Circuit board inspection of conformal coating is the most critical process in the complete production line.
Get this wrong and the reliability of the board can be significantly effected.
Inspection should be carried out on completion of the coating process. However, it is often a neglected stage of the conformal coating production line.
The International standards such as IPC A 610 and IPC CC 830 are explicit in their requirements for conformal coating inspection. Yet, few companies actually inspect to the required level or indeed understand what is required.
Nexus are to hold a series of webinars on conformal coating topics in the Autumn of 2012.The series of four online conformal coating webinars will cover the following topics and each last 90 min. The third of the online training webinars will be: Inspection & Quality of Coated Board Assemblies Monday 7th January
Conformal Coating Inspection standards exist from a variety of sources and are based mostly on practical experience. There are many cosmetic defects that are reworked in industry which are unlikely to actually impact on product quality. The majority of coated products are inspected manually but with the growing use of coating application in high volume, automatic optical inspection is gaining in popularity.
Topics will include:
Inspection standards and what is important
Understanding some of the coating terminology
Manual & automated inspection
Common process defects & why they occur
Recording & charting defects in a process
Cosmetic process indicators
“When considering the finish of the circuit board and what the quality of the coating is like we find customers perceptions and ours can vary. This is generally down to preconceived ideas of what a good quality coating should look like.”
“The standards state that when dipped a PCB can have coating dripping at a corner and have bubbles within. However, many customers don’t like this. The problem is that to reduce the dripping effect tends to have a knock on affect on the rest of the quality of the PCB like conformal coating thickness. So, educating the customer on what they should be inspecting and what is acceptable is key for the conformal coating inspection process”
“The UV brightener used in conformal coatings can vary from different coating manufacturers. The variation is due to volume or type of brightener used. Generally speaking a high brightness UV tracer will aid the automatic inspection system to identify that correct coating areas are covered and also look for stray material that may have entered critical keep out areas.”
“Some conformal coatings do not contain UV tracer and this is usually for a reason such as coating over LEDs or optical sensors where the UV tracer could affect light transmission. UV brighteners degrade over time with exposure to UV light therefore the coating on older PCBs may not show up quite as well depending on time and physical location of the board during its operational lifetime.”
“How can conformal coatings effectively and cost-efficiently be checked?”
“Applying conformal coating is the last step within the PCB production cycle. Similar to the previous production stages, quality assurance measures must be executed. After coating the assembly, proper wetting is mostly visibly checked by manual means. Because manual optical inspection has proven inappropriate for high PCB output, so called automated optical conformal coating inspection systems have been developed. They enable automatic testing of the correct wetting within the production line cycle.”
“Considering the Key Inspection Challenges faced by users, then continuity of what people actually require in terms of quality of finish is the biggest problem for conformal coating inspection. Using the standards available are fine but many companies don’t really read them and actually don’t understand them fundamentally.”
“For instance, is it really okay to have debris on the surface of a conformal coated PCB? The IPC IPC-A610 Acceptance Standard states it is okay as long as the, “Foreign material does not violate minimum electrical clearance between components, lands or conductive surfaces”. I guess that most would not consider this okay cosmetically even if technically and electrically this is fine. This discrepancy in understanding is key to inspection”
“Many users are looking to automate their inspection process, in order to increase their quality level. Systems are available that can integrate a camera and lighting, directly into a selective coating robot, or can be used in a standalone fashion separate machine.”
“There are numerous challenges involved in automating the inspection process, not least of which is the variation in brightness of fluorescence of the conformal coating materials, both batch-to-batch variations and between different conformal coating materials. Silicone materials in particular can have very low levels of fluorescence.”
“Other issues relate to the 3D nature of the substrate, light scattering leading to distortion of the images, background fluorescence of metallic solder joints/component leads and tall components or component bodies restricting the field of view, rendering it nearly impossible to inspect beneath or around through hole capacitors and connectors.”
“One of the interesting points is not all conformal coating defects and problems show up under UV light. You really need to inspect your PCB under white light and UV light to check for different defects. What can look like a perfect coating under UV can cosmetically look awful under normal daylight!”
“Also, you cannot tell how good coating adhesion is with visual inspection only. We examine the PCB as a coating service and if it looks good that’s fine. but without testing we cannot tell how well the conformal coating has adhered.”
“We find the customer wants to initially see everything. When we show them they realise generally how poor most automated processes like robots are at meeing IPC standards. We show them bubbles in the wrong places which an operator cannot see with no magnification and they then don’t know what to do. Its like they have opened Pandora’s box.”
“We then have to work with them to adjust their coating process and improve their production before we can effectively use automated conformal coating inspection at the end of the line.”
GOEPEL electronic has been developing Automated Inspection Systems for conformal coating, individually configured for special applications.
GOEPEL electronic offers a system for the automated optical inspection of fluorescing conformal coating. The TOM system (Teachable Optical Measurement) can be utilised for inspection of PCB coatings as well as PCB areas, which mustn’t be coated.The maximum PCB size is 460 X 400 mm.
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