Metal fusion bonding – Process – Plural joints
Reexamination Certificate
1999-07-12
2001-08-21
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Plural joints
C228S182000, C228S254000
Reexamination Certificate
active
06276593
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns apparatus and methods for forming printed wiring board assemblies.
DESCRIPTION OF THE RELATED ART
Solid-state circuits typically include transistors that are mounted to printed wiring board (PWB) assemblies. In a typical configuration, a PWB assembly is formed by bonding a pallet to the bottom surface of a printed wiring board. The pallet supports the PWB and acts as a heat sink to draw heat from the transistors and related componentry.
FIG. 1
shows a conventional assembly
5
in exploded isometric view. The assembly includes a pallet
14
that is bonded to the bottom surface of a printed wiring board (PWB). The PWB includes an opening
17
that is aligned with a corresponding pocket
15
formed in the pallet
14
. The pocket
15
is shaped to receive a transistor
18
. The transistor
18
is positioned in the pocket
15
and protrudes through the opening in the PWB
16
. The transistor leads
21
are connected to the PWB and the bottom of the transistor casing is directly to the pallet
15
.
The integrity of the solder joints formed between the transistor casing and the pallet
14
, and between the transistor leads
21
and the PWB
16
are critical to the successful operation of the assembly. Several factors are known to negatively impact the quality of the solder joints, and to thus shorten the operating life of the assembly.
One factor contributing to the premature failure of the transistor/pallet solder joint is the mismatched coefficients of thermal expansion (CTEs) between the transistor and the pallet
14
. Pallets formed of aluminum or magnesium are often preferred because they are lightweight and inexpensive to produce. However, the CTEs of these materials differ substantially from the CTEs of transistors commonly used in PWB assemblies. For example, the CTEs of high powered RF transistors, often used in used in amplifier circuits for wireless equipment, are roughly two times the CTE of aluminum.
Variations in solder joint thickness can also contribute to premature failure of the pallet/transistor solder joint, and the transistor lead/PWB solder joints. In one known method of assembly, a solder preform is placed between the transistor casing and a corresponding pallet. The transistor is pressed against the preform and pallet, as the solder is reflowed to join the components. Uneven load distribution on the transistor often causes the solder to be squeezed out from between the joined surfaces in an uneven fashion during reflow soldering. As a result, the solder joints between the transistor and the pallet, and between the transistor leads and the PWB are weakened and transistor performance is compromised.
Production levels for amplifier circuits are also limited using the above-described methods of assembly. In accordance with the described methods, each transistor must be individually positioned on a corresponding pallet and the leads and base of each transistor must be soldered or otherwise connected to the PWB assembly. Transistor leads can become misaligned (both horizontally and vertically) with respect to solder pads as the solder preform is reflowed to join the transistor casing to the pallet, requiring costly and time consuming rework of the assemblies.
Accordingly, there is a need for improved constructions and methods for forming PWB assemblies.
SUMMARY OF THE INVENTION
The invention provides a method for forming a printed wiring board assembly. In accordance with the invention, an insert is provided having a pocket containing one or more standoffs for supporting a device above the bottom surface of the pocket. The insert is mounted in the cavity of a pallet. A device, such as a transistor, having tinned leads and a tinned casing, is positioned in the pocket of the insert above the standoffs. The leads of the device are soldered to a printed wiring board, and the casing of the device is soldered to the pocket.
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Artaki Iris A.
Basavanhally Nagesh Ramamoorthy
Bergman Scott Allen
Picot Walter J.
Regn Marsha M.
Agere Systems Guardian Corp.
Duane Morris & Heckscher LLP
Dunn Tom
Johnson Jonathan
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