Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Patent
1996-09-06
1998-03-03
Ryan, Patrick
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
428901, 361728, 361746, 361749, 361750, 156264, 156291, 156578, 156629, 156634, 156656, 156902, 174254, 174259, H05K 306, H05K 300
Patent
active
057232059
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of PCT/U.S. Pat. No. 94/02437 filed Mar. 8, 1994.
The present invention elates to the fabrication of multilayer combined rigid and flex printed circuits having flexible printed circuits extending from the rigid board. In particular, the present invention relates to an improved process for the fabrication of a novel multilayer combined rigid and flex printed circuits, wherein the flex section has improved flexibility, thereby providing a rigid flex circuit with optimum flexibility in the connecting flex section.
Techniques for making multilayer rigid flex printed circuit boards are well known in the art. One early example of the prior art is disclosed in U.S. Pat. No. 3,409,732, assigned to the Assignee of the present application and whose teachings are incorporated by reference. Typically a rigid flex stacked printed circuit board includes flexible printed circuit cables extending from the periphery of the rigid section or sections. The rigid portions of the flex cables are typically used as sites for electronic components or mechanical hardware. It is important to note that the copper conductor in each plane or layer is fabricated from one continuous sheet of copper foil.
With improvements in electronic technology, there has been a constant need for advances in electronic packaging. This need has led to mere complex multilayer rigid flex printed circuit boards with many boards now using up to twenty-five, or even more, layers of circuitry. However, severe problems developed when the rigid circuit portions included many layers of conductors and holes plated through with copper to provide conductor barrels connecting the conductor layers.
One particular problem, reported on and discussed in U.S. Pat. No. 4,800,461, assigned to the assignee of the present Application, an whose teachings are incorporated by reference, described the fact that in multilayer rigid flex boards which included insulator materials such as acrylic adhesive and Kapton (Kapton is a trademark of E. I. dupont de Nemours and Company Inc. for polyimide film), the insulating materials placed a "Z-axis" stress on plated through holes. The coefficient of thermal expansion, it was reported, of the acrylic adhesive (Z-axis expansion) was the dominate influence. It was observed that because of the amount of acrylic required in many multilayer rigid flex applications, all plated through holes are stressed, with many cracking, making the boards unusable.
To overcome this problem, the '461 patent reported on a novel process to provide a rigid section incorporating insulator materials which, when subjected to elevated temperatures, did not expand in the Z direction to cause difficulties, including delamination and cracking of plated copper barrels. Stated another way, in the '461 patent, the materials causing undesirable expansion in the Z direction in the multilayer rigid section of the board, and the materials absorbing excessive amounts of moisture, such as acrylic adhesives and Kapton, were eliminated from the boards rigid section.
However, although the '461 patent was extremely successful in addressing the various problems recited therein, and in particular, the problem of thermal stresses described above, the process for fabrication of the rigid flex printed circuits has remained limited to the fabrication of a multilayer combined rigid and flex printed circuit board herein two circuit boards are always prepared from a basestock composition, and remain attached to one another via the prepreg. In other words, the process of fabrication according to the teachings of the prior art begin with the step of laminating two conductor layers (i.e. copper layers) to a single insulator layer (prepreg) followed by imaging and etching. Accordingly if one of the two bonded conductor layers was somehow improperly imaged, it was necessary to discard the entire lamination.
In order to solve the above mentioned problem, as reported on and discussed in PCT/U.S. Pat. No. 93/11684, filed Dec. 2, 1993, assigned to the assignee of the presen
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patent: 4687695 (1987-08-01), Hamby
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Caron A. Roland
Millette Lee J.
Thoman Joseph A.
Lam Cathy F.
Ryan Patrick
Teledyne Industries Inc.
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