Tag Archive | defect

What is conformal coating de-wetting and why does it occur?

Severe Conformal Coating De-wetting of a PCB

Severe Conformal Coating De-wetting of a PCB

De-wetting is where the conformal coating material is applied to a printed circuit board evenly but the coating refuse to stick to the surface that it has been applied to.

Factors that influence de-wetting usually involve non-ionic contamination such as:

  1. Residues from board manufacture including silicone surfactants from solder resist & HASL rinse contamination
  2. Component residues like mould release agents
  3. Silicone oil from adhesives in production.
  4. Soldering processes
  5. Cleaning bath contamination where rinsing has failed
  6. Operator handling adding contaminants

Click de-wetting conformal coating failure mechanism technical bulletin for further information and solutions on how to solve the problems.

Or contact us here at +44 1226 249019 or email sales@schservices.com

What is capillary flow in conformal coating and why does it occur?

Capillary / Scavenging on a printed circuit board where the coating has run away from an area around a component.

Capillary / Scavenging on a printed circuit board where the coating has run away from an area around a component.

Capillary flow (also known as scavenging) in conformal coating is where the conformal coating pulls or runs away from certain areas of a PCB to more favourable sectors due to a combination of effects. This can leave a patchy finish on the surface of the board.

Factors that influence the capillary effect during processing include:

  1. low viscosity of the conformal coating
  2. an abnormally high amount of coating applied
  3. a low surface energy of substrate
  4. the high surface tension of the conformal coating.

For further information Click capillary conformal coating failure mechanism technical bulletin

Or contact us here at +44 1226 249019 or email sales@schservices.com

What is the maximum height of components on the PCBs for a standard Conformal Coating AOI system?

This can be critical so important to know due to the length of the machine required and the imaging technology used in the conformal coating AOI.

Camera slightly to the side. Hidden areas behind the components are now shown clearly.

Camera slightly to the side. Hidden areas behind the components are now shown clearly.

The first question to ask is are the PCBs 3D in nature or are very flat? This helps determine whether we use camera or parallax free scanning technology?

Parallax is defined as a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.

How this relates to conformal coating AOI is that with a fixed camera or a standard scanner the camera is at a fixed point and the circuit board with its 3D components may have “shadows” where the camera cannot see down the side of the components. This can be seen in the two photos below.

Take a look at our Conformal coating AOI Parallax link here to understand the issues more.

Click For further information on Conformal Coating AOI Systems from SCH Technologies.

Or, contact us here at +44 1226 249019 or email sales@schservices.com

 

 

What standards can the SCH Technologies Conformal Coating AOI systems inspect to?

The systems have a variety of software options right up to inspecting TRUE IPC Class III standards. The reason we state true standards is that most people are not inspecting to this level even though they think they are. This is due to two reasons:

  1. It is very difficult to inspect to IPC Class III standards as it is under magnification, is time-consuming and challenging for operators.
  2. Also, it is very difficult generally for customers to produce conformal coating finish to this level without creating lots of defects.

The key to all inspection, manual or automatic, is to decide what you are looking for.

The surface energy of the substrate is lower than the conformal coating

The surface energy of the substrate is lower than the conformal coating

So,what type of conformal coating defects are you looking for? 

Are you looking for bubbles?Are you looking for foreign bodies? Or is it just coating in the right place and coating not in connectors? These questions are related to the inspection criteria you desire and it is very important to define exactly what are you actually looking for.

The software we use can measure all of these factors. The key is to understand whether your production process can stand up to the level of inspection / interrogation that the machine will provide?

Click For further information on Conformal Coating AOI Systems from SCH Technologies.

Or, contact us here at +44 1226 249019 or email sales@schservices.com

How do I know if my conformal coating and the process is compatible with my printed circuit board and its components?

There are many different ways the conformal coating can interact with the circuit board in a bad way. However, they can split into two types of interaction; direct and indirect.

Direct interaction is when it occurs during or just after application of the conformal coating in the drying / curing stage.Again, this can be split down into two areas. Damage to the circuit board and parts is the first.  Typical examples include solvent based coatings “melting” a plastic component sufficiently to cause integrity issues or the coating removing the inks marking components thereby rendering the component unidentifiable by inspection. The second is immediate deterioration of the performance of the coating or the masking materials themselves. An example include issues such as inhibition of the coating to cure or dry due to materials on the circuit board with certain coatings like heat cure conformal coatings or the circuit board creating conformal coating defects such as de-wetting 0r delamination. Another example is the coating attacking the masking materials which leads to harmful residues being let behind which then could lead to an indirect interaction.

Indirect interaction is where the coating or masking material has interacted with the circuit board materials and causes long term reliability issues. It could be stated that defect formation can also lead to this but what we are specifically talking about is a chemical reaction with the materials on the PCB that leads in the long term to field failures. A good example of this is lead free soldering materials which appear to be susceptible to interactions with coatings which lead to effects such as electrochemical migration and field failures.

So, how do you test to see if the coating is compatible?

Its fairly obvious to state but a series of tests is required to check the interactions. An examination of the component list and datasheets can quickly identify likely candidates which could be damaged by the coating and its solvents. You can also check to see if the cure schedule will damage the components. Will heating the PCB for 2 hours at 90C cause any failures? Other possible tests include over-exposure experiments of the individual components to the coating and its solvents  to see if they are damaged or absorb the solvents and increase in weight (submersion in thinners for a few minutes is likely to be worse than the coating application.

The next stage is to apply the coating to the PCB and see how the coating dries and cures. If defects like lack of adhesion or de-wetting occur then you may need to look at either bringing in cleaning or improving the cleaning process. Also, you can now check to see if the coating fully cures on the board?

Finally, you have the reliability test stage of the process. Techniques like Surface Insulation Resistance testing, thermal cycling and accelerated aging can shed light on the long term reliability of the product. If the product corrodes or fails after testing then you know you have an issue and you can examine directly if the coating actually improved the reliability of the circuit board or whether it contributed to its downfall.

For further information click conformal coating cleaning and reliability or contact SCH Directly to discuss your needs by the various options below

  • Telephone: +44 1226 249019
  • Email: sales@schservices.com
  • Click Contact Us to send us your requests.

Why do I get cracks in my conformal coating during processing?

Conformal Coating cracking is a  failure mechanism in processing where the smooth surface fractures into sections with the cracks in the coating leaving the area below exposed to potential contaminants.

Causes of Cracking in Conformal Coating include:

  1. Cure temperature too high
  2. Conformal coating heat cured too quickly without allowing enough time for room temperature (RT) drying
  3. Film thickness too great causing coefficient of thermal expansion (CTE) mismatch and cracks occurring in the coating
  4. Operating temperature too high or too low causing the conformal coating to flex too much & crack.

To find out more click conformal coating cracking technical bulletin.

Designing for Selective Robotic Conformal Coating Processing: Rule 2

Conformal Coating materials are liquids that flow in unpredictable ways on a circuit board due to surface tension affects.  These problems with flow can create defects like capillary (scavenging) and unwanted flow into connectors. Therefore, consider the following rule.

Rule 2

Maintain a minimum 2.5mm distance between MUST coat areas and MUST NOT coat areas.  If you do, be prepared for production people to attack you with a pitch fork as it may not be possible to prevent the liquid flowing into restricted areas.

The Rules

The Rules for Selective Conformal Coating are straightforward. Follow them and you can save money and time in your application process. However, if the Rules are not followed, the resultant circuit board design can challenge even the most sophisticated conformal coating system and its operator to achieve the finish desired.

Click Designing Circuit Boards for Selective Robotic Conformal Coating for further Rules.