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PCB LAYOUT FOR COMPENSATION OF COEFFICIENT OF THERMAL EXPANSION (CTE) DIFFERENCES

IP.com Disclosure Number: IPCOM000007870D
Original Publication Date: 1996-Nov-01
Included in the Prior Art Database: 2002-May-01
Document File: 3 page(s) / 115K

Publishing Venue

Motorola

Related People

Allen Hertz: AUTHOR [+3]

Abstract

Paging has entered a new product market with the addition of the capability for acknowledge back transmissions. The addition of this new feature requires an external antenna which is considerably larger than the current receiving antennas. As a pag- ing device, an acknowledge back pager is not car- ried in a manner allowing for use of an external antenna.

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Technical Developments

PCB LAYOUT FOR COMPENSATION OF COEFFICIENT OF THERMAL EXPANSION (CTE) DIFFERENCES

by Allen Hertz, David Tribbey and Charles Parr

BACKGROUND:

  Paging has entered a new product market with the addition of the capability for acknowledge back transmissions. The addition of this new feature requires an external antenna which is considerably larger than the current receiving antennas. As a pag- ing device, an acknowledge back pager is not car- ried in a manner allowing for use of an external antenna.

  Transmitting antenna technology for small trans- mitter antennas utilizes large pieces of ceramic mate- rials with etched laminates (copper) as a standard design. The transmitting antennas are generally pla- nar to the circuit supporting substrate and may be coupled directly to it. The interconnect would gen- erally be located at the center, with the ground and feed centered within the ground area. Due to the size of the ceramic substrate, it is difficult to manu- facture, and there is a high risk for mechanical fail- ure during field use as a result of the small intercon- nect to antenna area ratio. Additional pads may be incorporated at the two ends of the antenna for mechanical support, but the coefficient of thermal expansion (CTE) differences between the receiver board (FR-4, Polyimide, etc.) and the ceramic antenna base over a typical length of 2.5" suggests breakage during cooling aher reflow and/or prema- ture field failure. It is known that expansion diff- ences of as little as .OOl" are suspect to failure for pad to pad solder joints or component breakage. Calcu- lations verify the expansion of prior designs result in differences > ,001:'

CTE FR-4 (PCB material) is X = 14 ppm/C", Y = 13 ppm/C", Z = 85 ppm/C"

CTE TMM-10 (antenna material) is X and Y = 16 ppm/C", Z = 20 ppm/C"

  Based upon a temperature difference of 160°C (20°C room to 180" Reflow) and a 2.5" length, the receiver board (FR-4) would expand .0052:' and the antenna (TMM-10) would expand .0064: for a delta of ....