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May, 2016
Compliance: A Tough Approach to Replacing Conventional SMT Adhesives
By Jonathan Knotts, Brian Violette, and Daniel Morgan, Creative Materials, Inc.
lating circuit boards. With such a variety of processes, it is no surprise that many types of poly- mer composite compounds are needed. The many SMT materials currently available have been designed to meet minimum shear strength values, thermal shock or thermal cycle requirements, and minimum storage and operating temperatures. With the advent of tough compliant materials, the field of adhesives is changing and now offers more alternatives to customers. Leaning on proven products and the require-
D
ments that specify SMT and other board popula- tion materials is the current practice in creating new designs and products. This involves specifying high strength, good adhesion, and low moisture sensitivity. Although these requirements have served well for generations of electronics, they con- tinue to evolve as board technology changes.
Testing for Failure Relocation of components, due to the addition
of high-speed connectors, stacked die, embedded components, new sensor components, and MEMs,
Thinner boards, flex boards, less housing materials, and dynamic conditions are making products
more susceptible to strain, vibration and stress build-up.
all provide an ever-changing set of challenges when designing a product. Thinner boards, flex boards, less housing material, and dynamic condi- tions are making products more susceptible to
ie attach, staking, underfilling, glob top- ping, encapsulation, damming, and filling processes are all part of the job when popu-
strain, vibration and stress buildup. Many next-generation products, which have
driven advances in board and design technology, have also forced material suppliers to address fail- ures by developing new materials. The life of a board is determined through accelerated aging, which
synonymous, when the terms are used to describe mechanical properties, they are not at all similar. A “tough” material has the ability to strain or elon- gate and does not require the high forces to initiate its elongation that a “strong” material needs. From a conventional standpoint, a tough material has no merit in SMT applications. Failure mechanisms in SMT joints are most commonly cracking or delam- ination. Both of these are considered brittle fail- ures. The additional strain from substrate and component developments exacerbates brittle fail- ures as strong materials resist flexing, store, and magnify the energy of vibration and strain.
Compliance or Complaints? Compliant materials can be soft and weak,
New compliant materials are replacing
traditional rigid solders and provide an alternative to lead-free solder processes.
includes thermal testing, mechanical testing and harsh environmental exposure. This testing sets a minimum force to remove a component, and when the removal force falls below that threshold value, the end of its lifespan is reached. Materials with higher bond strengths can last longer before failing, which has led to a demand for more of these types of materials. But this also creates materials that have become much harder. The highest strength-to-break indicates a material with the highest modulus, or most rigid property set.
30/03/2016 13:26 Page 1
Strong Isn’t Tough Enough Although strong and tough can be considered
which is rarely acceptable for any SMT applications. However, other compliant materials are tough, hav- ing higher shear strengths, and have the ability to elongate significantly before reaching ultimate yield. A crack or delamination is in essence an ultimate yield. Since other materials effectively cannot strain, the pulling away from another surface, or from themselves, causes them to crack. These compliant materials not only maintain
adhesion and cohesion during extreme strain, which is the main reason for failure, but also have the ability to relax and dissipate the stress. An added benefit to the compliant nature of tough SMT materials is the ability to absorb and dissi- pate stresses from CTE mismatches. Current methods involve adjusting the CTE of an interface material to split the difference in the mismatched CTEs, which tightly constrains the design of the composite. This added degree of freedom of tough materials allows for more customization in rheolo- gy and other material properties.
Continued on next page SMT Nuremberg - 26-28 April, Stand 7-229 Nepcon China - 26-28 April, Stand 1D35 PILOT
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