Abstract:
A printed wiring board assembly is formed by mounting an insert having a pocket containing one or more standoffs in the cavity of a pallet. The pallet and the insert are both coupled to the bottom of a printed wiring board. A device having tinned leads and a tinned casing is positioned in the pocket of the insert above the standoffs. A solder preform is positioned in the pocket of insert, beneath the casing of the device. The assembly is placed in a soldering oven and heated to a at least a reflow temperature of the solder preform, whereby the device casing is joined to the insert and the device leads are coupled to solder pads on the printed wiring board.

Description:
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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the invention are more fully disclosed or rendered apparent from the following description of certain preferred embodiments of the invention, that are to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein: 
     FIG. 1 is an exploded isometric view of a printed wiring board assembly according to the prior art; 
     FIG. 2 is an exploded isometric view of a printed wiring board assembly including a metal insert mounted in a cavity formed in the surface of a pallet according to the invention; and 
     FIG. 3 is a cross-sectional elevation view of the PWB assembly shown in FIG. 2 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 2 and 3, a PWB assembly  10  in accordance with the invention includes an insert  12  mounted to a pallet  14 . The insert  12  and the pallet  14  are both bonded to the bottom surface of a printed wiring board (PWB)  16  with solder or adhesive  23 . The PWB  16  includes an opening  17  that is aligned with a corresponding pocket  19  formed in the insert  12 . The pocket  19  is shaped to receive an electronic component that, in the embodiment shown, consists of a high-powered RF transistor  18 . The transistor  18  is positioned in the pocket  19  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 soldered to the insert  12 . 
     More particularly, pallet  14  includes a cavity  20  having a substantially rectangular cross-section. The cavity is located on a top surface of the pallet  20  and is sized to receive insert  12 . Mechanical fasteners  22  are used to secure the insert  12  in the cavity of the pallet  14 . The fasteners  22  extend through vertical bores  24  provided in the pallet  14  and are received by threaded holes  26  provided in the insert  12 . The fasteners  22  may be screws or other equivalent fasteners known in the art. The pallet  14  can be formed of any material having suitable thermal and electrical properties and is preferably formed of aluminum. 
     Insert  12  may be formed of any electrically conductive and solderable material and preferably is formed of metal. Insert  12  has a profile which is complementary to cavity  20  provided in pallet  14 . A pocket  19  is provided on the top surface of insert  12  and is shaped to receive a transistor  18 . The pocket  19  may include one or more standoffs or lands  30  which support the transistor  18  in a predetermined position above the bottom surface of the pocket  19 . The standoffs  30  define both the thickness of the solder joint between the insert  12  and the transistor  18 , and the thickness of the solder joint between the transistor leads  21  and the PWB  16 . The standoffs  30  prevent the solder  23  from being squeezed out from between the transistor  18  and the bottom of the pocket  19  during soldering of the transistor to the insert  12 , thus ensuring that a uniform solder thickness is maintained between the casing of the transistor  18  and the pocket  19 . Experimental tests have shown that good solder joint performance and integrity are achieved using a uniform solder thickness of at least about 0.006 inches between transistor  18  and insert  12 , and preferably about 0.010 inches, and using a solder thickness of between about 0.004 and about 0.015 inches between the transistor leads  21  and the PWB  16 . Thus, the standoff height should be selected to provide solder joint thicknesses in these respective ranges. 
     In a preferred embodiment, four standoffs  30  are utilized, one at each corner of the pocket  19 . By locating the standoffs  30  at the periphery of the transistor  18 , the solder joint surface area can be maximized at the center of the transistor bottom, where the heat generated by the transistor tends be the greatest. This arrangement optimizes heat transfer between the transistor  18  and the metal insert  12 . It will be appreciated by those skilled in the art, that the location, size and number of standoffs provided in the metal insert  12  may be varied to suit a particular application or mounting component. 
     The solder preform  32  is shaped to fit in the pocket  19  of the insert with standoffs  30  projecting through cutouts or apertures  34  provided in the preform. The apertures  34  prevent the top of standoffs  30  from being covered with solder paste, thereby reducing the incidents of shorting between the transistor casing and the transistor leads  21 . 
     The metal insert  12  can be formed of any material having suitable properties of thermal conductivity and diffusivity and coefficient of thermal expansion (CTE). Preferably, the CTE of the metal insert  12  should approximate the CTE of the transistor  18  to maximize the integrity of the solder joint provided therebetween. Tests have shown that good solder joint performance is achieved when the respective CTEs of metal insert and the transistor are within 10 percent of one another. In a preferred embodiment, the metal insert  12  is formed of copper, and both the insert  12  and the pallet  14  are plated with gold to reduce incidents of galvanic corrosion. 
     A thermally and electrically conductive pad  28  is disposed between the bottom of metal insert  12  and pallet  14 . The pad  28  may be approximately 0.003 inches to 0.004 inches in thickness and is formed of cloth which is impregnated with metal. The pad  28  acts as a thermal interface between the insert  12  and pallet  14 , filling voids that result from poor contact between the mating surfaces, thus improving heat conduction between the surfaces. The thermal pad  28  may alternatively be formed of metal impregnated epoxy, thin sheets of metal or a layer of thermal grease. 
     In an alternative embodiment, the insert  12  may be eliminated from the PWB assembly, and the pocket  19  including standoffs  30  may be formed directly in the pallet  14 . 
     A PWB assembly in accordance with the invention is formed as follows. A thermal pad  28  is placed in the cavity  20  of pallet  14 . Thereafter, a corresponding metal insert  12  is inserted in the cavity and secured to the pallet  14  using a plurality of fasteners  22 . As the fasteners  22  are tightened, the thermal pad  28  is sandwiched between the insert  12  and the pallet  14 . The thermal pad  28  conforms to the mating surfaces of the insert  12  and pallet  14 , thereby eliminating any air gaps or voids between the surfaces. 
     After the insert  12  and pallet  22  are assembled, the PWB  16  is positioned on the pallet/insert sub-assembly so that the opening provided in the PWB  16  is aligned with the pocket provided in insert  12 . Next, the top surfaces of the insert  12  and pallet  14  are simultaneously bonded to the bottom surface of the corresponding PWB  16 . Bonding of the PWB to the pallet/insert subassembly is achieved at elevated temperatures using either solder or adhesive, and employing techniques commonly known to those skilled in the art. The bonding process serves to structurally and electrically connect the PWB  16  to the pallet  14  and insert  12 . 
     Once the PWB subassembly is formed, a transistor  18  or another electronic component can be mounted to the assembly. Before installing the transistor  18 , a solder preform  32  is placed in the pocket  19  formed in the insert  12 . The transistor  18  is then placed in the pocket  19  of the insert  12  on top of the solder preform  23  and positioned on the standoffs  30  so that the transistor leads are aligned with corresponding solder pads on the PWB  16 . Transistor leads  21  are straightened, prior to assembly, with a stamping die (not shown) to improve lead height tolerances. The transistor  18  is pressed against the solder preform  23 , using known fixture devices, as transistor leads  21  are connected to the PWB  16  and as the solder preform  23  is reflowed in order to join the transistor  18  to the insert  12 . Both the transistor casing and the transistor leads  21  are tinned to avoid potential embrittlement of the solder joint. 
     A number of advantages are achieved according to the subject invention. Soldering the casings of transistors  18  to copper inserts  12 , rather than directly to aluminum or magnesium pallets  14 , provides a number of performance advantages. In the case of high-powered RF transistors  18 , soldering the transistor casings to the inserts  12  has resulted in significant improvements in both thermal and RF performance. Additionally, soldering transistor casings to metal inserts  12  having closely matched CTEs has been shown to markedly extend the life of the solder joint as compared to prior known constructions. Standoffs  30  provided in the pocket  19  of the metal insert  12  ensure accurate positioning of transistor leads  21  with respect to the PWB  16 , and provide for a uniform thickness of the transistor/insert solder joint and the transistor lead/PWB solder joints. These attributes further enhance both the solder joint reliability and transistor performance. 
     A PWB assembly in accordance with the invention is particularly suited for an automated production environment. The use of standoffs provided in the pockets of the inserts combined with tight tolerances required for both the pallet assembly and the transistor leads, ensure accurate positioning of transistors and transistor leads with respect to PWBs. By eliminating the need for manual manipulation and positioning of individual transistors, the assembly process can be automated. Robotic arms may be used to position transistors and corresponding transistor leads on pallet assemblies in a quick, accurate and repeatable manner. Pallet assemblies containing transistors may then be placed in soldering ovens to simultaneously solder the transistor casings and transistor leads to PWB assemblies. Accordingly, the described methods and constructions allow hundreds of transistors to be simultaneously mounted to PWB assemblies in a reliable and efficient manner. 
     Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claim should be construed broadly, to include other variants and embodiments of the invention which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.