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www.us-tech.com
February, 2013
Assembly and Packaging
Materials Testing for Microelectronics
By Ian Mayes, Ph.D,
B.Met, DIC, C.Eng, MIMMM, Technical Manager — Bond Test, Nordson DAGE
ferent mechanical properties are joined together. Predictable and reli- able behavior depends on under- standing their properties and poten- tial failure modes. A wide range of methods exist to help improve this understanding. The simplest form of bend is
L
known as pure bend and this is what we aim to achieve in a four-point bend test. The strain is constant over a large part of the sample’s surface and this is particularly useful for straining board-level interconnect (IPC/JEDEC 9702). Bend testing is also used for
quality control of PC laminates and more general materials testing. Either 4-point (ASTM D 6272, EN ISO 14125) or 3-point (ASTM 790, EN ISO 14125) bending can be used. The well defined sample geometry permits the use of simple beam theo- ry and flexural modulus can be deter-
Bend testing is needed to assess the impact of sur-
face cracks and treatments on the strength of brittle materials, such as silicon.
mined from the initial slope of the load-displacement plot. Bend testing is not confined to measuring elastic properties, but can be used to meas- ure both yield and failure strengths of the material. The values obtained
ike buildings and bridges, elec- tronic circuits are structures in which materials with quite dif-
are not the same as those measured in a tensile test and are referred to as the flexural yield strength and flex- ural strength respectively.
high plastic strains being localized at the tip of the growing crack. Plastic flow plays a key part in the crack growth process and so this mecha-
there is no proven correlation between them. Fatigue of solder is a complex subject. The adhesion of tapes, PCB
tracks and solar cell ribbon bonds can be assessed using a peel test. The peel test measures the force per unit width (peel strength) to tear the flex- ible adherent from a rigid substrate. This is not a direct measure of adhe- sion strength as work is done in plas- tically deforming the flexible compo- nent, but if the sample and test con-
Peel testing measures
the force per unit width (peel strength) to tear the flexible adherent from a rigid substrate.
Straining board interconnect using 4-pt. bend. Micro bend is ideal for assessing
the impact of surface cracks and treatments on the strength of brittle materials, such as silicon. In these materials the high level of stress at the tip of a crack cannot be relieved by plastic flow and even small loads can lead to catastrophic failure. The standard way of assessing the delete- rious effects of dicing and grinding is to perform some form of bend test on a statistically significant population of samples, typically >25. Different bend methods result in different stress distributions and methods used include: 3 point (SEMI G86- 0303), 4 point, ring-on-ring and ball- on-ring (spherical bend). Bend testing can be applied to other microelectronic com- ponents, such as SMT resis- tors and capacitors made from brittle glass or sintered ceramics (IEC 60068-2-77).
Fatigue Failure Fatigue failure is gener-
Fatigue data for a repeated
bend test. Multiple measurements are made at each angle, with the plot representing the average value.
ally attributed to the initia- tion and subsequent growth of a crack under cyclic load or strain. More often than not cracks start at material defects which act as powerful stress concentrators, and with each cycle the crack gets longer until the material fails. Often the material shows no gross plasticity, the
nism of failure cannot account for failure in brittle materials, such as silicon.
Low Cycle Fatigue is generally
characterized by a fatigue life <104 cycles. Crack initiation is rapid and growth dominates the lifetime. The cyclic load-displacement plot can be used to high- light energy loss in the sample due to inelastic deformation and change in compliance due to crack growth.
Cycles Until Failure For a given set of con-
ditions, the number of cycles until failure will exhibit some variation and statisti- cal models such as the two- parameter Weibull distribu- tion are employed. This is to be expected as material defects and micro structure have a big influence on crack initiation and propagation. The simplest way of straining a
ditions are standardized, the peel test becomes a convenient quality control tool. Adhesive joints be tween rigid
adherents can be assessed using a lap shear test but in many cases, such as die adhesion a simple shear test suffices (MIL-STD-883H). Shear testing can also be used to test underfill adhesion and to measure
Glass fiber composite undergoing a 3-pt. bend test to assess rigidity and flexural strength.
solder joint is to bend a test board on which the component has been attached. Bond failure can be moni- tored by measuring the resistance of the connection. How ever, care must to be taken in interpreting such data to predict thermal cycling perform- ance. Mechanical tests performed isothermally are different from ther- mal cycling and, at the moment,
the adhesive strength between lead- frames and molding compounds (SEMI G69-0996).
Stud/Hot Pin Pull Stud pull and Hot Pin pull pro-
vide a simple way of applying tensile loads to difficult-to-grip, low geometry samples. In stud pull an adhesive is used to attach a metal stud. Hot Pin pull
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