Moisture Sensitivity/Desiccant Packaging/Handling of PSMCs
8
8.1
Introduction
This chapter examines surface mount assembly processes and establishes preconditioning flows
which encompass moisture absorption, thermal stress and chemical environments typical in the
variety of surface mount assembly methods currently in use. Also discussed are the standardized
moisture sensitivity levels which control the floor life of ...moisture/reflow sensitive PSMCs along
with the handling, packing and shipping requirements necessary to avoid moisture/reflow related
failures. Baking to reduce package moisture level and its potential effect on lead finish
solderability is described. In addition, drying, shipping, and storage procedures are included.
8.2
Moisture Sensitivity of PSMCs
This section addresses technical issues related to maintaining package integrity during board level
assembly processing using Plastic Surface Mount Components (PSMC). Surface mount processing
subjects the component body to high temperature and chemicals (from solder fluxes and cleaning
fluids) during board mount assembly. In through-hole technology the board assembly process uses
wave soldering which primarily heats the component leads. The printed circuit board acts as a
barrier to protect the through-hole package body from solder heat and flux exposure.
Note:
No component body should ever be immersed directly in the solder during the wave solder
operation. To ensure PSMC package integrity throughout the surface mount process, precautions must be
taken by both supplier and user to minimize the effects of reflow solder stress on the component.
Plastic molding compounds used for integrated circuit encapsulation are hygroscopic and absorb
moisture dependent on time and the storage environment. Absorbed moisture will vaporize during
rapid heating in the solder reflow process, generating pressure at various interfaces in the package,
which is followed by swelling, delamination and, in some cases, cracking of the plastic as
illustrated in Figure 8-1 and Figure 8-2. Cracks can propagate either through the body of the plastic
or along the lead frame (delamination). Subsequent high temperature and moisture exposure to the
package can induce the transport of ionic contaminants through these openings to the die surface
increasing the potential for circuit failure due to corrosion. Components that do not exhibit external
cracking can have internal delamination or cracking which impacts yield and reliability.
It should be noted that PSMC moisture sensitivity relates only to the risk associated with direct
exposure of components to reflow solder process stresses. No loss of package integrity is expected
for socketed parts or for through-hole mounted components not subjected to the solder reflow
environment. If through-hole components are exposed to SMT processing, then they can exhibit the
same moisture sensitivity as PSMCs. If through-hole devices are exposed to solder reflow
processes such as Convection, VPS, or IR, then they should be baked dry first, using the same
baking procedures described for SMT packages. Current data indicates that there is no negative
long term effects on reliability of PSMCs when package integrity is maintained through surface
mount processing.
2000 Packaging Databook
8-1
Moisture Sensitivity/Desiccant Packaging/Handling of PSMCs
The effect of moisture in PSMC packages and the critical moisture content which may result in
package damage or failure is a complex function of package design and material property
variables. These include: silicon die size, encapsulant thickness, encapsulant yield strength,
moisture diffusion properties of the encapsulant, and adhesive strength and thermal expansion
properties of the materials
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Packaging/Handling of PSMCs.