Conformal Coating for Traction and High Reliability Applications

Introduction
Electronics used in traction inverters are subject to a great variety of environmental conditions including dirt, corrosion, moisture, and chemicals. Additionally, high-voltage applications, above 3000V, require larger creepage-clearance distances to meet UL specifications. To avoid premature failure and ensure safe operation, the printed circuit board assemblies (PCBA) inside the inverter must be well protected.

Conformal Coating
Conformal coating adds a thin lacquer to all surfaces of the PCBA covering all exposed metal contacts. This service, that AgileSwitch provides for all of our products, can dramatically reduce the volume of an expensive enclosure and increase the dielectric withstand voltage of the product.

AgileSwitch recommends HumiSeal UV40 UV curable coating for a good balance of protection and cost. This material is MIL-I-46058C qualified, and IPC-CC-830 and RoHS compliant and recognized under UL File Number E105698. This material has excellent dielectric withstand voltage properties, humidity resistance, and chemical resistance. AgileSwitch also has experience with Humiseal 1A33 polyurethane and IB33 acrylic materials. Customers may also specify a material of their choice.

Approach
There are two approaches to conformal coating PCBAs: automated and manual.

Automated Process:
AgileSwitch issues a list of coating specifications, including material, thickness, and keep-out areas, to our subcontractor. The subcontractor programs a PVA Delta 6 Selective Coating/ Dispensing System with our specifications. The boards are washed in a batch cleaner and placed on a conveyor belt in the PVA machine. Each board is carried under an computer-controlled nozzle and the conformal coating is dispensed. The final coating is precise, repeatable, and very little material is wasted. This process does not require any manual masking.

You can see this process in action below on an AgileSwitch Gate Driver:

Top Side


Bottom Side


Manual Process:
The manual process follows a more conventional approach. Each board is cleaned in a batch cleaner and then a technician masks the keep-out areas using tape. Then, the boards are sprayed in a well ventilated area and cured. After curing, the masking tape is manually removed from all the boards to reveal the keep-out areas. This process is sufficient for many applications, but it requires hours of pre- and post-processing, it is not repeatable, and there may be inconsistencies in the coating thickness and coverage.

Summary
AgileSwitch offers conformal coating as a service for any of our products and highly encourages customers to use conformal coating for all high reliability applications.

Please contact AgileSwitch for more information.

AgileSwitch featured on PSDtv at PCIM 2017

In this episode of Power System Design TV from PCIM Europe 2017, Rob Weber, CEO of AgileSwitch discusses their portfolio of IGBTs and SiC MOSFET Drivers. And now introduces their full range of Stack Electronics.