A common defect that HumiSeal technical support deals with on regular basis is the wrinkling of high solids RTV silicone conformal coatings. If you are or have struggled with this issue in the past, this blog post will help you understand why this might be happening and what to do to correct it.
High solids RTV silicone conformal coatings provide various advantages that help to address many PCB design criteria. Some advantages that an OEM designer might look for the ability to withstand extremely high operating temperatures, good flexibility, and resistance to chemical attack. Fast cure makes these types of silicones attractive for the applicator for high throughput production. However, the fast cure of these silicones makes them prone to wrinkling and therefore compromise the material’s performance (lower SIR, moisture ingress, etc) and less than desired appearance. Furthermore, since silicones are tough to rework, stripping of the wrinkled coating becomes a challenge due to usage and disposal of stripping chemistries.
The root cause of wrinkling of the high solids RTV silicones is a difference curing stage between the two sides of coated film. In other words, the applied film surface (top or bottom) is curing at a much faster rate than the opposite side or middle of the coating. The appearance of wrinkles is caused by shrinkage of the cured silicone vs the still uncured part of the coating.
The four most common mistakes during the application of high solids RTV silicones are:
- Coating is too thick causing much longer cure time of the film on the inside than the surface
- Exposure to very high humidity during the cure cycle, causing unnatural cure acceleration of the surface
- Coating during extremely hot weather, which causes film to dry (not cure) on the surface
- Coating of boards with increased level of moisture present on the surface, such as if the boards were brought in from colder storage into a humid room. This would cause the bottom of the coating to cure along with the surface leaving middle partially uncured
Wrinkling corrective actions include:
- Reduction in applied film thickness. This thickness will depend on the ambient and curing conditions
- If a high build coating is required, an additional coating pass may be required rather than single high thickness
- Temperature and humidity controls are critical. Standard 65F to 75F room temperature with relative humidity of 30% to 60% are ideal
- Make sure that the boards are dry prior to application
- Choose a high solid silicone RTV with low shrinkage to reduce wrinkling susceptibility
HumiSeal silicone conformal coatings will help you alleviate wrinkling as our R&D team was able to reduce the post cure shrinkage of our high solids RTV coating. HumiSeal 1C48 is a 100% solids RTV silicone with less than 10 min tack free time. The 1C48 reduced shrinkage rates will help to reduce applicators venerability to wrinkling.
Wrinkling of the high solids RTV silicones is a common defect. HumiSeal technical support is here to fields these questions quite often, however, with the right material and a few simple corrective actions can assure you that your coating quality is free from these less desirable attributes. Don’t hesitate to call us and discuss with one of our experts your conformal coating needs.