I found that the wetting was a little slower but the overall results were the same. The SMD was easily lifted off the pads without any damage. The pads cleaned up equally as well using the recommended clean up procedure. Now I was ready to install the new 208 pin SMD. Using the same paste flux in a syringe, I applied a generous bead on all the pads. With a freshly tinned chisel solder tip and a popular brand no-lead solder, I accurately placed the SMD on the pads and tacked down 3 locations. As I started to drag solder across the pin, I found that the iron had to be moved a lot slower to allow each individual solder joint to be formed. Also I found that the cooling rate was slower. The solder joints definitely did not have the same appearance that we are use to. After careful inspection with good lighting and magnification, I found that my new lead-free attachment passed inspection. Now that I have had more experience in soldering and desoldering numerous different configurations of SMD's, I feel confident and comfortable performing lead-free rework. As lead-free circuit boards begin to reach your workbench, take the time to prepare and evaluate your existing equipment and supplies. This may be the time for some upgrading. Quality soldering is something that experienced technicians have always taken for granted. It is now time for a few practice sessions. The circuit board that you save may be your own.

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