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INC. Technical Developments Volume 20 October 1993
OPTIMIZED PRINTED-CIRCUIT PATTERN FOR MINIATURIZED TRIPLE-POLE SWITCH CONTACT
by Todd Roshitsh, Lawrence Pulliam, Giovanni Jaramillo and David Blatt
BACKGROUND AND PROBLEM STATEMENT
ASSUMPTIONS
The purpose of this invention is to provide an improved printed-circuit ("PC") contact pattern which ensures reliable operation of a triple-pole switch. For proper operation of such a switch in a keyboard application, the drive, sense, and ground lines must be shorted together simultaneously by the actuator. A PC pattern which improves the prob- ability of this occuring offers improved reliability over other patterns. Some factors which atfect switch reliability are noted in Table I.
The contact pattern may be on a flexible or rigid substrate, and possible mating components include a flexible membrane with printed conductive dots depressed by an actuator, or a conductive actuator. Specifically, advantages of the invention discussed here exist for miniature applications in which (1) there are demanding space or geometrical constraints which limit how large an actuator or actuator's tar- get contact area can be (2) there are limitations on how fine the printed-circuit trace widths and spaces can be, based on the technology and manufacturing method used to create them (3) three separate run- ners must be shorted together simultaneous&, though momentarily, for proper switch-circuit operation and
(4) the possibility exists ofmechanical misalignment between the switch actuator and the target position on the PC contact pattern. In addition, the contact pattern disclosed here eases PC routing of a typical keyboard arrangement consisting of keys located on an "X-Y grid," due to the symmetry and simplicity ofthe pattern.
1) Use of a circular actuator tip resulting in a circular contact area, which is common practice for keyboard assemblies due to the symmetry and etf- ciency ofthe geometry.
2) Use of equal lines and spaces, also a gener- ally accepted practice in PC design.
CRITERIA
1) All other things being equal, when compar- ing one PC contact pattern to another the pattern which is able to function effectively when formed by wider lines and spaces is more useful to the key- board designer since he is not as limited to PC line- line technology Therefore the pattern may be con- sidered more "reliable?
2) For given trace and space widths T, a pattern that will work with a smaller diameter D contact area is more reliable (assuming perfect actuator-to- contact registration).
3) Criteria 1 and 2 may be generalized by con- sidering the dimensionless ration R = D/T. A given PC contact pattern is more useful and reliable if it will function properly with minimized values of R.
4) For a given contact diameter, one PC pattern is more reliable than another ifgreater misalignment a of the actuator to pattern is tolerated over more orientations (i.e., at various angles to the pattern origin).
DESCRIP...
