PCB Assembly Rework

The introduction of Pb-Free assembly has been challenging for first-pass assembly, but even more challenges present themselves when PCB assembly rework is required. PCB repair in a Pb-Free environment can engender issues of cost, quality, timing and repeatability - all exaggerated because of Pb-Free demands.

AHI plans for all the issues that must be addressed including :

• Time and costs to train operators for Pb-free assembly, repair and inspection
• Material costs associated with procuring Pb-free wires, solder bars, wire core solder, etc. -all more expensive than traditional materials
• Processing temperatures for Pb-free assembly (approx. 30-35° C) require greater accuracy and precision
• Research and planning are essential in setting up the right procedures for PCB rework when confronted with a Pb-free requirement

AHI believes in best practices for PCB assembly rework, which recommend providing Pb-free training kits to aid in instructing technicians and developing profiles. Accurate and reliable reworks are more difficult in a PB-free environment because the PCB and the components closest to the part needing repair must endure several cycles of higher temperatures. To protect the PCB laminate, the preheat temperature should be set no higher than 10°C below the glass transition temperature (Tg) of the PCB material. Higher temperatures may be required for thicker Circuit Boards with greater Delta T across assemblies. Increased temperatures during this process means you may have to slow down the oven line speed during re-flow. All of these considerations require careful planning and adaptations and OEM carefully plans for all exigencies during each step in your project.

Circuit Board Repair Process

Once the rework parameters have been defined, the process begins the same whether it requires standard or Pb-free solder - the required steps are:

• Define and execute an accurate thermal profile
• The failed components must be removed
• The site is cleaned of all rust or solder residue and prepared for the new component
• Replace the component with new solder and flux and perform re-flow
• The re-work is thoroughly inspected

The next steps involved in the rework process vary depending on whether or not there is a Pb-free requirement. The differences in standard vs. Pb-free present a number of challenges that can only be resolved by introducing new or altered processes including tighter, more accurate thermal profiles and extreme precision throughout the PCB rework procedures. This will avoid a number of costly problems that can result from the varying thermal profiles. Assenbly House, Inc. implements the best practices procedures required to avoid errors such as blowholes on your Ball Grid Array ball, the separation of PCB substrate for the GA balls or micro-cracks from too much thermal stress.