16 Processing and Reliability of Solder Interconnections …
487
PoPoP components. This approach is far more cost-effective than is “inspecting-in”
performance and reliability at the quality assurance stage.
16.2.5 Underfill, Conformal Coatings, and Encapsulants
16.2.5.1 Underfill
Underfills can provide the added assurance that a stacked package will withstand
the mechanical loads caused by shock and vibration environments. This mitigation
has been particularly effective for preventing “drop-shock” failures of hand-held
consumer electronics [14]. In particular, underfills are an alternative to the so-called
shock-resistant, (low-Ag) Pb-free solders when the latter are not a viable solution
because acceleration forces are particularly high, or when the higher processing
temperatures of the low-Ag, Pb-free solders degrade the manufacturing yields of
PoP and PoPoP assemblies.
The use of underfills is not simply a drop-in step for the assembly process. Highvolume manufacturing processes take a significant “hit” with the incorporation of
an under underfill step. Although the actual application of the underfill to a stacked
package can be performed in less than a few seconds, ancillary factors such as curing
time and limited pot life of the underfill on the manufacturing floor add to the overall
process flow time [15]. Also, capital and recurring costs associated with dispensing
and curing equipment must be considered when implementing and underfill process.
High-reliability applications place less emphasis on drop-shock resistance. Rather,
PWAs and the stacked packages on them, must withstand the vibration and shock
environments associated with military, space, and satellite applications. Those
conditions include vibration during launch and shock loads resulting from staging
sequences. It is anticipated that these environments will necessitate the use of
underfills with PoP and PoPoP components.
However, these applications also require that consideration be given to the effects
of underfills on solder joint thermal mechanical fatigue (TMF) when exposed to the
temperature cycling conditions that are also inherent to high-reliability applications.
In fact, reliability under temperature cycling can be the controlling factor that determines, whether or not, underfills are a viable option for the stacked packages on
a PWA. More details are provided in Sect. 2 that address, specifically, solder joint
reliability and the effects of underfills.
The use of an underfill begins by identifying the appropriate material. Adapting
an underfill step to an assembly process will establish requirements such as pot life,
viscosity, and curing schedule details. Then, the underfill physical and mechanical
properties must accommodate the reliability requirements. Underfill materials, which
do not contain filler particles (e.g., silica) are generally better suited for drop shock
resistance [15]. On the other hand, those underfills with filler components provide
better resistance to solder joint TMF under temperature cycling conditions. The filler
particles tailor the underfill CTE to better match the CTEs of the packages (molding
487
PoPoP components. This approach is far more cost-effective than is “inspecting-in”
performance and reliability at the quality assurance stage.
16.2.5 Underfill, Conformal Coatings, and Encapsulants
16.2.5.1 Underfill
Underfills can provide the added assurance that a stacked package will withstand
the mechanical loads caused by shock and vibration environments. This mitigation
has been particularly effective for preventing “drop-shock” failures of hand-held
consumer electronics [14]. In particular, underfills are an alternative to the so-called
shock-resistant, (low-Ag) Pb-free solders when the latter are not a viable solution
because acceleration forces are particularly high, or when the higher processing
temperatures of the low-Ag, Pb-free solders degrade the manufacturing yields of
PoP and PoPoP assemblies.
The use of underfills is not simply a drop-in step for the assembly process. Highvolume manufacturing processes take a significant “hit” with the incorporation of
an under underfill step. Although the actual application of the underfill to a stacked
package can be performed in less than a few seconds, ancillary factors such as curing
time and limited pot life of the underfill on the manufacturing floor add to the overall
process flow time [15]. Also, capital and recurring costs associated with dispensing
and curing equipment must be considered when implementing and underfill process.
High-reliability applications place less emphasis on drop-shock resistance. Rather,
PWAs and the stacked packages on them, must withstand the vibration and shock
environments associated with military, space, and satellite applications. Those
conditions include vibration during launch and shock loads resulting from staging
sequences. It is anticipated that these environments will necessitate the use of
underfills with PoP and PoPoP components.
However, these applications also require that consideration be given to the effects
of underfills on solder joint thermal mechanical fatigue (TMF) when exposed to the
temperature cycling conditions that are also inherent to high-reliability applications.
In fact, reliability under temperature cycling can be the controlling factor that determines, whether or not, underfills are a viable option for the stacked packages on
a PWA. More details are provided in Sect. 2 that address, specifically, solder joint
reliability and the effects of underfills.
The use of an underfill begins by identifying the appropriate material. Adapting
an underfill step to an assembly process will establish requirements such as pot life,
viscosity, and curing schedule details. Then, the underfill physical and mechanical
properties must accommodate the reliability requirements. Underfill materials, which
do not contain filler particles (e.g., silica) are generally better suited for drop shock
resistance [15]. On the other hand, those underfills with filler components provide
better resistance to solder joint TMF under temperature cycling conditions. The filler
particles tailor the underfill CTE to better match the CTEs of the packages (molding