by Naresh Virmani NASA Parts and Packaging Staff, 301-731-8902 naresh.virmani@gsfc.nasa.govThe following summary has been extracted from a report entitled "Effectiveness of Parylene Coatings on Moisture Protection of Plastic Encapsulated Microcircuits and Assemblies". For a copy of the complete report please contact Nick (Naresh) Virmani at 301-731-8902.
The use of plastic encapsulated microcircuits (PEMs) in space flight applications has been a long-standing issue. PEMs are reputed to offer significant availability, size and weight advantages over their hermetically sealed counterparts and PEM proponents believe that these devices can operate as reliably as hermetic devices in many space applications. On the other hand, PEM opponents maintain that, despite marked improvements in PEM construction techniques and in associated reliability data, there still is insufficient long-term storage and usage data.
The most undesirable characteristic of PEMs is that they are permeable to moisture due to the hygroscopic nature of the epoxy molding compounds (EMC). The amount of water absorbed by a PEM package depends on a combination of factors such as: package size and configuration, volumetric ratio of the molding compound to the leadframe metal and die, formulation of the EMC, and the operating environment. If preventive measures are not taken, any absorbed moisture will diffuse through the encapsulant via capillary action and will eventually reach the die/epoxy interface. If the moisture picks up any ionic contamination it can lead to serious problems. Delamination along the leads and/or cracks in the encapsulants can facilitate moisture entry into PEMs. Contaminants, such as activated fluxes, cleaning solution residue, or other corrosive agents can then be mobilized by this moisture and can ultimately cause corrosion-induced failures. Absorbed moisture can also contribute to serious problems when PEMs are soldered to boards. Heat from the soldering process can lead to delamination and cracking, typically referred as "popcorning", and can result in premature failure.
The trend towards smaller package geometries and thinner protective layers of encapsulants has resulted in greater vulnerability of these packages to moisture induced problems. Reliability studies have shown that the problem of moisture absorption in PEMs can be lessened by applying a conformally coated thin film of Parylene (developed by Union Carbide Corporation) over the devices. [1,2] The time-to-first-failure and mean-time-to-failure of standard PEM package assemblies can be improved by at least a factor of two using this technique. Parylene's water repelling nature makes it an outstanding barrier to migrating ions (e.g., sodium and chlorine). As a result, Parylene is often preferred among conformal coatings as a moisture barrier.
Parylene is applied using a unique Vapor Deposition Polymerization (VDP) formation process. It conforms precisely to the topography of printed wiring assemblies (PWAs), resulting in a continuous, pin-hole free shield. The Parylene coating completely covers sharp edges and corners and is known for its ability to penetrate and cover hard-to-reach inner surfaces. As a result, a Parylene coating over a PEM provides protection against moisture and contaminants, helps prevent corrosion of metallization, and provides protection against mechanical damage to boards and PEMs.
Parylene coatings maintain good adhesion to epoxy molding compounds. The coefficients of thermal expansion (CTE) of Parylene is similar to those of epoxies. (The CTE of Parylene is approximately 35 ppm/degree C, while the CTE for typical molding compounds is approximately 27-30 ppm/degree C). Parylene's modulus of elasticity is such that a thin coating over a molded epoxy package will retain very little thermal cycling generated stress. Epoxy gel-coat surfaces between conductor traces are of particular importance for the preservation of electrical function in Printed Wiring Assemblies. Good adhesion here is essential to preserve electrical isolation between conductors in humid environments. In this situation, Parylene's VDP process offers a special advantage because the Parylene monomer is soluble in the epoxy gel-coat. As a result, the first Parylene polymer is formed under the surface of the epoxy, resulting in an interpenetrating polymer network, which is an effective mechanism for adhesion.
The available studies clearly indicate that the reliability of
PEMs coated with Parylene is significantly better than that of unprotected
PEMs. However, studies to-date have not evaluated thin small outline packages
(TSOPs). TSOPs are considered the most difficult application of PEM technology.
An evaluation of Parlyene coated TSOPs could yield additional information
and is recommended to determine the acceptability of this coating for space
flight use.
References:
[1] "Development for Application of Parylene Coatings," Report No.
TP75-331, Hughes Aircraft Company, December 1975.
[2] "Comparison of Plastic and Hermetic Microcircuits Under Temperature
Cycling and Temperature Humidity" Bias, L. Condras, et al, IEEE Transactions
on Components, Hybrids and Manufacturing Technology, Volume 15, Number
5, October 1992.