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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 used in the package. The PSMC moisture sensitive phenomenon has
been identified as a contributor to delamination related package failure mechanisms including bond
lifting, wire necking and bond cratering, as well as die surface thin film cracking and other
problems. External package cracking is commonly treated as the most visible and severe form of
moisture sensitivity. It should be noted that internal cracking/delamination can be present even if
there is no evidence of external cracks. Intel has evaluated PSMC moisture sensitivity for its
current portfolio. Package moisture level has been measured as a function of temperature and
relative humidity. Critical moisture level limits to avoid cracking/delamination and other internal
damage have been determined and products susceptible to cracking/delamination have been
identified. Intel implemented handling procedures to ensure that these products are delivered to
users so that packages will not incur damage that could affect yield or reliability, during user solder
reflow processing. The user must take responsibility during storage, board mount assembly and
board rework to avoid package overexposure to moisture by following precautions recommended
in the following pages. These steps help to ensure that package integrity is maintained throughout
the surface mount process.
Figure 8-1. Package Crack Mechanism
Moisture Absorption
During Storage Die
Minimum
Plastic Thickness
Lead
Frame
Plastic
Encapsulant
Note:
Moisture saturates the package to a level determined by storage RH, temperature, time and
plastic moisture equilibrium solubility.
241187-1 A5736-01
8-2 2000 Packaging Databook
Moisture Sensitivity/Desiccant Packaging/Handling of PSMCs
Figure 8-2. Package Crack Mechanism (continued)
Crack Generation During Solder
Moisture Vaporization
During Heating
Pressure Dome Delamination Void
Note:
Vapor Pressure and Plastic Expansion Combine to Exceed Adhesive Strength of Plastic Bond
to Lead Frame Die Pad. Plastic Delaminates From Pad and Vapor-Filled Void Expands, Creating
a Characteristic Pressure Dome on the Package Surface.
Plastic Stress
Fracture
Crack Collapsed
Void
Note:
Crack Forms and Pressure Dome Collapses, Emanating From Boundary of Delamination Area at
Frame Pad Edge. Remaining Void Area Acts to Concentrate Stresses in Subsequent Temperature
Cycling, Leading to Further Crack Propagation.
241187-3 A5737-01
8.2.1 Surface Mount Assembly Processes
Traditional insertion (through-hole) assembly technology involves relatively few process steps and
minimizes the exposure of components to harsh processing environments. Modern surface mount
assembly can be very complex, especially when mixed technologies (surface mount and insertion)
are used on the same board. Furthermore, the components are fully immersed in the solder heating
media (vapor phase, convection heating or infrared reflow) in surface mount mass reflow processes
whereas solder heat during solder dip (or wave solder) is applied only to leads of insertion mount
packages. The circuit board shields the package body from the heat of the solder wave. Component
exposure in both reflow solder and wave solder process environments is illustrated in Figure 8-3.
2000 Packaging Databook 8-3
Moisture Sensitivity/Desiccant Packaging/Handling of PSMCs
Figure 8-3. Component Exposure to Wave Solder and Reflow Solder Environments
Through-Hole SMD
40