A recurring theme at Bay Materials has been dealing with water. More precisely, not a problem with water itself, but keeping water out of things. Several recent projects have revolved around the difficulty of finding adhesives, encapsulants and sealants that can resist water at elevated temperatures.
Picking encapsulants and adhesives isn’t easy to start with; you need surface compatibility with the materials in question, acceptable cure and application/pot life/thixotropy properties, as well as appropriate CTE and rigidity to survive expected conditions for higher-performance needs. Things only get harder when you throw water into the mix.
Many encapsulants or adhesives (we won’t go pointing fingers just yet) that appear good based on manufacturer testing for water resistance or MVTR, fail under immersion conditions. Claims of water resistance made based on 85C / 85% relative humidity tests often do not apply full immersion conditions. Some adhesives are tested at high humidity and at high temperature, but not both at the same time, which is exactly when things tend to fall apart.
If the data isn’t available, the best way to find out if an adhesive will hold up under the conditions of your process and product use conditions is to test it. Depending on adhesive chemistry, some analytical methods can indicate cure completeness directly. Accelerated aging, water uptake studies, and immersion testing give real data on how well it will survive, projections on how long and what failures to eventually expect.
Humble immersion testing is a powerful and inclusive tool. The weight gain the adhesive undergoes while soaking indicates how much water it absorbs. This is an indicator of the stress it will be subjected to during wet/dry cycling, as well as a measure of how resistant it will be to immersion. If the adhesive lost weight when it is dried back out, the cure conditions may have been insufficient, or the stoichiometry of a two-part adhesive may have been incorrect.
Water uptake of several adhesives at 50C.
Some products that survive immersion conditions fail quickly under slightly elevated temperatures, such as 40-50C. Salt water has the additional problem of chloride ions rampaging around just looking for things to react with. For example, standard wire bonds protected with an industry-standard single-part epoxy encapsulent failed in about two days in salt water at 80C. The same wire bonds survived many weeks in solution with a tripolyphosphate counter ion. So, make sure to take testing far enough and specific enough; if you expect NaCl conditions, test under NaCl conditions.
Here are some rules of thumb we’ve developed for screening adhesives that will be immersed:
- Test at the top of your thermal envelope early. Things can fail here surprisingly, and ruling candidate materials out early on can save you lots of study time. If your planned adhesive has a thermal transition temperature lower than expected conditions, make sure you test on both sides.
- Don’t ignore the urethanes. Nearly everyone thinks “ewww Urethanes are wimpy” but increasingly modern “high-performance” urethanes offer surprisingly good adhesive and water resistance properties.
- Cure conservatively. Assuming it is feasible for your anticipated manufacturing process, test using the most conservative cure. If the bottle says 1 hour at 95C, try overnight at 95C.
- Be sure to check final dry weight against the initial weight when doing a water-uptake study. Adhesive loss during the immersion can mask undesirably high water uptake, and sufficient water uptake can mask adhesive loss that is indicating an improper cure, or unstable adhesive.
- Consider multi-coat approaches. Particularly with delicate parts like wire bonds you often have a dilemma: strong solvent-borne barrier systems will shrink and damage the delicate parts during cure, and often have very low viscosity, making them hard to apply. No-solvent systems (often epoxy) may have great CTE and application properties, but just not be a good enough barrier. Filling the area with the epoxy, and then coating the epoxy with a high barrier nylon or other resin can provide the best of both worlds.
If anyone else has brilliant solutions or interesting problems dealing with water resistant encapsulents or adhesives, drop us a line in the comments.
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