498
P. Vianco and M. Neilsen
• Coefficient of thermal expansion (CTE),
• Elastic modulus,
• Glass transition temperature (T g ), and
The CTE mismatch between the encapsulant, conformal coating, or underfill
generates additional shear and tensile loads on the solder joints, depending upon
the extent to which they couple to the stacked package and the PCB. The preferred
situation is that the material CTE closely match that of the package or PCB to minimize the additional loads. Materials that fill the gap between packages should have
a CTE that matches that of the solder.
The elastic modulus is an important factor. A more compliant material, that is, one
of lower elastic modulus, can reduce residual stresses by elastically deforming under
the applied loads. Unfortunately, the tradeoff of having a reduced elastic modulus is
that the material typically has a higher CTE value. Therefore, the design engineer
must select a material, the properties of which, minimize solder joint TMF. Computational modeling is a valuable tool to make a “first-cut” in choosing the appropriate
encapsulant, underfill, or conformal coating for an assembly.
The glass transition temperature is also key parameter for encapsulants, conformal
coatings, and underfills vis-à-vis solder joint reliability. It is preferred that temperature fluctuations, whether those experienced in service or, those used in accelerated
aging, avoid crossing T g . When the temperature excursion crosses T g , there is a
significant change to the physical properties of the material. For example, there
are increases of CTE and decreases of elastic modulus, with a positive temperature ramp. The effects are reversible with the negative temperature change. In either
circumstance, the consequence is potentially sharp increases in the loads put on the
solder interconnections. However, a greater discrepancy can take place in reliability
predictions, when T g is crossed in accelerated aging; but, it is not crossed in the use
environment. In effect, the aging tests are potentially exciting an irrelevant failure
mode.
16.3.2.2 Geometry
Besides material properties, geometry poses two contributing factors to the solder
joint reliability of PoP and PoPoP components. First, the stacked package has a
higher profile than other devices. The taller height is a particular concern in the case
of encapsulants, because it allows the latter a greater area to “grab” the package and
exert more loads on the PoP or PoPoP component that could potentially damage the
solder interconnections.
The second geometry factor is the size of the gaps between two packages as well
as that between the bottom package and the PCB. The gap height determines whether
the encapsulant or conformal coating will partially, or completely, fill the gap, topto-bottom. When either material bridges the gap, as opposed to simply wetting one
or both surfaces, the confined geometry causes the material to exert very high tensile
stresses on the solder joints when the expanding material pushes the two surfaces
P. Vianco and M. Neilsen
• Coefficient of thermal expansion (CTE),
• Elastic modulus,
• Glass transition temperature (T g ), and
The CTE mismatch between the encapsulant, conformal coating, or underfill
generates additional shear and tensile loads on the solder joints, depending upon
the extent to which they couple to the stacked package and the PCB. The preferred
situation is that the material CTE closely match that of the package or PCB to minimize the additional loads. Materials that fill the gap between packages should have
a CTE that matches that of the solder.
The elastic modulus is an important factor. A more compliant material, that is, one
of lower elastic modulus, can reduce residual stresses by elastically deforming under
the applied loads. Unfortunately, the tradeoff of having a reduced elastic modulus is
that the material typically has a higher CTE value. Therefore, the design engineer
must select a material, the properties of which, minimize solder joint TMF. Computational modeling is a valuable tool to make a “first-cut” in choosing the appropriate
encapsulant, underfill, or conformal coating for an assembly.
The glass transition temperature is also key parameter for encapsulants, conformal
coatings, and underfills vis-à-vis solder joint reliability. It is preferred that temperature fluctuations, whether those experienced in service or, those used in accelerated
aging, avoid crossing T g . When the temperature excursion crosses T g , there is a
significant change to the physical properties of the material. For example, there
are increases of CTE and decreases of elastic modulus, with a positive temperature ramp. The effects are reversible with the negative temperature change. In either
circumstance, the consequence is potentially sharp increases in the loads put on the
solder interconnections. However, a greater discrepancy can take place in reliability
predictions, when T g is crossed in accelerated aging; but, it is not crossed in the use
environment. In effect, the aging tests are potentially exciting an irrelevant failure
mode.
16.3.2.2 Geometry
Besides material properties, geometry poses two contributing factors to the solder
joint reliability of PoP and PoPoP components. First, the stacked package has a
higher profile than other devices. The taller height is a particular concern in the case
of encapsulants, because it allows the latter a greater area to “grab” the package and
exert more loads on the PoP or PoPoP component that could potentially damage the
solder interconnections.
The second geometry factor is the size of the gaps between two packages as well
as that between the bottom package and the PCB. The gap height determines whether
the encapsulant or conformal coating will partially, or completely, fill the gap, topto-bottom. When either material bridges the gap, as opposed to simply wetting one
or both surfaces, the confined geometry causes the material to exert very high tensile
stresses on the solder joints when the expanding material pushes the two surfaces