August, 2017
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Page 55
Managing Thermal Energy with a Finely-Tuned Adhesive Continued from previous page
Single-component epoxies have Shore
hardnesses of approximately D85, where modi- fied two-component epoxies have Shore hard- nesses of A55 — comparable to a pencil eraser. Modified epoxies do not require heat to cure, which can offer further processing efficiencies. Typical properties include thermal con-
ductivity of 1.5 W/m-K and a dielectric strength of 18 to 20 kV/mm. Modified epoxies cure upon mixing. Pot life and set time vary across the product range, but are related to each other. One cannot have super fast set time without a faster pot life. For example, a product that achieves han-
dling strength in two to three hours would have a set time of less than 25 minutes, provid- ed that dispensing is continual. This type of product is excellent for high-speed production lines. However, after 25 minutes of non-use, the dispense noz- zle should be discarded.
Time is a Factor When considering the tempera-
ture limitations of adhesive bonds, time is as important as the actual tem- perature. Of the three types of adhe- sives —single-component epoxy, struc- tural acrylic and modified epoxies — the single-component materials have the highest temperature resistance. However, it is important to fac-
tor in the coefficient of thermal expansion (CtE). The CtE relates the change of a material’s size to a change in temperature. Over time and thermal cycles, bonds between two materials with different coeffi- cients of thermal expansion can cause the adhesive to crack. Cracks in the adhesive will alter the insulating and conducting strength. The higher the CtE, the more “give and take” the adhesive has, resulting in a reduced likelihood of cracking. Permabond offers materials
with a range of CtE values. The ES578 is a 1k epoxy with a CtE of 45 x 10–6 mm/mm/°C. TA4392 is a struc- tural acrylic with a CtE of 80 x 10–6 mm/mm/°C. Modified epoxy MT3821 has a CtE of 110 x 10–6 mm/mm/°C. Permabond 820, an instant cyano- acrylate, has a CtE of 90 x 10–6 mm/mm/°C. Note that CtE exponent values
are generally listed to the –6 power, but not always. Be sure to check the exponents before comparing the coef- ficient of thermal expansion of vari- ous materials. These same considerations of
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Best possible soldering results with and without vacuum
Typical applications of adhesives and epoxies include heat-sink bonding, potting and encapsulation of com- ponents, and bonding SMDs, among others.
thermal conductivity, flexibility, fire retardan- cy, and the ability to be processed apply not only to bonding heat sinks, but also to potting and encapsulating components, bonding SMDs and heat exchangers, along with electrical transformers, coils and windings. By using the proper adhesive, and fine-
tuning the characteristics of the material to fit a particular process, production efficiency can benefit greatly. This not only saves time during manufacturing, but also boosts the reliability of the material’s performance during processing, which ultimately increases product quality. Contact: Permabond, LLC, 14 Robinson
Street, Pottstown, PA 19464 % 732-868-1372 fax: 732-868-0267 E-mail:
info.americas@
permabond.com Web:
www.permabond.com r
VisionX-Series Convection Soldering
CondensoX-Series Condensation Soldering
High
Performance
Soldering
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