Leadframes

Wire Interconnect Materials. Wire-bonding is accompHshed by bringing the two conductors to be joined into such intimate contact that the atoms of the materials interdiffuse (2). Wire is a fundamental element of interconnection, providing electrical connection between first-level (ie, the chip or die) and second-level (ie, the chip carrier, or the leadframe in a single-chip carrier) packages. 

Leads serve as the input—output interconnections between the component package and the mounting platform. Sometimes leads also aid in the dissipation of heat generated in the package. In the case of plastic packages, leads are formed from the leadframe, which also acts as a heat-dissipation path and a mechanical support for the die. 

Many devices, particularly high-power devices, require good heat dissipation to prevent the device from overheating and degrading the device performance. This has generally been obtained through the use of the proper leadframe material and/or the use of an encapsulant that has a high thermal conductivity (e.g.,... 

The moldability/1344 indicates the number of samples which, upon visual inspection, show flaws such as surface voids and rounded corners. Wire sweep and internal voids were determined by x-ray screening. These data may reflect a somewhat greater reactivity of the new molten resin. A further increase in resin viscosity may lead to wire sweep and other imperfections such as internal voids. A low resin bleed count supports this idea. (Resin bleed is resin material protruding onto leadframes beyond the rectangular shape of the resin mass). Linear devices were also molded over CuA leadframes at 175 and 150°C. The test yields were 98.0 and 98.8% respectively. These yields, as well as those obtained with A-42 leadframes (Table IV) are identical to the yeilds observed when commercial encapsulants are used. 

All devices made with both copper alloy (CuA) and A-42 leadframes and encapsulated with the developmental resin were subjected to thermal cycling between -65°C and 150°C. Samples were kept for 5 min. at each temperature,... 

Failures as a function of pressure chamber time for the linear amplifiers made with alloy A-42 and copper alloy (CuA) leadframes are given in Table V. Even though the resin can be molded at 150°C, the test results indicate that devices encapsulated at 175°C out-perform all others. 

In addition to the linear devices, accelerated moisture tests under identical conditions were also performed on the digital devices on A-42 leadframes. The results are given in Table VI. 

Table VI. Failures Upon Accelerated Life Testing of Devices Encapsulated with GE s Developmental Molding Compound. Digital Devices on A-42 Leadframes...

A critical point that was discovered during this assembly development was a design rule for the height of the die with respect to the leadframe, the wirebond, near the leadframe, may be higher than the top of the dam. Both methods have passed exposure testing to harsh chemical environments, including nitric acid and salt water, at elevated temperature. 

Figure 1.3 Single chip, adhesive-attached and wire bonded to leadframe.

After mechanical attachment to a substrate, a leadframe, or to the inside of a package, bare die or chip devices are electrically connected by one of five methods wire bonding, flip-chip bonding, TAB, solder attachment, or attachment with electrically conductive adhesives. Fig. 1.6 shows some of these methods. 

The die may be attached and wire bonded or flip chip bonded to a small rigid interconnect substrate such as a ceramic or a plastic laminate, for example, bisma-leimide triazine (BT). Conventional wire bonding and overmolding processes, as in leadframe CSPs, are also used. 

Adhesives on etched leadframes bleed more than on stamped leadframes this is believed to be due to the presence of residual cutting oils on the stamped versions. 

Thermally conductive films are used to bond heat dissipating components to heat sinks. Electrically conductive silver-filled epoxy films when used to attach substrates also serve as ground planes and provide RF/EMI shielding. They are also used in the high production automated attachment of leadframes to chips and to bond chips vertically in chip stacks. 

Plasma-cleaning equipment range from small laboratory models having 6-inch-diameter chambers to large-capacity automatic production units. Equipment from March Instruments (a Nordson Co.) is reported to enable a user to process 500-1,000 leadframes per hour. Table 4.11 provides further detail for some commercially available plasma cleaners. 

March Instruments AP-1000 High throughput system 600/ 13.56 PC bond pad cleaning 20 leadframes/... 

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