There are three main causes for bubble generation on solvent based coatings: Air entrapment, Solvent flash-off and Too fast cure thick sections of coating.

In regard air entrapment, it is very difficult to apply coating to the topography of a circuit board without trapping air under some components. If you are accelerating the cure with heat these bubbles may not appear until the module is placed in the oven and the gas under the device expands due to heating.

The air, which remains under the component after cure, is not usually a problem: in some cases it is of benefit since it can act as a cushion during thermal cycling. A problem can arise when a portion of this air finds its way to the leads of a device and "pops" during cure leaving an exposed area. Also if the bubble is between pins or conductive paths this may lead to a weak point that may eventually allow corrosion, tin whiskers or dendritic growth.

There are several solutions: cold conformal coating (A CC with a lower temperature, 50-60 degrees F for example, will have a much longer batch time allowing the air to naturally escape and still have the coating flow to the space left by the busted bubble; A brief puff of air will often cause bubbles at the surface to pop, though care must be taken not to blow the coating off the area being coated. Even Rotation of the board or gentle agitation will assist bubbles to rise. The solution will be dependent on the module's design.

For bubbles generated by solvent flash-off by providing a period of time for the excess of solvent to flash off before taking the boards to a curing oven, the bubbles are minimized or eliminated. Also having again the coating a slightly lower temperature than the standard temperature (50-60 degrees F) will help with this issue.

Initial coating temperature as well as after-dispense cure temperature will affect both the solvent flash rate and the cure rate. Air movement around the board can similarly affect the flash rate and cure rate.

Finally for bubbles generated by an accelerated cure rate of the coating or extremely thick sections, mainly by trapping the cure byproduct some easy solutions can be applied. Provide the board some time before entering an oven will help.

Also heating the coating with a gentle curve, in opposite to as hot as it can get in a short time will minimize this kind of bubbles. If the wet thickness is above 15 mils you may need to cool the coating a little to reduce its skin over speed.

There are dozen of other probable causes however rarely seen, like gas absorption, coated that was pressurized for long time, organic contamination, etc. The suggestions above should most likely take care of the issue. Selecting also a low viscosity solvent borne silicone coating will help with this issue.

Wayne Wagner President, Krayden Inc. Wayne Wagner has over 25 years in the conformal coating industry and is the president of Krayden Inc., a leading distributor of engineered materials.

It sounds like you're getting poor initial surface wettability between the circuit card and your low VOC silicone compound when you apply it, which allows bubbles to form at the card and silicone interface, and then remain in the silicon compound after curing.

Considering the extreme environments you describe, it is not surprising that the circuit cards have poor wettability when you receive them. Using a suitable argon, oxygen or argon-oxygen plasma treatment would allow you to activate the surface of the circuit cards for better bonding and improved adhesion, and reduce or eliminate your bubble problem.

A relatively low-cost batch plasma system would likely meet your needs in this regard. Such a plasma system can accommodate multiple parts for each plasma cycle, and can be easily loaded and unloaded by an operator between plasma cycles. In any case, you should contact Nordson MARCH to run some samples in order to demonstrate the improvement that a suitable plasma treatment can provide.

Scott D. Szymanski Global Marketing Manager, Nordson MARCH Mr. Szymanski works to expand strategic alliances, strengthen partnerships with equipment suppliers, and develop future product offerings tailored to the semiconductor market. NOTE: Mr. Szymanski is no longer working at Nordson MARCH

Without knowing specifics it I shard to solve the problem. However, here are a few 'tips':

• Is there any chemistry in the curing which results in evolution of a byproduct, which may volatilize? Such as acetic acid, water, depending on the temperature, these types of by-products will cause bubbles;
• Is it possible to use a vacuum to "outgas" the adhesive or the repair, before curing it? Any dissolved air will be removed, before the curing step. This procedure is often done even with coatings from cast films, whenevr bubbles will damage the cast product.
  
  
  

It is difficult to help here without knowing the cure mechanism of the coating most silicones generate what is called a leaving group which is volatile and can cause bubbles if the cure is too fast.

What I have found to be helpful with bubbles in surface coating is localized heat to the surface of the coating in the vicinity of the bubble. The heat should only be applied for a short period of time this will cause the bubble to expand and burst as a result.

If your coating is a thermal cure this approach could be counter productive if the heat is applied for too long some experimentation will be required.

The reason that you have bubbles between and behind component legs is probably due to the viscosity (high) and thickness (100 micrometers +) of the material applied.

What tends to happen is the coating dries on the surface and solvent is trapped in the coating, if this coating contains solvent. This is also probably accentuated by the fact the coating is not flowing under the device due to high viscosity and is trapping air under the device.

I would suggest a water based coating, acrylic or polyurethane, if you need low or no VOC, or a solvent based coating if solvents are not an issue for you.

The lower viscosity combined with a reduction in coating thickness will eliminate the bubbles.

I presume the original boards are coated with silicone, if not I would only suggest using a silicone coating if the units that you coat have a continuous operating temperature over 150 degrees C.

Below 150 degrees C and if the original boards are not coated with silicone I would suggest acrylic if only moisture protection is required and polyurethane if moisture and chemical protection are required.

If you need further information please do not hesitate to contact me, cpalin@chasecorp.com

Chris Palin European Manager, HumiSeal Chris Palin is currently managing European sales and support for HumiSeal Conformal Coatings. His expertise is in test & reliability, solder technology, power die attach and conformal coating.

Bubbling normally occurs because the solvent in the coating does not get a chance to evaporate off before the surface skins over. If you apply it in large amounts or if the item is hot then bubbles will be trapped in the bulk by this surface skin.

The answer is to apply it in thin coats and build it up to the thickness required. This may need you to thin down the mix somewhat and you should experiment with this.

Trying to cure it too fast can also cause this so I would recommend building it up in thin coats with 30 mins or so air cure between coats.

Bryan Kerr Principal Engineer - CMA Lab, BAE Systems Bryan Kerr has 35 years experience in providing technical support to PEC assembly manufacturing. His experience ranges from analysis of materials and components to troubleshooting and optimizing, selecting reflow, cleaning, coating and other associated processes.