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tech.com
September, 2016 Production
Common Causes of Adhesive Problems
By Barry Siroka, Business Development Manager, EpoxySet H
FROM YOUR PERSPECTIVE, THIS IS MUCH BIGGER THAN IT LOOKS.
igh-tech adhesives are generally very reliable. When used correctly, they can resolve many design issues, saving money, time and effort. However, when issues do occur there are a variety of potential reasons
for their failure. Common failures include no adhesion upon cure, reduced ad- hesive strength, or loss of adhesive strength over time. When any problem oc- curs, the adhesive is often one of the first components to be examined. Find- ing the cause of a failure can be difficult, especially when it is intermittent or began after a long period of successful operation. Contamination should be one of the first considerations. Any grease, oil
or other impurity on the surface can cause the loss of adhesion. Dirty sub- strates are an obvious potential problem but contamination can be inadver- tent. Oils from skin contact, especially on very small parts, could be enough to cause a problem. Components may have been improperly cleaned. Sometimes, component manufacturers change a production process that does not affect the component performance or tolerances but can unintentionally affect its bonding. Controlling the cleaning process at the usage site is the best method to ensure that this problem does not arise. Surface preparation is essential for bond consistency. There are many
methods used to prepare surfaces, including washing, abrasion and plasma/corona treatment. The best surface preparation will depend upon the specific substrate and adhesive chemistry used. The mix ratio for many two-part materials can be a major issue. Some
systems are very sensitive to minor changes in the mix ratio. Many materials are stoichiometrically balanced and an off-ratio mix may cause the material to cure erratically or not perform well. Incompatibility with substrates can also contribute to adhesive failure.
Certain plastics may contain plasticizers which could seep up to the surface over time, causing a bond to fail. This seepage could be inconsistent from lot to lot so that some parts never fail while others lose adhesion. Some sub- strates can actually interfere with the cure mechanism of an adhesive. This is usually an issue that is addressed when selecting the adhesive but can appear later.
Even some materials that are not as sensitive to the mix ratio may ex-
hibit slightly different characteristics when the ratio is varied. Materials that do cure with an off-ratio mix may have slightly different finished hardness and tensile strength thus affecting final performance. The mixing of two part adhesives is a basic process function but is essen-
tial for these adhesives to work properly. Insufficient mix may result in a par- tial chemical reaction which leads to partial curing. A material cured insuffi- ciently will likely result in poor bond strength. Application technique is also critical. If material is applied manually, en-
sure that the amount is consistent for each unit. Most adhesives work best with an appropriate bond line. Too much or too little material could produce variable strengths when cured. If possible, make sure that the adhesive is ap- plied to the same location on every component. If automated placement is used, ensure that the equipment is clean
It’s just one small connection. Often smaller than a millimeter wide. But done right, it can save you millions. At Alpha, we place even the smallest electronic assembly challenge in a bigger light. As part of an assembly process that turns particles of material
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Visit us at SMTA International, Booth #723
when starting and remains clean during use. Some adhesives can partially cure in the dispensing apparatus, clogging the unit and preventing a consis- tent amount of material from being applied to each part. Air entrapment can be a source of failure especially in small applica-
tions. Removal of air from a system prior to application may be a necessary processing step. Air gaps could prevent the adhesive from interfacing fully with the substrate surface, resulting in decreased strength. Cure time and temperature are major factors that contribute to incom-
plete cure and performance of many systems. Some materials must be exposed to certain temperatures in order to cure. Many ovens can vary considerably and an adhesive may not seem to cure in its allotted time. Too high a temper- ature may cause a material to polymerize incorrectly. Products cured with UV energy have the same issues. They must be
cured with the correct wavelength and energy level. Certain chemistries will not cure with low light exposure and others can burn if subjected to a high dosage. Some of the UV chemistries must be heat post-cured to achieve max- imum performance. Environmental conditions should also be noted as many materials can be
affected by their surroundings. For example, high moisture levels can prevent cure in some materials while it speeds up the reaction in others. Carbon diox- ide, pH, oxygen and environmental temperature may adversely affect the fin- ished adhesive. As stated earlier, intermittent problems are the hardest to resolve. These
are almost always due to a factor other than the adhesive itself. Of course a batch of adhesive may have been produced incorrectly, but this is actually quite rare as manufacturers try to use reliable raw materials and process con- trols to ensure a consistent finished product. In the end, acting on an aware- ness of the common causes of adhesive failure will contribute to lasting device
performance. Contact: EpoxySet, Inc., 1 Industrial Circle, Lincoln, RI 02865
% 401-726-4500 fax: 401-726-4502 E-mail:
info@epoxyset.com Web:
www.epoxyset.com r
See at SMTAI, Booth 723
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