Pinholes in epoxy coating powders

Epoxy Powder Pinholes — Repairing them in Capacitors, Resistors & Thermistors

Why blow holes suck (or why suck holes blow)

And now that I got your atten­tion (and prob­a­bly attract­ed a few con­fused weirdossor­ry guys), let’s talk about Pin­holes, a com­mon issue with epoxy pow­der coat­ings, what we know about it and how we can elim­i­nate it.

Pinhole definition and types

Pin­holes are those lit­tle annoy­ing pop-corny marks that you might see on varis­tors, capac­i­tors or any type of epoxy coat­ing pow­ders and resins. Porous mate­ri­als (par­tic­u­lar­ly very porous tan­ta­lum) are most sus­cep­ti­ble to pin­holes, but even less porous sur­faces can also be affected.

There are two types of pin­holes that can be observed when using epoxy coat­ing powders:

  1. Blow” holes
  2. Suck” holes

Those apt­ly named defects are caused by air bub­bles that are trapped under the sur­face of the epoxy. 

Blow holes are caused by air bub­bles that expand when heat­ed but get trapped as they try to escape through the coat­ing powder.

Suck holes are caused by air bub­bles that con­tract when cool­er than the sur­round­ing area, draw­ing with them the epoxy inward.

Pin­holes in action

Please note that in our case there is no dis­cus­sion on pin­holes caused by sol­vents since all our prod­ucts are 100% solids and there­fore have no sol­vent relat­ed pin­hole defects.

Troubleshooting Pinhole Causes

To resolve defects caused by pin­holes, we address the ways in which the bub­bles are caused and can be avoid­ed. There are three major prob­a­ble caus­es and those are either the Equip­ment, the Process or the Materials.


It can hap­pen that the heat­ing ele­ment is not hot enough or the flu­idized bed is not uni­form and caus­es exces­sive bubbles. 

Solu­tion: The two obvi­ous steps are to turn the tem­per­a­ture up on the heat­ing ele­ment and also adjust the flu­idized bed to be more uni­form. Still not enough? Maybe you need to add more pow­der (with fin­er par­ti­cles) to even out the flow.


There is no “catch all” rec­om­men­da­tion for process­es. All of them dif­fer and there are just too many things that can go wrong (or right if you are one of those pesky, glass full, peo­ple). But we can still point out the most com­mon risk factors.

Speed. Are you too fast or even too slow on the trig­ger between heat­ing and dip­ping? That can be cause for con­cern. Try adjust­ing the time win­dow between heat­ing and dip­ping to achieve the best results.

Heat. This is not lazy c/p advice. Heat is incred­i­bly impor­tant so just like with speed, the heat­ing ele­ment can be inad­e­quate and might be too hot or even too cold for its own good. Try adjust­ing the tem­per­a­ture up/down until you observe the ide­al results.

Con­t­a­m­i­na­tion. Sub­strates can poten­tial­ly be con­t­a­m­i­nat­ed by pre­vi­ous process steps or even envi­ron­men­tal caus­es. Make sure that your sub­strate is con­t­a­m­i­na­tion free before coating.

Blow­holes in epoxy — Src


And that’s where CAPLINQ comes in. Some sol­id advice when pick­ing the right mate­ri­als for your appli­ca­tion is to look into the following:

  • Coat­ing pow­ders should have an “opti­mum” stick point tem­per­a­ture to ensure air escapes pri­or to gel
  • Coat­ing pow­ders should have a low melt­ing tem­per­a­ture of solids to be sure pow­der does not block air passage
  • Melt vis­cos­i­ty should be low to allow air to pass through before the epoxy is cured
  • Epoxy should have “self-heal­ing” flow prop­er­ties that allow gaps to flow and be closed once pin­holes appear

Hav­ing issues iden­ti­fy­ing a prod­uct that works well for you? Then dis­cuss with your CAPLINQ rep­re­sen­ta­tive to make sure you have select­ed the cor­rect prod­uct for your appli­ca­tion. We have a large vari­ety of epoxy coat­ing pow­ders to choose from and we are col­lab­o­rat­ing with world lead­ing epoxy coat­ing pow­der manufacturers.

Can’t find a suit­able prod­uct? Our Research and Devel­op­ment team is open to cre­ate a prod­uct adjust­ed to your needs. Con­tact us for more information.

About George Kountardas

George is a Jack of all trades with an unappeasable inquiring mind. Obsessed with new products and technologies, he is always pushing forward for better, faster and more efficient applications. Always learning something new.

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