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Willis, SMART Group, NPL Offer Cleaning, Coating Seminars
February 5, 2014 |Estimated reading time: 4 minutes
An increasing interest in conformal coating and cleaning for the electronics industry has lead to many recent workshops and seminars run by SMART Group and NPL in the UK and by other organisations worldwide. Over the last 12 months, I've been rekindling my interest and knowledge on the subject for a special feature area and seminars sponsored by SMT Magazine at National Electronics Week (NEW) in the UK.
Things have moved a long way since my time as an apprentice using brush and manual spray coating techniques within GEC. Producing operator instructions and inspection criteria for the number of brush hairs allowed in a coated seems like a lifetime ago.
Simple process control checks can be invaluable during conformal coating. These may vary depending on the type of product, material, volume, application, and design density. Checking the wet coating thickness on the surface of the board after application or alternatively a blank panel, will make measurement simpler. Using a wet depth gauge, check the coating in two or three areas on a sample board. Ideally, one should check more than one sample in a batch. Record the reading, location, material, and process parameters for future reference.
Coating a blank test board prior to running production or using a proprietary matrix test card, like those used by Nordson, is a good way of checking spray process parameters. The test board or card can be inspected with a long wave UV light for coating definition, overspray, or coating run on. Process parameters like solids content, temperature, nozzle type, speed, height, and pressure can be recorded with the sample and retained for reference.
A test card from Nordson is shown with and without UV light illumination on spray patterns.
Measuring dry coating can be performed on a test label applied to the surface of a board prior to coating. The thickness of the dry label can be recorded prior to coating--subtracting the thickness of the label provides the coating thickness in that location. The label can be retained for future reference and is ideal for engineers to review during process audits. Digital thickness measuring systems are available for logging data for SPC.
With care, coated boards can be examined directly after spray coating using a UV light source on an out feed conveyor. Care needs to be taken to avoid contamination and disturbance of the coating. Surface contamination with hairs, fibres, or other particulate can be a problem with a delay before curing. Recent introduction of automatic optical inspection (AOI) specifically designed for coated boards can be used as off line or in line inspection with software which has been specifically developed for coating defects.
When coated boards are cured in a convection or IR oven the temperature profile must be established based on the material supplier’s recommendations. The profile must also be considered in relation to the coating thickness. Temperature profiling procedures should follow the techniques used for reflow soldering with the correct attachment of thermocouples. Attempts to speed up the curing cycle is a common cause of failure.
Inspect sample boards under UV lighting for complete coating coverage. Check areas of the board after removal of any masking for contamination or lifting of the coating at the masking interface. The coating should be compared with any work instructions, alternatively a golden coated board assembly should be available for reference. Obviously in high volume is where AOI systems will have an impact in the future.
During sample inspection of boards all defects, touch up of the coating must be recorded for future review. Inspection should be based on documented inspection criteria or customer specification. Where new defects or PCB design specific issues are seen they should be photographed and added to inspection criteria, training updates or included with customer specific instructions.
Having images of common process defects and examples of satisfactory coating to include in PowerPoint training modules or for onsite work instructions is an ideal resource for process and quality engineers tasked with training sessions. This type of reference material is available online and offered free to new seminar attendees. IPC will also be supporting the feature area and providing free cleaning and coating poster guides to feature attendees.
IPC free guides to cleaning and conformal coating with related standards.
For further information and to register for your free seminar place at the NPL/ SMART Group Conformal Coating & Cleaning Experience April 8-10, 2014 in Birmingham, UK, click here.
Conformal Coating and Cleaning Seminars
Tuesday, April 8:
- 10.30 Design Rules for Conformal Coating and Cleaning
- 11.30 Clean or No Clean for Conformal Coating Reliability
- 1.30 Selecting a Coating Based on Product and Environment
- 2.30 Reworking Conformal Coating and Component Damage
- 3.30 Process and Field Failures with Coatings
Wednesday, April 9:
- 10.30 Dip or Spray Coating Advantages and Disadvantages
- 11.30 IPC, ESA and NASA Inspection Requirements for Coating
- 1.30 In Process Quality Control for Conformal Coating PCBs
- 2.30 Cleaning Printed Board Assemblies for Coating
- 3.30 Confirming Component Compatibility with Cleaning Materials
Thursday, April 10:
- 10.30 In House or Contract Coating – Cost and Implementation
- 11.30 PCB Contamination Testing Options for Coated Boards
- 1.30 Test Methods for Conformal Coating Reliability
- 2.30 Conformal Coating Defects Causes and Cures