THIN MULTILAYER COATINGS FOR ADVANCED PROCESSING OF ELECTRONIC PACKAGES. I. V. Kadija J. A. Abys

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1 THIN MULTILAYER COATINGS FOR ADVANCED PROCESSING OF ELECTRONIC PACKAGES I. V. Kadija J. A. Abys AT&T Bell Laboratories 600 Mountain Avenue Murray Hill, NJ Abstract Current trends in electronic packaging are dominated by two major factors, the economic thrust towards lower costs and an increasing demand for higher level performance. This market environment requires a higher product output per machine and more complex products. Increased outputs typically require higher operating temperatures which could be problematic when utilizing traditional metal finishes for packaging. We have evaluated a palladium multilayer system and its capabilities in meeting the requirements of the semiconductor industry. 157

2 INTRODUCTION The surface finishing of components utilized in advanced electronic packaging is becoming an increasingly critical technology. It directly impacts on the reliability of electronic devices through its performance at interfaces within the package. The packaging processes typically involve a variety of attachment techniques such as: soldering, wirebonding, die-attach, brazing, welding, gluing and encapsulation. Clearly then, the interactions of the surfaces which are being bonded depends on the physico-chemical characteristics of each operation and compatibility of the surfaces in question. The requirements of these bonding operations (e.g. wirebond pull strength) is seldom, if ever, met by the bulk materials (e.g. Alloy 42) utilized to manufacture the components. In most cases, the substrate materials have entirely incompatible physico-chemical properties which prevent the bonding operations. For example, it is certainly difficult, if not impossible, to reliably wirebond a gold wire to an Alloy 42 lead frame. This situation is further complicated when a specific piece part is required to form a variety of bonds as is the case of a lead frame where die-attach, wirebonding, encapsulation and soldering must be accomplished in order to manufacture a device. To successfully meet the requirements of the various bonding operations, the semiconductor industry has relied heavily on the use of selective plating to deposit metals such as Ag, Ni, Au and SnPb. Recently, however, to meet new packaging requirements and in an attempt to reduce cost, the use of palladium as an overall plated finish has been investigated 1-6. In this regard, we have developed a palladium multilayer finish, PallaTech MLS, which can meet the most stringent requirements of solderability and wirebonding in the as plated, steam aged and thermal aged conditions 4-8. Furthermore, these finishes survive oxygen plasma treatments utilized in PWB and MCM manufacturing operations. In this paper, we describe the performance of the PallaTech MIS in solderability and wirebonding operations particularly at the higher temperature range. Custom designed multilayer finishes were demonstrated to efficiently enhance the solderability and wirebonding reliability and production throughputs at acceptable costs.

3 BACKGROUND INFORMATION Wirebonding is one of the most critical packaging operations which requires very high yields in order to be cost effective and have a positive impact on the overall reliability of the electronic components. For thermosonic bonding processes critical parameters include: Power Applied To Bonding Tip (Watts) Force Applied To Bonding Tip (g) Substrate Temperature ( o C) Bonding Time (ms) A successful bonding operation requires that the above parameters be selected properly for established substrate material, bonding tool, wire material and thickness. Choosing the proper settings is essential to assure a proper bond with the desired bond strength. However, there are limitations due to the choice of substrate and wirebond materials. For example, the temperature of the bonding tip can have a marked effect on the vibratory characteristics of the equipment which affects the wirebond performance 7. Likewise, the temperature of the substrate must be limited if bonding a chip-on-board to a plastic laminate. In this case, the insulation material could deteriorate leading to PWB distortion and loss of utility. In practice 7, bonding at lower temperatures utilizes longer bond time which in some cases limits the production speed. Alternatively, a solid copper alloy leadframe can withstand a considerable temperature range without affecting the bulk properties of the substrate. Nonetheless, one must consider the effect of temperature-on the surface finish of the leadframe. If this material tends to oxidize at elevated temperatures or if interdiffusion of base metal is of concern, then, once again, careful consideration of the temperature of operation is essential. Higher temperatures are usually favored for the purpose of moisture elimination from the substrate. While it was demonstrated that wire bonding can be successfully performed at liquid nitrogen temperature 8, it is generally accepted that the higher temperature will allow acceptable bonding in shorter time. The chart in Figure 1 shows the relationship between the time and temperature as experienced in several industrial applications in wirebonding with 1 and 1.25 mil gold wire. 159

4 As indicated by Figure 1 at the nominal force range of g, one can reduce the wirebonding time from 60 ms to 12 ms by increasing the substrate temperature from 150 to 33O O C. The techno-economical aspect of this parametric change is quite important. This increased wirebonding speed can be directly related to an increase in production output and consequently to reduced manufacturing costs. In order to take advantage of the possibility to enhance the production capacity by increasing the substrate temperature, the substrate finish must be capable of withstanding this temperature, without a deleterious effect on its performance. Following such thermal excursions, the integrity must be preserved. Furthermore, the thermally aged finish must meet the solderability requirements. Soldering is determined to be one of the most sensitive steps in the majority of packaging applications 7. Typically, environmental factors such as thermal excursions, oxygen and organics cause deterioration of the finish. The result is an absence of solderability and product failure. With this in mind we have developed a series of multilayer finishes which can withstand higher temperatures and other environmental attack such as plasma treatments and still remain solderable. WIREBONDING AND SOLDERABILITY OF MULTILAYER FINISHES As discussed above, the metal finish of many electronic packaging piece parts must withstand temperature excursions in excess of 200 C and in some instances survive oxygen plasma cleaning and other manufacturing steps which expose it to organic contaminants 6. For these conditions, the generic nature of the PallaTech MLS finishes permits the design of structures that meet a wide range of thermal and plasma treatment conditions. In order to test our MLS finishes 6 prepared under manufacturing condition, we chose a range of multilayer finishes as presented in Table

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6 Soldering is determined to be one of most sensitive steps in electronic packaging 6 when exposed to thermal excursions, oxygen and organics which can cause deterioration of the finish. The result is an absence of solderability and product failure. With this in mind, we tested a series of multilayer finishes which can withstand higher temperatures and other environmental attack such as plasma treatments and still remain solderable. The following are examples of performances obtained with finishes presented in Table 1. CONCLUSIONS Multilayer finishes can be designed to pass wirebonding and solderability of products exposure to meet a wide range of thermal and cleaning treatments of the substrates. Samples 1,4,5 and 6 which range in overall palladium thickness from 7 to 13 microinches maintain solderability up to 5 hours at 2OOC, exception being sample 6 (7 microinches) which fails at 5 hours. At 250 C, these samples remain solderable after 10 minutes of thermal aging. Samples 2,3, 7 and 8 shows excellent solderability after 15 minutes at 6 162

7 250C. The latter 4 finishes include a hyper-thin gold coating over the multilayer palladium finish. The benefit of this modification largely outweigh the added cost since the solderability can be maintained in some instances even through the 450C aging for up to 2 minutes, as is the case with Sample 3. All samples also show acceptable solderability after 1 minute at 33O O C. For extreme thermal conditions, Sample 9 shows that a multilayer finish including 10µ" of gold can meet specifications which are typically reserved for 100µ" of gold finish, i.e. 45O O C for 5 minutes. The wirebonding and solderability tests show that these multilayer finishes are quite economical and yet meet the substrate temperature requirement for high speed wirebonding. In conclusion, we have demonstrated that with minor modifications of a generic finish configuration one can obtain a wide range of solderable finishes which can be also utilized for other packaging applications like wirebonding, die-attach and encapsulation. 163

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10 Paper Not Available In Time For Publication I Thin Multilayer Coatings for Advanced Processing of Electronic Packages Dr.Igor V. Kadija & Dr. Joseph A. Abys, AT&T Bell Laboratories, Inc., Murray Hill, NJ Current trends in electronic packaging are dominated by two major factors-the economic thrust towards lower costs and an increasing demand for higher level performance. This market environment has increased the pressure in manufacturing of electronic devices; higher product output per machine and more complex products are sought. Increased outputs require higher operating temperatures, causing failures in typical finishes. A multilayer system and its capabilities in meeting these requirements have been evaluated. Materials and handling cost reduction, and enhanced functional performance, will be discussed in detail. for copies of this paper please contact Dr. lgor V. Kadija AT&T Bell Laboratories, Inc. 600 Mountain Ave. Murray Hill, NJ Ph. 908/ FAX 908/