Coating processes – Electrical product produced
Reexamination Certificate
1999-10-26
2001-11-27
Buttner, David J. (Department: 1712)
Coating processes
Electrical product produced
C428S413000, C438S127000, C525S113000, C525S122000, C525S482000, C525S484000
Reexamination Certificate
active
06322848
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an epoxy encapsulating material that has unique flexibility characteristics.
Electronic components often must operate under severe or harsh environments such as in automotive, marine and aerospace applications. In order to protect these sensitive components they often are coated with a protective encapsulating material.
It is known to use various epoxies as flexible encapsulating materials for electronic components. Encapsulating materials can be exposed to continuous temperatures of up to about 140° C. and intermittent temperatures of up to about 150° C. Such exposure is referred to herein as “thermal aging”. However, the known epoxies such as EPOLITE FH-1432 commercially available from H.B. Fuller Co. and XN-2248 Resin/XY-2233-1 Hardener commercially available from Nippon Pelnox Corporation exhibit substantial increases in hardness and modulus with thermal aging. For example, according to measurements obtained by the present inventors, EPOLITE FH-1432 has a flexural modulus of 1111.6 measured at −40° C. after thermal aging of 504 hours at 140° C. and a percent hardness change of 30.77 percent after thermal aging of 504 hours at 140° C. and XN-2248 Resin/XY-2233-1 Hardener has a flexural modulus of 850.2 measured at −40° C. after thermal aging of 504 hours at 140° C. and a percent hardness change of 32.30 percent after thermal aging of 504 hours at 140° C.
Failures occur because of stress induced on an electronic component over time due to changes that occur in the encapsulating material's mechanical properties or viscoelastic properties. These changes occur during thermal cycling and will induce extreme pressure on electronic components. Below −20° C., the conventional chemistry for flexible epoxies causes them to become very rigid and have very high modulus. Also, the hardness of these materials increases dramatically when exposed to high temperatures of 130° C. or more for any length of time. This is due to the additional crosslinking of the epoxy resin and curative. This combination of limited glass transition temperature, very high modulus below the Tg and thermal age hardening causes poor thermal cycle performance and dramatically increases stress on electronic components. The varying stress fatigues the solder or the component itself and eventually causes an open circuit or a short. Consequently, it would be very advantageous to have an epoxy encapsulating material that continues to exhibit flexibility after thermal aging.
SUMMARY OF THE INVENTION
There is provided according to the present invention a flexible encapsulating material resulting from a mixture that includes at least one epoxy novolak resin and at least two other epoxy resins wherein the material has a percent hardness change of less than 20% after thermal aging of 504 hours at 140° C. According to another embodiment of the invention there is provided a flexible encapsulating material resulting from a mixture comprising an epoxy novolak resin having less than 3 epoxy groups per molecule and at least one other epoxy resin. The encapsulating material is used to encapsulate electronic components.
An important feature of the invention is the superior properties exhibited by the cured encapsulating material are maintained even after long term or intermittent exposure to high temperatures. The cured material imparts very low stress on electronic components due to its robust flexibility. In particular, the cured material can exhibit a percent hardness change of less that 20%, preferably less than 10%, after thermal aging of 504 hours at 140° C. In addition, the cured material has a flexural modulus of less than 1100 MPa, preferably less than 450 MPa, measured at −40° C. after thermal aging of 504 hours at 140° C., and a Tg of less than −40° C., preferably less than −55° C. The flexibility properties of the material invention distinguish it from hard molded epoxy encapsulated materials.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Unless otherwise indicated, description of components in chemical nomenclature refers to the components at the time of addition to any combination specified in the description, but does not necessarily preclude chemical interactions among the components of a mixture once mixed.
While not being bound by any theory, it is believed that the epoxy novolak performs two key roles in the system of the invention. It unexpectedly dramatically increases the tensile and tear strength of the cured material without adversely effecting the low hardness, low Tg or low modulus. This increase in strength without an adverse property effect is surprising since an epoxy novolak typically is used to produce a very hard, high Tg material due to its highly functional structure. It also surprisingly acts as a catalyst to improve the cure rate of the composition at elevated temperatures. Consequently, amines (such as diethylenetriamine, triethylenetetramine and tetraethylenepentamine) that typically would be used as catalysts but are known to adversely effect thermal aging characteristics can be avoided.
Epoxy novolaks are well known materials that typically are made by reacting an epoxy-containing compound with a novolak. The novolak can be made from a variety of phenolic compounds such as phenol, cresol and other substituted phenols. It has been found that epoxy novolaks having less than 3, preferably less than 2.5, epoxy groups per molecule are particularly effective. The “average” number of epoxy groups per molecule is determined by dividing the total number of epoxy groups in the epoxy-containing material by the total number of epoxy molecules present. A particularly useful epoxy novolak is 1,4-butanediol diglycidyl ether of phenolformaldehyde novolak (such as those available under the tradenames DEN-431 and DEN-438 from Dow Chemical Co.).
The amount of epoxy novolak that is present can be 1 to 15, preferably 5 to 10, weight percent, based on the total amount of epoxy compounds in the composition. If the epoxy novolak is present in an amount greater than 15 weight percent the hardness, modulus and Tg would increase undesirably. If the epoxy novolak is present in an amount less than 1 weight percent, the tensile strength and tear strength would decrease undesirably and cure time would increase undesirably.
In addition to the epoxy novolak, the invention includes at least one other epoxy compound. The additional epoxy compound(s) should be present because they provide the desired flexibility, Tg and modulus. If only an epoxy novolak is present these properties would be too high.
It is especially useful to include at least one mono-epoxy functional ingredient and at least one di- or poly-epoxy functional ingredient. The mono-epoxy functional ingredient adds exceptional flexibility to the cured material, but is relatively slow curing. The di- or poly-epoxy functional ingredient adds chemical and moisture resistance, thermal stability and flexibility.
The additional epoxy ingredient(s) include monomeric epoxy compounds and epoxies of the polymeric type and can be aliphatic, cycloaliphatic, aromatic or heterocyclic. These materials generally have, on the average, at least 1.5, preferably at least 2 polymerizable epoxy groups per molecule. The polymeric epoxies include linear polymers having terminal epoxy groups (for example, a diglycidyl ether of a polyoxyalkylene glycol), polymer skeletal oxirane units (for example, polybutadiene polyepoxide) and polymers having pendant epoxy groups (such as a glycidyl methacrylate polymer or copolymer). The epoxies may be pure compounds but are generally mixtures containing one, two or more epoxy groups per molecule. The molecular weight of the epoxy compound may vary from 130 to 4,000 or more. Mixtures of various epoxy compounds can be used.
Epoxy-containing materials that are particularly useful include those based on glycidyl ether monomers of the formula:
where R′ is alkyl or aryl and n is an integer of 1 to 6. Examples are di- or polyglycidyl ethers of polyhy
Phenis Michael T.
Tavares Manny
Buttner David J.
Dearth Miles B.
Lord Corporation
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