| I have been put incharge of developing a process to rework | a doublesided SMT PCB with DCA/COB (direct chip attach, chip on board ) components. The rework will consist of removing some 603-1206 chips on both sides of PCB, oh yeah | its has a polyurethane conformal coating on both sides. | | I have some questions that I need some answers | | 1. What is the best way to remove conformal caoting from | selected areas? | | 2. Will I create problems by reflowing the DCA/COB | encapsulant, or create some intermetallics by reflowing | solidifeid solder joints of non-rework parts? | | 3. Will reflow change the properties of the conformal? | (some conformals are solder through or burnoff) | | 4. Will an aqueous wash on partially stripped conformal | PCB's be a problem? (moisture entrapment in PCB) | | Thanks for any help! | Tom: Responses to your questions are:
>1. What is the best way to remove polyurethane conformal coating from selected areas?
Your conformal coat material supplier is best equipped to tell you how to remove the coating. Polyurethane conformal coating is not easy to remove. Without in-put from your supplier, in order of preference, the methods of removing polyurethane conformal coating are:
1 Thermal 2 Peeling 3 Micro blasting 4 Solvent
>2. Will I create problems by reflowing the DCA/COB encapsulant, or create some intermetallics by reflowing solidified solder joints of non-rework parts?
2a. Create problems by reflowing the DCA/COB encapsulant: Your DCA/COB encapsulant supplier is best equipped to tell you the impact of reflowing the encapsulant. Some encapsulants can be damaged at temperatures below 150C.
2b. Create some intermetallics by reflowing solidified solder joints of non-rework parts: If you have "good solder joints" on your board, you have created an intermetalic layer already. Will you remove that layer during rework? Probably not. Will you have a thicker layer after rework? Most likely.
>3. Will reflow change the properties of the conformal? (some conformals are solder through or burnoff)
Your conformal coat material supplier is best equipped to tell you the impact a reflow cycle on the coating. Some polyurethane conformal coating can be damaged at temperatures below 150C.
>4. Will an aqueous wash on partially stripped conformal PCB's be a problem? (moisture entrapment in PCB)
Your bare board and possibly some of your components is (are) hydroscopic. It (they) will absorb moisture in the air and certainly during water washing. De-moisturizing the reworked assembly, prior to re-applying the conformal coating, is a good idea.
What is the process flow that you envision? Will it go like:
1 Remove conformal coating around 0603 - 1206 parts 2 Remove 0603 - 1206 parts 3 Level solder on pads of removed parts 4 Remove coating that was under removed components 5 Hand deposit side #1 solder paste 6 Hand place side #1 components 7 Reflow side #1 8 Clean side #1 flux residues 9 Hand deposit side #2 solder paste 10 Hand place side #2 components 11 Reflow side #2 12 Clean side #2 flux residues 13 Bake 14 Test 15 Conformal coat assemblies
I�m wondering if you shouldn�t skip the "remove conformal coating" step. Take some side cutters to the center of the components, break �em, remove the end caps with a rework tool, and push-on with the clean-up and component replacement process.
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I have been put incharge of developing a process to rework...
- Dec 07, 1998
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