I've recently been placed in control of operations of our SCS conformal coat sprayer and I must eliminate problems with wicking of humiseal 1b73 into surface mount connectors while also trying to minimize masking as much as possible. We use a 55:45 ratio of 1b73 to PGMEA for our process. How can I alleviate this problem while minimizing end of line touch up? Also are there any good masking tapes/dots to minimize dewetting available?
Typically, there are three main approaches to avoid coating wicking into press-fit connectors (and other keep out areas). Wicking is a phenomenon caused by a combination of low viscosity of conformal coating material and strong capillary forces generated by the low standoff gaps in the SMT process.
1. Use of masking materials. Masking materials are typically available in liquid, tape and dedicated rubber push on 'boots'. Simply apply the masking material (let it dry if liquid) and then you should be able to apply the coating as normal. Sometimes, especially in dipping processes, where the complete immersion increases the opportunity for masking materials to leak, it may be necessary to use a combination of masking materials.
2. Use of thixotropic gel versions of standard coatings. Some suppliers can supply a thixotropic gel version of the same coating you are using, which can be dispensed directly around the keep-out area, and due to thixotropic nature, material will not flow or wick into components and coating can be applied over and around the dispensed gel.
3. Use of a higher viscosity UV curable material. The combination of higher viscosity and snap cure will prevent the material from being able to wick into the component. Care must be taken however, to ensure the viscosity of the coating material is above 500 cPs or so, or else the material may be prone to wicking.