PCB Assembly Rework

First-pass Pb-free assembly is sufficiently challenging but circuit board repair is even more challenging. The issues of time, cost, quality, and repeatability associated with PCB repair are now exacerbated with Pb-free. Extra time and cost are involved to re-train operators to perform Pb-free assembly, rework, and inspection. Also, the cost of Pb-free materials like solder bars, wires, and wire core solder is more expensive compared to traditional eutectic soldering materials. Since the operating window for Pb-free assembly and rework is considerably smaller and tighter in terms of processing temperatures (approx. 30 – 35º C), more precision and accuracy are required to develop the profile and procedure for Pb-free rework. This means more research time is needed to investigate and set up proper circuit board rework procedures.

Good PCB assembly practice calls for Pb-free training kits to train people and develop the profiles. Maintaining a high quality rework is more challenging because the PCB and the adjacent component to the targeted rework component are subjected to multiple cycles of higher temperatures. To maintain the integrity of the PCB laminate the maximum preheat temperature is set at approximately 10º C below Tg (glass transition temp.) of the PCB material. Higher thermal preheat temperature minimizes the potential thermal distortion and shock to the PCB during the reflow process. Thick circuit boards such as 0.093 mils thick or more require more heat, have greater Delta T across assemblies, and may require slower re-flow oven line speed. As overall circuit board size increases, the process window tends to shrink.

Circuit Board Repair Process

PCB-Rework2The circuit board rework process, regardless whether it involves eutectic or Pb-free solder, is the same. It begins by having a good thermal profile done, removing failed components, cleaning and preparing the site, and removing rust or solder residues. Next, the component is replaced with new flux and solder; re-flow is performed; and inspection is the last step.

This is where similarities end and several major differences emerge between Pb and Pb-free rework. These differences introduce a host of challenges and newer or altered practices to resolve them. With the higher temperature differences between eutectic and Pb-free solders, Pb-free rework mandates tighter processes, better thermal profiles, and newer rework practices involving greater precision provided by advanced PCB rework stations. Otherwise, a host of different rework problems can occur. Examples include the creation of a blowhole on a Ball Grid Array (BGA) ball due to the excessive heat required for Pb-free rework, BGA balls separating from the PCB substrate due to insufficient heat required for Pb-free, and/or micro-cracks occurring on a BGA and PCB caused by excessive thermal stress.

To learn more about PCB Assembly Rework, visit our PCB Rework Strategies Pay page.