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December 20, 2017

Epoxy Wicking up Wire Insulation

We have 13 gauge insulated wires soldered directly into plated holes in a circuit board. The board is placed inside a plastic housing and filled/potted with epoxy. During the curing process, the epoxy "wicks" up the wire inside the insulation and hardens. This reduces the flexibility of the wire beyond the filled area. Can you suggest ways to eliminate the wicking of the epoxy up the wires so the wires remain flexible?

T.J.

Expert Panel Responses

Higher Viscosity epoxy. Or softerhardness.

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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.

You don't say how many wires per circuit board, or how manycircuit boards you need to do this to, so the solution I suggest may not bepractical. I saw this issue in an automotive components manufacturer whoneeded to pot the CCA inside of a plastic housing just to protect theelectronics from the harsh under-the-hood environment, and they also had anissue with the loss of a flexible harness at the point where the wires exitedthe potting material. The good news is that there are potting materials that are curedinstantly using UV radiation. A two-stage potting process can be used. In thefirst stage, an automatic dispenser can be used to dispense a fairly thickUV-curable potting component on top of the solder joint at the point where theinsulation of the 13-gauge wire ends. Then the CCA is subsequently passed underthe UV curing conveyor which flash-cures the first-stage potting, sealing theinsulation before any has time to wick up the wire underneath the insulation.Then the entire CCA can be potted inside of its enclosure, and the first-stagepotting prevents any of the second-stage potting material from wicking underthe wire. CAUTION: Selection of the first-stage pottingmaterial requires qualification. It must be compatible with the solder joints,the insulation on the wires, and lastly the 2nd-stage(currently-used) potting material. For example, there are certain UV-curableRTV compounds that can be used, but they MUST be electronics-grade RTVmaterials, as some RTVs will attack solder joints. I suggest you talk to your current potting vendor if you haven'talready. They may already have a solution to this issue, because this is not uniqueto your product. If they don't have a solution, contact the folks at one of thefollowing companies. They both represent a large number of product lines, andboth have experts available that can help you find a compatible solution. Justremember, qualify whatever product you choose. You may need to run somehumidity testing as well as other testing to ensure the materials are indeedcompatible and will withstand the environment in which your product is intendedto be used. You don't want to trade your current problem with a worse onethat does not manifest itself until much later! Contact the folks at oneof the following companies for help: www.hisco.com/ and www.ellsworth.com

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Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

Heat-shrink sleevewould that work???

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Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

Three opportunities present themselves:
  1. Modifyor change the epoxy such that it does not wick
  2. Modifythe curing process for the current epoxy
  3. Sealthe wire-to-board interface to prevent epoxy intrusion
The first path may or may not be feasible. The same propertythat causes the wicking (moderate viscosity, good wetting and capillary flow)are necessary to ensure that the potting material fills small gaps adequatelyand that gas bubbles are purged easily. If you are willing to evaluatealternative materials, you may be able to find a material that has lesstendency to wick while maintaining other desired properties. The second option also may be feasible; if you are using anelevated-temperature curing process, the elevated temperature can drasticallydrop the viscosity of the un-cured potting material, which will dramaticallyincrease wicking. Dropping the temperature of the curing process, at leastuntil partial cure has been attained, may be a solution. If you are usinga vacuum to de-gas after potting, this can also dramatically increase wicking.It may not be possible or desirable to eliminate the vacuum step, however. The third option is a sure fix, but requires an additional material, andpotentially an additional curing step, to seal around the wires.

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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.