May, 2016
www.us-
tech.com Replacing Conventional SMT Adhesives Continued from previous page Open-minded professional engineers have
found vast improvements to device life and relia- bility with materials from hardnesses of low shore D to high shore A. In the past, shore D values of 80-90 have been specified for nearly all SMT mate- rials. The hardness is the most visible indication of modulus and can be evaluated easily by the end user or the contract manufacturer. Modulus values corresponding to these hardness values range from double-digit GPa to single-digit MPa. Each component and board design should be
evaluated for its operational temperatures, envi- ronments, expected lifespan, and type of housing to properly match its modulus. Having the modu- lus matched to the expected conditions will nearly always allow for a compliant material to out-per- form its rigid counterpart.
Alternative Materials Liquid and paste processes offer
a variety of advantages when com- pared with overmolding processes. They cost less to implement because they do not require expensive custom molds or tooling, the same careful handling to avoid imperfections, or the associated storage costs of large pieces of equipment. The necessary equipment for liquid and paste appli- cation can be as small as a desktop dispenser and box oven, even for pro- duction-sized batches. One of the notable benefits of
using liquids and pastes is the ability to operate at room temperature. Molding processes require high tem- peratures to melt the resin used in the shot. Once injected into the mold, the material cools rapidly. Pastes and liquids have large processing windows where multiple layers can be built up, or parts can be realigned. Electrically conductive adhe-
sives (ECAs) offer a variety of bene- fits over traditional solder attach- ment in SMT processes. The need for high-temperature soldering ovens or irons can be replaced with much lower temperature ovens for curing adhesives, and the curing parame- ters of an ECA often provide multiple avenues for cure. ECAs offer an alternative solution to lead-free sol- der replacements when they do not perform to specification, as well as being free of dangerous materials.
Protection Over Time Failure modes of mechanical
attachments can often be attributed to dropping a finished product. Drop - ping handheld or smaller devices containing surface-mount compo- nents may cause fractures and crack- ing in the bonds of more rigid, less compliant solders and traditional die attach adhesives. Devices destined for the con-
sumer market have increasing relia- bility requirements to protect against drops. When a device is dropped it introduces an enormous amount of stress on its components. In typical strong SMT materials these forces can be localized into points of extreme stress and cause the materials to fail. Tough materials have the ability to absorb this stress and dissipate it. The ability to avoid this kind of
damage is currently demonstrated in materials with high strength — where the force of the impact is weaker than the bond. Eventually, however, the overall strength of the bond will decrease, due to repeated stress and age. Embrittlement due to aging is a common cause of failure. Affected by factors such as thermal, mechanical or chemical fatigue, aged
An example of a brittle connection failure.
devices are more likely to crack or delaminate when under even small amounts of stress. As materials experience repeated stress it is
common for fractures to begin to develop. Bond strength of traditional materials is most important when determining the lifespan of the final compo- nent or device. More compliant bonds are able to
absorb repeated stress and have a higher tolerance for cracks and fractures, which is typically observed in tougher materials. Tougher materials may not survive a higher fall, but would certainly survive more falls. The increased demand for handheld devices,
such as smart phones and tablets, as well as the demand for more durability is moving the industry away from rigid solder. Tough materials have a lower modulus that gives them the ability to last much longer before becoming brittle and failing. This allows for a significantly longer device lifes- pan. Tough materials are often the best choice for devices that are subjected to accelerated aging and other conditions that may cause premature failure in typical SMT materials. Contact: Creative Materials, Inc.,
12 Willow Road, Ayer, MA 01432 % 978-391-4700 E-mail:
info@creativematerials.com Web:
www.creativematerials.com r
Page 65
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