Looking for a solution to silicon conformal coating wicking up into connectors. We build IPC-A-610 class 2. We are using a PVA 650 with Dow Corning 1-2577 conformal coating. We have solder joints that must be coated 1.25mm from the connector's pads. Any tips on controling viscosity and any input on thixotropic gels, or dielectric grease would be greatly appreciated.
Humiseal says: 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.
We have tried using pookie to form an air tight seal on top of the connectors without success. We have tried filling the connector with dielectric grease with some success. Havent found any info on 1-2577 in gel form. Has anyone found this product? Has it worked?
I have come to the realization that the root of all of our coating issues stem from one source. We are unable to control the viscosity of our coating. I am ignorant on the subject. Is there a proven method to consistantly produce the identical viscosity? Currently we are adjusting our programming to match our coating's properties, sometimes multiple times a day. This is not even close to effective. I rejected 50% of our last run. Any suggestions?
Peelable masking has been our solution here recently. We an automovite customer that requires coating on both sides of the boards with many keep out areas. We are currently building a dispense robot that will apply tech spray wondermask 2218. The wondermask cures in 40min. and can withstand reflow temps. If that's not an option you have to look back at your equipment and it's capabilities. What modes are you using?(ie swirl, bead, lines, mono) Fluid pressures? Line speeds? Dispense times? There are so many variables it makes me sick. I haven't left the Asymtek machine in about two weeks. It's a constant battle. Any particual questions let me know.
Nordson has a viscosity control system and a laser fan width control option. We use the viscosity control system on our machine. Basically it heats the material to a set temp. The laser fan width measures the spray pattern width with a laser and automatically adjusts spray pressure to maintain a set width. I don't remember if PVA has any of these options. These are the only two coating machine brands I am familiar with.
tombstonesmt: Ammonia based masks can create solderability issues after they are removed. You can get non-ammonia based latex mask material but down side is slower cure and cost a little more as they have higher levels of latex etc
If we were to apply a thixotropic gel into a keep out area what would the removal process be? We have keep out areas that are later used for mounting purposes so the customer requests no residue in these areas. As I previously stated, we're using masking for keep out areas. This is applied after all soldering has been done on our assemblies.
We've got the same problem here, what we do is to apply peelable solder mask around the connectors or the exclusion zone. But be carefull! Sometimes the soldermask get inside the connectors and you can't remove it so you have to change it. Another thing to do is implementing in your DFM a quote that say that you can't put via, test point, component at less than 3mm from your connector. It's what we do and it seems to work.
Have you tried to preheat the board or the contact area? Many times with the heat cured materials it will start the cure process and slow the flow of the material. Our Protect-R-Shield silicone product was formulated to be a much thicker viscosity with different thixotropic properties to avoid this issue and others. We use the Asymtek 940 and can be extremely precise with the application to specified and keep out areas. Unfortunately we are a coating service provider and do not sell the product. Good luck. Don
We dont heat cure any of our assemblies. We coat relitively few boards and just let them cure at room temp. Perhaps we should start heat curing. In fact, we have an old, fully functional, heller 1500 just sitting in storage. Hmmm. . . .
There are different types of cures for conformal coating such as RTV which stands for Room Temperature Vulcanising which means they are cured by the humidity in the air. This type of cure does not need heat to cure. Then there are heat cures, UV cures and some have combinations. If yours is RTV it is not advisible to add heat for curing. This can have a negative effect.
There are 1 and 2 part heat cures. If you are not applying it on LEDs, then you won't have to worry about long term yellowing. The 2 part systems require mixing but have a longer shelf life. Feel free to give me a call @704-216-2430 if you need additional information.