A FEASIBILITY STUDY OF CHIP COMPONENTS IN A LEAD-FREE SYSTEM

Transcription

1 A FEASIBILITY STUDY OF CHIP COMPONENTS IN A LEAD-FREE SYSTEM Chrys Shea Dr. Leszek Hozer Cookson Electronics Assembly Materials Jersey City, New Jersey, USA Hitoshi Kida Mutsuharu Tsunoda Cookson Electronics Assembly Materials Hiratsuka, Japan Abstract A study was performed to understand the assembly constraints of the newest miniaturized package chip components, (or 0402 metric). The study included all facets of surface mount assembly: pad design and location, printability of the small deposits, placement tolerances on the components, and reflow capability of the small, lead-free deposits in an air environment. This study was a joint effort between two manufacturers: a solder paste developer and a pick and place machine supplier. The study tested multiple solder deposit sizes as part of an effort to characterize the repeatability of the paste prints and the paste s reflow fusion capability in air. It also tested several component placement orientations and offsets, and multiple padstacks. The result shows strong feasibility of devices in modern assembly systems, and discusses the parameters that produced the best results in the system under consideration. Keywords: Lead-Free, SMT Assembly, Miniaturization, Introduction Miniaturization in electronic products continually drives the need for smaller and smaller component packages. Smaller components bring solutions to designers seeking smaller form factors, but they also bring challenges to the assemblers. Several years ago, the package size known as 0201 (Imperial) or 0603 (metric) presented new challenges to assemblers. Many studies were performed to show the feasibility. Now, the package size known as (Imperial) or 0402 (metric) is now available. The 0402 is approximately one-fourth the size of an The relative sizes of chip components are shown in figure 1. Figure 1. Relative sizes of chip components Experimental Setup Four major tests were included in the experiment. Characteristics that were tested for included Printability Reflow Capability Placement Accuracy Self-Alignment A diagram of the test vehicle is shown in figure 2. Because the details of the design are difficult to see in the small graphic, a larger view is shown in Appendix A. The test vehicle combined varying rectangular pad dimensions, spacing between pads, and spacing between components.

2 C=0.22 C=0.18 C=0.14 P=0.05 P=0.07 P=0.07 A=0.22 P=0.05 P=0.07 G=0.12 Pad sizes: C: vertical A: horizontal G: space between pads P: space between components G=0.18 P=0.05 P=0.07 P=0.07 A=0.14 G=0.12 P=0.07 P=0.05 P=0.05 P=0.07 A=0.14 P=0.05 P=0.07 G=0.08 G=0.12 G=0.08 G=0.18 G=0.12 G=0.08 G=0.18 G=0.12 G= x 0402mm components 0402 in each yellow area No stencil apertures Figure Test Vehicle. Larger view can be seen in Appendix A. A laser cut stainless steel stencil was used for the print tests. It was 80um (3mil) thick. The rectangular apertures were reduced by 10% of pad area; the circular apertures were designed with the diameter equal to the shorter side of the rectangular pad. The printability test used visual inspection, as the deposit size was too small for accurate automatic inspection. The major criterion was deposit shape. A Yamaha YVP-Xg printer used the following parameter settings: Print Speed, 50 mm/sec; Squeegee type, both metal and poly; Print Pressure, 50 N; Separation Speed, 18 mm/sec; and Wipe Frequency, 1 (each print). Three different Type 5 pastes were used. Specifications of the pastes can be found in Appendix B. Two different aperture shapes and 3 sizes were used, for a total of six deposit types per paste. Reflow capability was determined by the smallest deposit to fully fuse in an air-environment reflow process. The reflow equipment was a Tamura TNR25-537PH 7-zone oven. Two profiles were used: one that soaked at 170C for 2 minutes and one that soaked at 180C for 2 minutes. Peak temperature for both was 230C. Profiles can be seen in figures 3 and 4. Figure C soak profile A chip placing test was performed to determine the necessary accuracy of the placer and explore the limits of the process window with respect to defects and selfcentering. A Yamaha YV88Xg was used to perform the tests. It has a mounting accuracy of 0.05mm (2mil). Placement pressure, location and rotation were varied. Placements were programmed with each combination of following offsets: 0 and 0.05mm in X and Y; -0.2mm, 0, and +0.2mm in Z, and 0 25 degrees theta in 5 degree increments. Tolerances of the test boards, stencil, and components that were used in this test are shown in figure 5. Figure 5. Tolerance stack up of test assembly components Figure C soak profile Results Three Type 5 Lead-Free pastes were tested. They were labeled A, B and C. Paste C provided the best printability. Its smallest acceptable print size was 0.16mm (6.3mil) square or 0.18mm (7.1mil) round with metal squeegee, and 0.13mm (5.1mil) square or 0.14mm (5.5 mil) round with rubber squeegee. Photographs of the smallest acceptable prints with metal squeegee with Paste C are shown in figures 6 and 7. Photographs of all test prints can be seen in Appendix C.

3 Figure mm (6.2mil) square features Figure mm (7.1mil) round deposits after reflow Paste C exhibited the best print and reflow characteristics, therefore it was used for the remainder of the tests. A nitrogen reflow environment was not included with this test. It is hypothesized that smaller features, on the order of 0.13mm (5mil) square and 0.14mm (5.5mil) round would reflow under nitrogen. Recall that the smallest acceptable print deposit size was 0.16 and Therefore there was no need to explore reflow properties of a deposit size that could not be repeatably printed. Figure mm (7.1mil) round features Of the three pastes tested for reflow characteristics, Paste C also performed the best, with the finest features to completely reflow in air sized 0.16mm (6.3mil) square or 0.18mm (7.1mil) round. This coincided with the finest feature this paste could repeatably print. Photos of reflowed deposits are shown in figures 8 and 9. Placement tests showed that the current capability of the equipment is sufficient for 0402/01005 placement. Even offsets of 0.05mm (2mil) in both X and Y resulted in centered components after reflow. Figure 10 shows the post-reflow state of the offset components. Theta offsets as much as 25 o also self-centered in the reflow process; however, offsets of 10 o or less are preferred. Figures 11 and 12 show 10 o rotations before and after reflow. Figure mm (6.3 mil) square deposits after Figure 10. Post-reflow alignment of 0402/01005 devices placed with 0.05mm offsets in X and Y.

4 Figure /01005 components placed with 10 o theta offset. Figure /01005 component floating on solder due to insufficient placement pressure. The solder reflowed, but did not wet to the component floating on top of it. The next larger force, 0mm Z, did squeeze some paste out from under the component, but eliminated floating issues. The largest placement force, +0.2mm Z, squeezed too much paste out from under the component. This effectively broke up the solder deposits, making them smaller than their reflow capability, and left some paste unfused. Figure /01005 components that were rotated 10 o theta self-centered during the reflow process Placement pressure of -0.2mm Z was the weakest force tested, and showed good placement characteristics with very little paste spread. Placement pressures this light require good tack force. The paste in this test had a tack force of 100fg, which appeared to be sufficient to maintain the components positions throughout the normal conveying process on the assembly equipment. Observations made during this test included mispicks, where the component was left in the tape pocket, but once a component was successfully picked, no issues were noted with vision alignment or dropping of the devices prior to placement. No tombstones or blown off components were observed. With respect to design guidelines for the PWB itself, this experiment showed a minimum end-to-end spacing of 0.07 mm (2.8mil). Figures 14 and 15 show the results of pad spacing at 0.07mm and 0.05mm. This force may be a little light, as several components floated on top the solder during the reflow process. An example is shown in figure 13. Figure /01005 components placed with 0.07mm gap between pads. No solder bridges observed.

5 Figure /01005 components placed with 0.05mm gaps between pads. Notice the solder bridges. Conclusion Technological feasibility of assembling component package sizes known as Imperial or 0402 metric has been proven in a lead-free alloy system. Type 5 solder pastes were used with demanding, extended-soak reflow profiles. Typical assembly equipment was used. With the exception of a special nozzle size for the component, no modifications were made to the printer, chip placer, or reflow oven. Typical print parameters were used with a standard laser cut stencil. Of three pastes, the best performer in both print and reflow characteristics was candidate C, which is now commercially known as ALPHA OM-325. The finest features to print repeatably and reflow completely in air were 0.16mm (6.3mil) square and 0.18mm (7.1mil) round. Initial definition of the process window shows that with the proper material set, the components may be placed off-center by as much as 0.05mm (2mil) in both X and Y axes. It may also be misplaced by up to 25 o in theta, but less than 10 o is preferred. Placement pressure is critical. Too little may result in component floating. Too much may cause solder deposits to break up and not fully fuse in reflow. Gross misplacement of the components may also have the same effect on reflow characteristics. Component locations on the circuit board should be spaced no closer than 0.07mm (2.8mil) to eliminate the defect mode of solder bridging. Acknowledgement The authors wish to express their thanks to the Yamaha Japan Technical Support Group for their participation in this study.

6 APPENDIX A Diagram of Test Vehicle C=0.22 C=0.18 C=0.14 P=0.05 P=0.07 P=0.07 A=0.22 P=0.05 P=0.07 G=0.12 Pad sizes: C: vertical A: horizontal G: space between pads P: space between components G=0.18 P=0.05 P=0.07 P=0.07 A=0.14 P=0.07 P=0.05 G=0.12 P=0.05 P=0.07 A=0.14 P=0.05 P=0.07 G=0.08 G=0.12 G=0.08 G=0.18 G=0.12 G=0.08 G=0.18 G=0.12 G= x 0402mm components in each yellow area 0402 No stencil apertures

7 APPENDIX B Specifications of Solder Paste Candidates PASTE A PASTE B PASTE C Solder Material 96.5Sn/3Ag/0.5Cu 96.5Sn/3Ag/0.5Cu 96.5Sn/3Ag/0.5Cu Metal content 88.3wt 88.2wt 88.5wt Solder Size m m m Viscocity 2000 P 2000 P 2000 P TI index Ionic Halide Present NO NO NO Hot Slump mm Solder Spread Ratio % (non JIS method - stencil print) Tackiness (gf) 1h 2h 1h 2h 1h 2h

8 APPENDIX C Photos of Print Tests PASTE A 0.20mm 0.16mm 0.13mm 0.22mm 0.18mm 0.14mm

9 PASTE B

10 PASTE C 0.20mm 0.16mm 0.13mm 0.22mm 0.18mm 0.14mm