Lead Free Solder for Flip Chip Zhenwei Hou & R. Wayne Johnson Laboratory for Electronics Assembly & Packaging Auburn University 162 Broun Hall, ECE Dept. Auburn, AL 36489 USA 334-844-1880 johnson@eng.auburn.edu Erin Yaeger, Mark Konarski & Larry Crane Loctite Corporation 1001 Trout Brook Crossing Rocky Hill, CT 06067 USA 860-571-2599 Larry_Crane@loctite.com Abstract The electronics industry is moving to replace lead in electronic assemblies. The driving factors are potential legislation, primarily in Europe, and global market pressures for more environmentally friendly products. The search for a drop-in solder alloy replacement has been ongoing for more than 10 years and none have been found. The current industry trend is to use the eutectic (or near eutectic) Sn-Ag-Cu alloy. The melting point of this alloy (~217 o C) is significantly higher (34 o C) than eutectic Sn-Pb (183 o C). This will impact the assembly process. In addition, the lead-free alloys have a higher modulus than eutectic Sn-Pb. This may change the stress distribution during thermal cycle testing, affecting reliability and failure modes. This paper examines the assembly process for flip chip die with Sn-Ag-Cu solder bumps and initial reliability testing. Key Words: Flip Chip, Lead-free, Assembly, Reliability Introduction The elimination of lead in electronics assembly has been discussed since 1990. Initially, the driving force was a proposed legislative ban in the U.S. At the time no solder alloy replacement alloys were identified and the legislation was dropped under strong pressure from the electronics industry. However, increasing restrictions on hazardous materials in landfills, recycling requirements and manufacturer responsibility for products from cradle-tograve have kept the topic of lead in the mind of manufacturer s. Today, with proposed legislation in Europe and global competitive market pressures, particularly in Japan, the elimination of lead in many, if not all, electronic products appears imminent. The successful introduction of electronic products assembled with lead-free solders belies the arguments that it can not be done. However, just because some assembly types and reliability requirements can be satisfied with lead-free solder, does not translate to all products and all reliability requirements. Much work remains to be done. One of the first challenges to the industry was the selection of a replacement solder alloy. The National Center for Manufacturing Sciences (NCMS) concluded in 1997, after a major four year research effort that there were no drop-in replacements for eutectic Sn-Pb [1]. The International Tin Research Institute (ITRI) [2] and the National Electronics Manufacturing Initiative [3] are both recommending the Sn- Ag-Cu eutectic (or near eutectic) alloy for reflow solder applications. Momentum does appear to be building for this alloy selection. The Sn-Ag-Cu eutectic has a melting point of ~217 o C, significantly higher than eutectic Sn-Pb (m.p. 183 o C). This will require higher peak reflow temperatures that may in turn impact fluxes and flux residue. For flip chip applications, the interaction between the underfill and the flux residue may degrade thermal cycle and thermal shock reliability. The higher Young s modulus of the Sn-Ag- Cu alloy (46 GPa versus 33 GPa for eutectic Sn-Pb) will also alter the stress distribution on the assembly, potentially impacting reliability and failure mechanisms. This paper examines the assembly process for flip chip die with Sn-Ag-Cu alloy bumps and initial thermal shock testing. Die with eutectic Sn-Pb solder were used for controls in the experiments.
Assembly Two test vehicles were used to develop the assembly process. The first test vehicle was the FB250 test die from Delphi Delco and test board from Flip Chip Technology. The FB 250 die is 6.35mm x 6.35mm with bumps on 0.457mm pitch. The second test vehicle was the PB8 test die from Delphi Delco and test board from Flip Chip Technology. The PB8 die is 5.1mm x 5.1mm with solder bumps on a 204µm pitch. Both test boards are a four-layer construction with ten die sites. The design has a trench in the solder mask to define the solderable pads for flip chip attachment. The PWB surface finish was electroless nickel/immersion gold. Two fluxes were used in the assembly evaluation, a tacky, no-clean flux and a low residue, no-clean flux. The tacky flux was applied using the rotating flux dipping station on the Siemens F5 pick and place system. The depth of the flux film on the rotating drum and the dip time controlled the tacky flux volume transferred to the solder bumps. The low residue flux was applied by spraying prior to die placement. All die were placed with the Siemens F5. The reflow profile used for the eutectic Sn-Ag-Cu is shown in Figure 1. Thermocouples were placed on the board surface and under a die between the flip chip die and the board surface. The temperature variation between locations was slight (<2 o C). The peak temperature was 240 o C. The test vehicles were reflowed in a nitrogen atmosphere (<25ppm O 2 ) using a Heller 1700 oven. solder sites were visually inspected. Samples were also mounted and crosssectioned. Figures 2-4 show a series of cross sections with FB250 die, Sn-Ag-Cu solder alloy and flux dip depths from 45µm to 65µm using the tacky, no-clean flux. The hold time was 3 seconds in all cases. For consistent solder wetting, the 65µm depth was required. For comparison, a flux depth of 55µm was used with eutectic Sn-Pb solder. Figure 2. Cross-Section of Sn-Ag-Cu FB250 Solder Ball with 45µm Flux Depth Using the Figure 3. Cross-Section of Sn-Ag-Cu FB250 Solder Ball with 55µm Flux Depth Using the Figure 1. Reflow profile for Sn-Ag-Cu Solder Alloy. After reflow, die were sheared for initial evaluation of solder wetting. The The low residue, no-clean spray flux also achieved good wetting with the Sn-Ag- Cu solder alloy (Figure 5). For thermal shock testing, PB8 test die and boards were assembled. Again, die were cross-sectioned after reflow to examine the solder joints. Typical examples with each flux are shown in Figures 6 and 7.
Figure 4. Cross-Section of Sn-Ag-Cu FB250 Solder Ball with 65µm Flux Depth Using the Figure 7. Cross-Section of Sn-Ag-Cu PB8 Solder Ball Using the Low Residue, Noclean Flux. Closer examination revealed defective plating on some of the PB8 test boards. The electroless nickel was thin and did not plate onto the sides of the copper pad on some boards. The Sn-Ag-Cu completely wet to the electroless nickel, but not to the exposed, unprotected copper on the sides of the pads Figure 8). The FB 250 boards had the proper nickel plating and this problem was not observed. The problem does not appear to be related to the solder alloy, flux or assembly process. Figure 5. Cross-Section of Sn-Ag-Cu FB250 Solder Ball Using the Low Residue, Noclean Flux. Solder Nickel Copper Figure 8. Close-up of Left Side of Figure 8 Showing Nickel and Solder Wetting on the Top of the Pad, but No Nickel or Solder on the Side. Figure 6. Cross-Section of Sn-Ag-Cu PB8 Solder Ball with 65µm Flux Depth Using the When examining Figure 7, there was initial concern that the Sn-Ag-Cu solder did not completely wet the sides of the copper pad. Some cross-sections revealed voids in the solder joints (Figure 9). Unassembled die were cross-sectioned and voids were found in the solder balls as-received (Figure 10). Thus, void introduction during the reflow process is not suspected. The effect of these voids on thermal shock reliability will
be investigated during failure analysis after completion of the thermal shock tests. Figure 9. Cross-Section Showing a Void in the Solder Joint after Assembly. Figure 10. Cross-Section Showing a Void in a Sn-Ag-Cu Solder Ball 'As-Received'. After reflow assembly, the die were underfilled with Loctite 3563, a fast flow, snap cure underfill. The die were underfilled using a Camalot 3700 dispense system. The substrate temperature was 95 o C. After dispensing, the underfill was cured in a conveyor oven with a peak temperature of 165 o C for 5 minutes. A Sonix scanning acoustic microscope was used in C-mode to inspect the underfill. (Figure 11). No voids were observed. Figure 11. Example C-SAM Image. Thermal Shock Testing Liquid-to-liquid thermal shock was used to evaluate the reliability of the underfilled test vehicles. Two boards (20 die) with each flux (tacky and low-residue) and solder alloy (Sn-Ag-Cu and Sn-Pb) were tested. The cycle was from -40 o C to +125 o C with 5 minutes at each extreme and a 1- minute transition time. In-situ resistance monitoring is required for accurate thermal shock or thermal cycle testing of underfilled flip chip die. The authors have observed failures (high resistance/open circuits) with daisy chain test die in the hot bath hundreds of cycles before the failure is observed at room temperature or at the cold extreme. This can be explained using the illustration in Figure 12. From finite element modeling, the maximum stress on the solder joint occurs at the low temperature extreme of the cycle since the stress-free temperature should be near the cure temperature of the underfill. However, at the cold temperature extreme the contraction of the underfill (the CTE of the solder is lower than that of the underfill) will hold the cracked solder joint together maintaining electrical continuity. Upon heating to the high temperature extreme, the expansion of the underfill will open the crack and a failure can be measured. Thus periodic measurements for continuity at room temperature will not discover high temperature intermittent opens and will over estimate the reliability of the connections.
Room Temperature (a) References 1. National Center for Manufacturing Sciences Lead-Free Solder Project Final Report, National Center for Manufacturing Sciences, Ann Arbor, MI, August, 1997. 2. Kay Nimmo, Worldwide Environmental Issues In Electronics And The Transition To Lead-free, Proceedings of the IPC Lead-Free Summit, Minneapolis, October 1999 3. NEMI News Release, January 24, 2000, www.nemi.org/pbfreepublic/index.html High Temperature (b) Figure 12. Illustration of the Need for In-situ Monitoring. The thermal shock test is ongoing. At the time of this writing, 1445 cycles have been completed with no failures. The test will be continued until >90% of the die have failed and failure analysis will be performed. Summary An initial assessment of the flip chip assembly process with Sn-Ag-Cu solder does not indicate any significant change in processing will be required with capillary flow underfills. A higher reflow temperature must be used, but appears to be compatible with existing PWBs and fluxes. The effect of the new alloy and higher processing temperature on reliability must still be determined. Thermal shock tests are underway.