Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1996-03-22
2001-02-13
Le, H. Thi (Department: 1773)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C106S626000, C428S330000, C428S331000
Reexamination Certificate
active
06187852
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates generally to the laminates which are widely used to make printed circuit boards. In one application, the invention is applied to the composite material used as a substrate for such circuit boards, particularly those made with glass fiber reinforced epoxy resms.
The generally recognized types of laminates are discussed in
Printed Circuits Handbook
, Coombs ed., Third Edition, McGraw-Hill Book Co., 1988. The lowest cost materials typically use phenolic resin impregnated paper (FR-2) and are used where the cost is more important than the electrical and physical performance. FR-3 is a paper composite which has been impregnated with epoxy resins rather than phenolic resins. CEM-1 is a composite which is more expensive than the FR-2 and FR-3 materials, but which provides improved electrical and physical properties. For CEM-1 an epoxy resin is used to coat paper as in FR-3, but the core is covered with glass fiber reinforced epoxy resins outer layers. FR-4 is made with a fabric of glass fiber yarns impregnated with epoxy resins. The present invention has application to all types of laminates, but in particular to FR-4 laminates.
Inorganic fillers are disclosed in U.S. Pat. No. 5,264,065 to be useful in controlling the coefficient of thermal expansion in the Z-axis of laminates, generally using 30 to 100 parts of filler per hundred of resin. Such fillers have been used in laminates for other and related purposes, such as to limit resin flow and to improve punchability according to the Japanese patent publications (JP 222,950 (1989), JP 199,643 (1982) and 7,044 (1984), JP 97,633 (1989), JP 120,330 (1990)) discussed in the '065 U.S. patent. In U.S. Pat. No. 4,960,634 zinc oxide is disclosed as an additive for improving thermal conductivity. In JP 133,440 (1990) “burned” talc is disclosed to improve dimensional stability of laminates. The talc was heated to 1000° C.-2000° C. to remove the water of crystallization. Talcs heated to below 1000° C. were stated to lack the desired improved dimensional stability.
The present inventors have been concerned with problems arising from the use of epoxy resins having relatively high glass transition temperatures (Tg), particularly about 150° C. or higher. These resins have advantages not available with those having lower Tg values, but they tend to be more brittle and more difficult to work mechanically. They produce epoxy resin dust when laminates are punched, cut, or drilled during fabrication of printed circuit boards. Dust is undesirable for various reasons, but particularly because it affects the precision with which circuit patterns can be made.
The present invention provides a means to significantly reduce the amount of dust produced, with the attendant advantages in manufacturing printed circuit boards. The invention also provides laminates which may be more readily drilled. Typically, laminates are stacked and drilled at the same time for efficiency. If the holes are not drilled cleanly, reworking or rejecting the board may be necessary. Alternatively, the drilling process may be modified, but this may reduce the speed with which the laminates are drilled. An additional advantage of the invention is that the epoxy resin has lower coefficient of thermal expansion (CTE) in the Z direction, which reduces circuit failures due to differential thermal expansion. These advantages are attained by including minor quantities of talc in the epoxy resin. However, it has been found that not all talcs can be used for some cause a substantial reduction in electrical strength.
SUMMARY OF THE INVENTION
The invention in one aspect is a method of improving laminates, in particular those made with epoxy resins having a Tg of 150° C. or higher, by incorporating in the epoxy resin layers up to about 20 wt. % of talc particles, preferably about 10 to 15 wt. %, which have a maximum particle size of about 40 &mgr;m. The talc should be relatively pure and approximate the theoretical formula 3 MgO .4 SiO
2
.H
2
O with less than about 0.01 wt. % (100 wt. ppm) water extractable anions. Particularly useful are Montana platy talc particles (>96 wt. % talc). They may be either untreated or have a surface treatment such as a silane. It is an important characteristic of the talc particles of the invention that they do not cause a significant reduction of electrical strength relative to laminates which contain no talc.
In another aspect the invention is a laminate for printed circuit boards which comprises a reinforcing material in an epoxy resin matrix, particularly those having a Tg of 150° C. or more, and containing up to 20 wt. % of talc particles having a maximum particle size of about 40 &mgr;m and which do not significantly reduce electrical strength relative to laminates which contain no talc. A preferred talc is Montana platy talc, which has a relatively high purity (>96 wt. % talc) and approaches the theoretical composition 3MgO .4 SiO
2
.H
2
O with less than about 0.01 wt. % (100 wt.ppm) water extractable anions. Optionally, the talc may be surface-treated, e.g. with a silane.
DESCRIPTION OF THE PREFERRED EMBODMENTS
The addition of solid particles to epoxy resin compositions has been suggested to lower the cost of laminates and to gain other advantages. Inorganic particles such as clays, talc, etc. typically are less expensive than epoxy resins and have been suggested as fillers. Ideally, solid particles could improve toughness of brittle materials by preventing the propagation of cracks. They have the disadvantage that they may be abrasive and cause additional wear to cutting and punching tools used in the manufacture of circuit boards.
For use in epoxy resins there are a number of requirements which should be met. First, the solid particles must be compatible with the components of epoxy varnishes formulated for application to paper and glass fiber, for example, they should be resistant to aggressive solvents such as DMF and acetone. The particles should be of a size which permits good dispersion through the varnish and they should not be too large relative to the distance between the circuit lines. They should not agglomerate and their density should be similar to that of epoxy resin so that they neither tend to settle nor float on the surface. It is important that the particles not degrade the electrical properties of the resin, particularly the electrical strength required of laminates. They should not have a significant effect on high temperature tests which laminates must meet, such as solder float. The inventors have found that not all solid particles satisfy these requirements and have discovered certain talcs which provide enhanced performance when used in laminates, but which do not degrade electrical properties.
Talc
Talc is a naturally occurring mineral having a theoretical formula 3 MgO .4 SiO
2
.H
2
O. Since it is a natural material, the composition varies. Of talcs mined in the U.S., those from Montana are considered to be among the purest, (e.g. >96 wt. % talc), while many others contain significant amounts of impurities, which may affect their ability to be used in certain applications. This appears to be the case in the present application, as will be seen in the examples below, where the electrical performance of laminates can be degraded by less pure talcs.
A major use of talcs is as reinforcing fillers in plastics, where they provide improved stiffness and creep resistance. They have been suggested for use in the laminates which are important components in printed circuit boards, along with many other types of particulate fillers, as has been noted above. However, the present inventors have found that all talcs are not equally useful.
After being mined, talc deposits are processed to purify them and to reduce them to fine particles. Commercial products may have a maximum particle size of about 70 &mgr;m, with finer materials having particles no larger than about 10 &mgr;m. Pure talc is very soft and it has a hardness of 1 on the Moh scale. The particles have various crystal fo
Conn Peggy Mae
Kamla Jeffrey Robert
Richgels Scott Harold
Tungare Aroon Vishwanath
Isola Laminate Systems Corp.
Le H. Thi
McDonnell & Boehnen Hulbert & Berghoff
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