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EFFICIENT MANUFACTURING OF FLIP CHIP PACKAGES
                    FOR SMART CARDS AND RFID PRODUCTS

                     ...
with stencil printed bumps (e.g. 100 micron diameter bumps) than the much
larger mass used in DSC (Differential Scanning C...
FIGURE 1


      Antenna Printing &
       Batch Curing Line
       (not required for
        etched-copper
          ante...
excitation within the resins themselves and from thermal conduction supplied by
the overlying silicon chips.

Since the fr...
TECHNICAL SUMMARY

1. UNDERFILL CURING

Equipment

The underfill dispense was performed by hand using a pneumatic EDF
disp...
Sample Prep and Process Conditions

The PFC smart card assemblies were printed onto 12” x 8.5” flexible polymeric
sheets (...
Visual Inspection

To evaluate the underfill cure with respect to voids generated during VFM-curing,
a Lambda Technologies...
Heat Flow (Arbitrary)




                                      40   60   80   100   120 140 160       180   200   220   2...
Raytek non-contact
                                          temperature probe




             ABS
             substrate...
Line Balance and Equipment Considerations

The target RDIF-tag throughput defined by PFC is 2000 units per hour. Based
on ...
compatible with the standard MicroCure 5100 VFM system. The major benefits
of VFM processing for this application that dir...
Top fixture


                                                                         ABS
                               ...
3. CURING OF FLIP CHIP BUMPS ON ETCHED-COPPER PET SUBSTRATES:

PET sheets, containing etched-copper antenna, are primarily...
The first process step in assembling these tags is to stencil print two small
conductive bumps on the active pads for each...
processing pallets were designed as separate top and bottom panels that held
the sheets in the middle. Cut-outs machined i...
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APPLICATIONS REPORT

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Transcript of "APPLICATIONS REPORT"

  1. 1. EFFICIENT MANUFACTURING OF FLIP CHIP PACKAGES FOR SMART CARDS AND RFID PRODUCTS BY JIM CLAYTON, POLYMER FLIP CHIP CORPORATION MARK KEENE, POLYMER FLIP CHIP CORPORATION ZAK FATHI, LAMBDA TECHNOLOGIES ABSTRACT The demand for RFID and smart card products is projected to provide one of the fastest growing markets over the next few years. To be competitive in this arena, technology companies and manufacturers of flex circuits and assemblies are being forced to look at increased production rates and lower cost manufacturing techniques. Polymer Flip Chip (PFC), of Billerica Mass, has successfully addressed this requirement with the selection of a new process technique. By implementing a Variable Frequency Microwave (VFM) curing process, PFC found the potential to dramatically increase process throughput capability, permit use of lower cost substrates, while simultaneously reducing inventory cost, utility cost, fixture expense, and space on the clean room assembly floor. VFM was initially chosen for curing because it selectively heats and enables the use of cost effective, low deformation temperature materials. When using VFM, the Flip Chip modules are selectively heated above the temperature of the substrate by 25 to 50°C. Furthermore, the VFM process significantly accelerates the cure of both conductive and non-conductive adhesives, enabling faster throughput and continuous flow cure processes. A technical review of process development, comparing convection with the VFM cure results will be presented. Optical microscopy, DSC, IR imaging and electrical functionality were evaluated during the qualification process. BACKGROUND Conductive polymer inks continue to gain widespread interest as a viable high- volume solution for low-cost, flip chip interconnection. The processing steps required for both wafer bumping and chip assembly are simple, yet reliable. However, one significant disadvantage has been the longer work-in-process (WIP) times needed to fully cure both conductive epoxy bumps and non- conductive underfill epoxies. Convection oven curing times for polymer adhesives may range from as little as 30 minutes to longer than 8 hours, depending on the cure temperature and exothermic reaction of the constituent resins of the epoxy system. Conductive epoxy bumps, in particular, appear to be “mass sensitive”, meaning that it takes an inordinate amount of time to cure the smaller mass associated Page 1
  2. 2. with stencil printed bumps (e.g. 100 micron diameter bumps) than the much larger mass used in DSC (Differential Scanning Calorimeter) sampling when deriving the recommended curing schedules. This difference is believed to be attributable to the weaker exothermic reaction contributed by the low percentage of organic resin mass present. Published curing schedules, therefore, need to be viewed as generalizations only, when attempting to establish a conventional oven cure schedule for bump interconnections made with conductive polymers. Recent development work, based on VFM (Variable Frequency Microwave) oven curing, has demonstrated that epoxy bump cure rates may be reduced to as little as 3 minutes (and even faster) for the same epoxy systems which previously required eight or more hours to cure. VFM curing through microwave energy provides, in general, faster processing and greater control over the curing process than that achieved using conventional heating methods, such as convection or IR ovens. In particular, the VFM technique enables very selective heating of materials while avoiding the typical problems associated with electrical-arcing of metallic components or circuitry. The MicroCure™ VFM product line, developed by Lambda, provides a remarkable new tool for rapid curing of polymer conductive adhesives inks and liquid encapsulants within the electronic packaging industry. Two primary types of Smart Card and RFID inlets or flex circuits are presently being fabricated at Polymer Flip Chip Corporation and are discussed in this paper: (a) screen-printed conductive-ink antennas on ABS-plastic and (b) etched- copper antennas on PET. ABS-plastic is the least expensive material for these types of products and is primarily used for ISO-type Smart Card inlets assembled with 8 to16-bit read-write chips. Copper-etched PET flex circuits are used primarily for RFID applications. These circuits provide better line density, required when routing traces beneath the smaller 8-bit chips, and lower resistance needed for higher-Q antennas. Since both of these flex materials deform easily when exposed to temperatures exceeding 90 deg-C, the epoxy materials chosen for assembly must cure below this temperature to be compatible. Unfortunately, low-temperature curing epoxies typically require several hours to fully cure and have a shorter, 1 to 2- hour, processing window for screen printing before the hardening agents begin to increase the ink viscosity. A typical batch process flow using conventional oven curing is illustrated in Figure 1. As a result of longer cycle-times required for low temperature curing, the total process time often exceeds 9 hours, excluding time required for loading and unloading panels from the vertical carts. In contrast, a total in-line process flow can be substituted using VFM curing, as illustrated in Figure 2. Page 2
  3. 3. FIGURE 1 Antenna Printing & Batch Curing Line (not required for etched-copper antennas) Screen Print Load Panels Cure & Cool Un-load Panels onto Vertical Panels in from Vertical Antenna Traces Carts Walk-in Oven Carts Antenna Bumping, Chip Placement & Batch Curing Line Load Panels onto Cure & Cool Un-load Panels Screen Print Pick & Place Vertical Carts Panels in from Vertical Antenna Bumps Chips Walk-in Oven Carts Chip Underfill and Batch Curing Line Underfill Load Panels Cure & Cool Un-load Panels Chips onto Vertical Panels in from Vertical Carts Walk-in Oven Carts Not required for etched- FIGURE 2 copper antenna process line Screen Print VFM-cure Antenna Screen Print Pick & Place VFM-cure Antenna Underfill VFM-cure Chip Antenna Traces Traces Antenna Bumps Chips Bumps Chips Underfill HOW VFM-CURING WORKS VFM-curing uses a variable swept frequency to apply microwave energy into a closed chamber for curing. Polymer resins, containing polarized molecules, will couple and absorb with the microwave energy, causing rapid polymerization of the resins as the energy is dissipated in the form of heat. Different materials and shapes respond in different ways to microwave irradiation, either to absorb or reflect the energy, and these differences can be engineered into an optimum combination of materials and pallet designs. Silicon and carbon, for instance, act as good absorbers for microwave energy and heat quickly. FR-4, ABS and PET materials, however, are fairly transparent to the same energy and will heat more slowly. Selecting FR-4 material for pallet construction will insure that the inlet substrates remain 50-60 degrees cooler than silicon RFID or Smart Card chips. Energy for curing either the epoxy bumps or underfill comes from both molecular Page 3
  4. 4. excitation within the resins themselves and from thermal conduction supplied by the overlying silicon chips. Since the frequency is rapidly swept, arcing problems associated with metal or semiconductor components is eliminated. Arcing is the result of excessive charge build-up in metallic materials in the presence of standing wave patterns. In the VFM technique, the electric fields are electronically stirred and the microwave energy is not focused at any given location more than a fraction of a second. The dynamics of charge build-up that leads to arcing are never achieved. Hence, bare semiconductor devices are not adversely affected by the microwave energy, resulting in defect-free processing. In the course of this evaluation, it became apparent that different epoxy materials exhibited dramatic differences in VFM-curing properties. Due to the close association between Polymer Flip Chip Corporation and Epoxy Technology, a new class of polymer adhesives, both electrically conductive and non-conductive underfill epoxies, are being formulated specifically for rapid microwave curing. QUALIFICATION OVERVIEW The objective of this work was to evaluate the VFM process for curing silver-filled conductive polymers (Epo-Tek No’s. E2101-PFC and EE162-5) and non- conductive underfill epoxy (Epo-Tek No. U-300) on ABS and PET substrates. Primary goals for this project were to significantly reduce the existing process cycle time, ensure equivalent or enhanced throughput (line balance), equivalent or improved material and product properties using minimum equipment floor space. Three different applications were tested: 1. Curing of U-300 Underfill on PFC smart card and RFID assemblies. 2. Curing of EE162-5 printed antennas (5 turn coil) on ABS substrates. 3. Curing of E2101-PFC flip chip bumps on etched-copper PET substrates. A summary of the results are as follows: 1. The curing of screen-printed antennas was performed using a special microwave fixture. Five-turn ISO standard antennas were cured within 15 minutes as opposed to 120 minutes at 90-95°C. Recent formulation adjustments in the conductive ink are expected to be further reduce the total process cycle in the range of 7 minutes or less. 2. The curing of conductive epoxy bumps on PET with etched copper traces was accomplished in as little as 3 minutes, a process currently carried out at 80°C for 8.5 hours. 3. The two-part Epo-Tek underfill was cured as rapidly as 30 seconds, a process currently carried out at 75°C for 30 minutes followed by 95°C for 30 minutes. Page 4
  5. 5. TECHNICAL SUMMARY 1. UNDERFILL CURING Equipment The underfill dispense was performed by hand using a pneumatic EDF dispenser. The ABS substrates were placed on a hot plate pre-set at 75°C. Dispense pattern followed an “L” shape. The variable frequency processing for the PFC underfill curing trials was performed in a VFM cure station (Lambda Technologies, Inc. model MicroCure 5100). The frequency range utilized during heating was 6.425 ± 0.575 GHz with a sweep rate of 0.1 seconds. A Raytek infrared pyrometer (refer to Figure 3) is incorporated in the MicroCure 5100 control system and was used to control the surface temperature of one of the flip chip die. A Nortech fiber optic contact probe is also incorporated in the MicroCure 5100 control system and was used to monitor the temperature of the substrate during the flip chip underfill curing. The power was pre-programmed to produce a multi-step profile summarized in Table 1. Table 1. VFM profiles used for processing of PFC parts VFM Time (min) Time (min) Time (min) Time (min) Total Time Profile at 60 °C at 70 °C at 80 °C at 90 °C (min) VFM P1 - 1 - 4.5 5.7 VFM P2 - 1 - 4 5.2 VFM P3 - 1 - 2.5 3.7 VFM P4 - 1 - 2 3.2 VFM P5 - 1 - 1 2.2 VFM P6 1 - 1 - 2.2 Figure 3. Illustrates the IR non-contact monitoring system. The Raytek pyrometer is mounted on a gimbal that facilitates the movement of the device and enables the operator to point it to the die of interest. Page 5
  6. 6. Sample Prep and Process Conditions The PFC smart card assemblies were printed onto 12” x 8.5” flexible polymeric sheets (ABS). These sheets were then cut into individual circuits for the underfill trials and were processed one at a time. For these trials, all of the die on the test vehicle were 8-bit, 13.56 MHz chips, underfilled using pre-mixed Epo-Tek U300. Figure 4 shows two of the test vehicles placed on the conveyor rails inside the VFM oven. A red laser spot illuminates the chip on the right sample and is used for accurate alignment of the Raytek infrared pyrometer. A total of 4 test vehicles were used as set-up parts to determine both dispense and VFM process profiles. Fourteen test vehicles were then processed using VFM profiles summarized in Table 1. These profiles are multi-stepped to obtain a gel state of the underfill before proceeding to fully cure the epoxy. This technique can be used with epoxy materials that tend to excessively out-gas when rapidly heated. VFM power levels ranged from just 60 watts up to a maximum of 80 watts for these profiles. The temperatures for the step profiles were based on the temperatures used conventionally. All the VFM processed samples were fully cured. Figure 4. Digital image of representative PFC test vehicle. The ABS sheets were cut to provide a large number of trial samples. The individual antennas were placed on a microwave transparent (FR-4) substrate. Post Cure Analysis Following cure of the test vehicles, visual inspection was performed and all die sites appeared cured. The clear underfill turns to an amber color upon curing in both VFM and convection heating. Page 6
  7. 7. Visual Inspection To evaluate the underfill cure with respect to voids generated during VFM-curing, a Lambda Technologies test vehicle, with 3 glass die per board, was dispensed and cured. The glass die were then inspected under a microscope to check for the presence of voids. Figure 5 displays an image, which shows the Lambda Technologies test vehicles adopted for visual inspection after cure. No voids are exhibited after the accelerated cure of the VFM process. Epo-Tek U300 underfill dispensed and VFM-cured under glass die for visual Figure 5. Digital image of a Lambda Technologies test vehicle. No voids were visible under the glass die after the rapid VFM cure. DSC, Extent of Cure Analysis To confirm cure extent with the profiles used in the PFC process trials, the Epo- Tek U300 epoxy was dispensed on a test die and cured with the same process cycles described in Table 1. A similar test die was cured in a convection oven using the conventional 60-minute cure cycle. Figure 6 shows the wet adhesive DSC curve. As indicated in the figure, the reaction temperature range is between 70 and 160C. The measured heat flow (J/g) was found to be (437.77). The VFM processed samples were also run through DSC. No peak exotherm was collected after cure. Cure extent was calculated to be in excess of 96%. Heat flow release (J/g) after VFM cure was in the range of (0.7J/g). In summary, the rapid VFM process cycle leads to full, complete curing. Page 7
  8. 8. Heat Flow (Arbitrary) 40 60 80 100 120 140 160 180 200 220 240 Temperature (C) Figure 6. Overlay of Differential Scanning Calorimeter (DSC) scansFor fresh adhesive and VFM-cured adhesive. Temperature Monitoring During the VFM process cycle, temperature is monitored by a non-contact IR system, focused on one die in the test vehicle. The VFM process inherently ensures a uniform distribution of the microwave energy over the various die. MicroCure systems are also provided with a surface contact, fiber-optic temperature measurement system for profiling and set-up. An ABS PFC test vehicle was subjected to VFM profile # 5 (refer to Table 1). The temperature of the substrate and that of the die during cure was measured to evaluate the temperature gradient between the die and the ABS substrate; hence, the selectivity of VFM heating. Figure 7 illustrates the locations where temperature was monitored. The temperature of the die was monitored by the Raytek system with an emissivity value pre-set at 0.77, the value recommended when referencing from silicon. The emissivity was kept at this value for subsequent processing. The die temperature followed the thermal profile described in VFM profile #5, while the temperature of the substrate directly beneath the die lagged behind by 25°C. This gradient in temperature is beneficial in this process because it leads to less risk of substrate deformation (beyond its viscoelastic threshold) during process. Furthermore, the substrate temperature away from the die, as illustrated in Figure 7, remained at room temperature during the entire process. Again, the selectivity of VFM heating is beneficial for this application. Page 8
  9. 9. Raytek non-contact temperature probe ABS substrate Nortech contact Nortech contact temperature temperature Figure 7. Schematic representation of temperature measurements made on the die and substrate. The thermal gradient was measured to be 25C. Figure 8. Illustrates an IR image of a VFM heated ABS test vehicle. As illustrated in this image, the thermal energy distribution over the test vehicle is selective and focused by the underfill/die area. This same phenomenon is to be expected for curing liquid encapsulant (glob-top) curing for chip on board applications used in smart card assemblies. Figure 8. IR-image illustrating selective heating of the smart card components. Page 9
  10. 10. Line Balance and Equipment Considerations The target RDIF-tag throughput defined by PFC is 2000 units per hour. Based on the results of these preliminary trials, the standard MicroCure 5100 should be capable of achieving this goal. The MicroCure 5100 can host up to five (5) pallets at a time, or as many as 100 RFID devices per cycle, depending on the width of the pallets and size and spacing between individual antennas. Based on the 2 minute total process cycle and assuming a one-minute combined load and unload time, or a total cycle time of three (3) minutes, this would provide for 20 cycles per hour, or a throughput of 2000 UPH. In the case of the underfill process, the antennas can potentially be stacked in a specially designed cassette (microwave transparent - made of either FR4, G10, or G11). Up to 4 sheets could potentially be stacked at a time. With the assumption that 5 cassettes are loaded with 4 sheets each, the UPH in this case can be quadrupled. Stacking would permit a throughput up to 8000 UPH from the single Microcure 5100 VFM system. The MicroCure 5100 is SMEMA compatible and operates in-line with dispense and buffer modules, as desired. Figure 9 shows an example of pallets carrying 48 X 78mm ISO antennas that have been screen printed on ABS plastic film. Figure 9. Photograph of screen printed antenna sheets on the edge-rail conveyor as viewed from service access door. Conclusions The feasibility for rapidly curing flip chip underfill epoxies for smart card applications has been demonstrated. Several VFM profiles were determined to be satisfactory for the product and pallet configuration and the process is directly Page 10
  11. 11. compatible with the standard MicroCure 5100 VFM system. The major benefits of VFM processing for this application that directly respond to PFC objectives include the following: • Rapid cure permits in-line and just-in-time line balance versus the existing batch process. • Work in process is reduced by 95%. • Liability for defects occurring between QC stations is therefore dramatically reduced. In addition to the above direct benefits, the following factors are also significant in comparing VFM to existing batch process methods: • Reduced floor space for curing process (50 inches and in-line). • Reduced energy and heat discharge into manufacturing environment. • The MicroCure system only heats the epoxy so the oven exhaust is minimized and the prime power is only 3-4 kilowatts. 2. CURING OF CONDUCTIVE INK PRINTED ANTENNAS (5 TURN COIL) ON ABS SUBSTRATES: The major objective for this particular application is curing the thick films (inks) without substrate deformation. There are two routes that can be taken to achieve this objective: 1- the substrate material needs to be made of a material that can withstand the cure temperature without deformation and 2- use a cure process that is selective in nature. The substrate material choices are dictated by the economics involved. The VFM was investigated as an enabling technology for the curing method compatible with the economically viable materials. The following paragraphs describe different configurations and the resistance measurements that were performed to confirm the degree of curing. In general, the lower the antenna resistance, when measured end-to-end, the better the curing process. The curing of conductive inks (antenna printed with E162-5 epoxy) was performed using special microwave fixtures. These fixtures placed silicon wafers or metal plates above and/or beneath the sheets of ABS and aided in providing radiant heat to the antenna ink. In some instances the bottom fixtures contacted the sheets, in others the sheets were suspended above the surface of the bottom fixtures using a sheet of Kapton (polyimide) film. The curing of antenna was carried out using different configurations as illustrated in Figure 10. Full curing of the 5 turn coils was accomplished in 15 minutes as opposed to 120 minutes at 90-95°C. Recent testing of conductive inks specially formulated for VFM curing has shown promise that this process can be further optimized to yield a total process cycle in the range of 7 minutes or less. Page 11
  12. 12. Top fixture ABS Substrate material Bottom Fixture Figure 10. The various configurations used for curing antenna on ABS substrates. Configuration 1 The top fixture was silicon, the bottom fixture was metal. A first test vehicle was heated to 95°C and held at temperature for 15 minutes. The resistance was found to be 6.5 Ω. A second test vehicle was similarly heated to 95C and held for 20 minutes and yielded 3.6Ω. Configuration 2 Both top and bottom fixtures were made of silicon. A first test vehicle was heated to 95°C and held at temperature for 15 minutes. The resistance was found to be 2.0 Ω, and seemed fully cured. A second test vehicle was similarly heated to 95°C and held at temperature for 14 minutes and yielded a resistance of 2.3 Ω. A third test vehicle was held at 90°C for 12 minutes and yielded a resistance of 2.4 Ω. Conclusion Several other configurations were tried and found to have similar results. Curing within 15 minutes, using standard off-the-shelf ink and without substrate warpage, is quite possible. Better fixtures that answer the production concerns and can further reduce the cure times need to be explored. Further study is also needed to gain a better understanding of which curing profiles work best for this type of VFM application. Page 12
  13. 13. 3. CURING OF FLIP CHIP BUMPS ON ETCHED-COPPER PET SUBSTRATES: PET sheets, containing etched-copper antenna, are primarily used for RFID inlet assembly. A typical antenna, designed for a 4-bit, 13.56 MHz chip, is illustrated in Figure 11. The inside and outside loops of the antenna terminate at two small pads which are connected to the two active pads of the RFID-chip, located in opposite diagonal corners of the chip. Before the chips are diced on the wafer, conductive epoxy bumps are stencil printed and cured in the corner of each chip of the wafer. An example of conductive bumps on a chip, are shown in Figure 12. In addition to the two active pads on the antenna, two non-functional pads are situated between the antenna loops, and are used to physically balance the chip during flip chip placement. Figure 11. Example of a copper-etched RFID antenna showing the chip placement pads. Figure 12. Philips I-Code RFID chip showing the polymer conductive bumps in each corner. Only the upper-left and lower right bumps are connected to “active” pads of the chip. These bumps are stencil printed and cured before the wafers are diced. Page 13
  14. 14. The first process step in assembling these tags is to stencil print two small conductive bumps on the active pads for each antenna etched on the PET sheet. Figure 13 shows the appearance of the wet epoxy bumps following this first step. Then, using automated pick and place equipment, the active bumps of the RFID chips are aligned and pressed into these wet epoxy bumps. When assembled, the RFID chip bridges across the antenna traces, as shown in Figure 14. Because the resulting “bump-to-bump” connections are electrically conductive at this point, the antennas can be functionally tested both before curing and again after curing, allowing the VFM-curing process yields to be easily identified and tracked. Figure 13. During assembly, conductive bumps are stencil printed onto the antenna pads. The chips are then aligned and placed to establish “bump-to-bump” interconnections. Figure 14. Appearance of the RFID-inlet after flip chip placement. Hoping to eliminate the long, 8.5-hour bump cure-cycle mentioned earlier, VFM- curing was viewed as being very strategic to lowering WIP, handling, and ultimately unit cost. The through-put goal for this process was also 2000 UPH. A variety of cure profiles were tested in combination with differing pallet materials. To maintain flatness of the PET sheets during chip placement, the Page 14
  15. 15. processing pallets were designed as separate top and bottom panels that held the sheets in the middle. Cut-outs machined in the top panels provide access to the antenna pads where the chips are placed. Consistent curing results were obtained by automatically ramping to110 deg-C, at a rate of 1.0-2.0 degrees per second, and holding at this temperature for approximately 1 minute. Since the temperature is referenced off the top of one of the silicon chips on the antenna sheet, there is a thermal lag of approximately 25- 50 degrees between the PET sheet and silicon chips. The total process time varies, dependent upon the total number of pallets with antenna sheets being simultaneously cured, but typically ranges from 3-4 minutes. This through-put falls short at 1500 UPH, assuming four panels with 25 antennas per sheet and a 4 minute cycle time, but recent process enhancements have shown that the cure cycle can be reduced to approximately 2 minutes. After substituting the VFM-curing process in place of the conventional oven- curing process, PFCC saw an immediate improvement in assembly yields. Production yields now average a solid 99% compared to fluctuating yields between 85-95% using the older process. These results are surprising in light of the fact that this production line has only been running about 12 weeks and is not under statistical process control yet. Some of the yield increase may be attributed to reduced handling, but some previous types of failures have now disappeared altogether. As a result of the successful qualification of VFM curing for in-line production of smart card and RFID inlet products, Polymer Flip Chip Corp. purchased a LAMBDA Model #5100 VFM oven and intends to add a second unit to their RFID assembly line shortly. Figure 15 shows the VFM unit installed at PFCC’s manufacturing facility in Billerica, Massachusetts. Figure 15. LAMBDA Microcure Model 5100 as installed on the RFID-inlet manufacturing line at Polymer Flip Chip Corporation. Page 15

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