Lexan cracking during Assy Process

[CelTech54]

This may be an unusual request. I work at an Electronics contract MFG. We assemble circuit boards. On one of the boards two LED displays are installed. My problem is the Lexan lens on one or both of the displays crack. The Len houses an 8 segment LED display and the display is encapsulated/potted using a Bisphenol A- epichlorohydrin (25068-38-6). The viewing area of the display has no sign of cracking however the vertical sides of the lens will crack or develop multiple micro fractures. The lens is molded and the cracks primarily show up in the gate area of the lens. There are also four molded in standoffs on the lens that will occassionally develop a vertical crack. The cracks are vertical. The displays are fastened to the circuit board using a Selective Solder wave which is a small solder fountain that moves on a XY axis. We have monitored the temps the lens is exposed to in the soldering process and seems it never exceeds 165F. The boards were then allowed to cool to room temp then washed in an In-Line washer. The wash/rinse/dry process takes 15 minutes. Wash consists of Vigon A200 and DI water (concentration is 15% Vigon) for 3 minutes then three rinse cycles consisting of 100% DI water. All solutions are heated to 140F. Then a dry zone that is 160F. We see approx 60% failure (cracks after wash). Boards have been inspected after solder process and before wash and not seen any cracks. We have tried lowering water temp to 100F still using Vigon at both 10% & 15% concentration but still get cracking. We use IPA to clean during rework process (use swabs) but this can yield mixed cleaning results if fluxes are burnt on. IPA is one chemical that is deemed compatible. Any recommendations to eliminate the cracking and a safe cleaning solution. RMA flux is used in the soldering process and the leads are under the display/lens body. We need a solution to safely remove the RMA flux residue and not crack the Lexan lens. IPA can work but I believe it would need to be pumped through a spray wand to flush flux from under the component body. Fumes and flammability are a concern.

Thank You

This has been an on going problem an I have little hair left to pull out.

[DwightDixon]

It does seem the Virgon (whatever that is) is causing the PC to crack where it is stressed due to both molded in stress and the assembly stress. You might try annealing the parts after they are molded. The Lexan Tech people or website should tell you the temperature. You can use polarized light to analyze how much assembly stress there is and change the assembly technique to minimize that. You should also find out what is in Virgon and talk to the Lexan folks about it compatibility. Whoever makes Virgon may have a suggestion as to whether another product of theirs might be less aggressive.