evaluation of stencil foil materials, suppliers and ... - IPC Outlook

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As originally published in the SMTA International Conference Proceedings.0.55 and >120% for 0201s with ARs of 0.65. Those arerelatively high numbers that merited further investigation.A potential reason for the excess volumes could be poorgasketing between stencil and the PCB caused bymisalignment, so positional accuracy of the prints fromsuppliers C & D was queried in the SPI database.Table 7. Average print offsetsStencil StencilNo. TypeOffsetX (in)OffsetY (in)23 4‐D ‐0.0001 ‐0.001322 4‐D 0.0005 ‐0.000721 3‐D 0.0004 ‐0.000620 3‐D 0.0006 ‐0.000525 5‐D 0.0007 ‐0.000824 5‐D 0.0004 ‐0.000617 Eform ‐ D 0.0004 ‐0.001726 Eform ‐ D ‐0.0001 ‐0.001119 Laser Ni ‐ D 0.0004 ‐0.000618 Laser Ni ‐ D 0.0005 ‐0.000110 Eform ‐ B 0.0018 ‐0.001811 Eform ‐ B 0.0016 ‐0.00178 Eform ‐ B 0.0006 ‐0.00219 Eform ‐ B 0.0005 ‐0.002015 Laser Ni ‐ B ‐0.0001 ‐0.002016 Laser Ni ‐ B 0.0001 ‐0.00233 Laser Ni ‐ B 0.0004 0.00002 Laser Ni ‐ B 0.0003 ‐0.0007Table 7 shows the average print offset of stencils as reportedby the SPI machine. The majority of the prints from the SSstencils are displaced from the centers of their pads by lessthan 0.001”. The electroformed stencils’ prints are alldisplaced by more than 0.001”; half of them are displacedby 0.002” or more. While the measured positional offsetsare not conclusively the root cause of excessively highsolder volumes, it is probable that an average aperture-padmisalignment of 0.002” would cause excessive paste to bedeposited on the PCBs. Note that supplier C’s stencils arenot included in this portion of the analysis; the productswere eliminated from contention prior to the investigation ofpositional accuracy.Process CapabilitiesMost of the stencils tested produced acceptable Cpks basedon the control limits used in production. BGA Cpks wereall above 1.67. All 0201 Cpks, except those associated witha pair of laser-cut nickel stencils from supplier C, also metthe 5-sigma threshold.YieldsTable 8. Yield comparisonStencilNo.8910114141727321516131219185123226721202524StencilType1 ‐ Bcoated1 ‐ Bnot coated1 ‐ Bcoated1 ‐ Bnot coated1 ‐ Ccoated1 ‐ Cnot coated1 ‐ Dcoated1 ‐ Dnot coated2 ‐ Bcoated2 ‐ Bnot coated2 ‐ Bcoated2 ‐ Bnot coated2 ‐ Ccoated2 ‐ Cnot coated2 ‐ Dcoated2 ‐ Dnot coated3 ‐ Acoated3 ‐ Anot coated3 ‐ Dcoated3 ‐ Dnot coated4 ‐ Acoated4 ‐ Anot coated4 ‐ Dcoated4 ‐ Dnot coated5 ‐ Dcoated5 ‐ Dnot coatedComponent AR TEBGA 0.58 96%0201 0.70 121%BGA 0.55 90%0201 0.67 113%BGA 0.60 67%0201 0.71 95%BGA 0.66 81%0201 0.78 109%BGA 0.46 55%0201 0.54 91%BGA 0.45 59%0201 0.54 91%BGA 0.55 85%0201 0.67 125%BGA 0.54 106%0201 0.66 127%BGA 0.55 68%0201 0.65 98%BGA 0.55 81%0201 0.64 97%BGA 0.54 77%0201 0.63 109%BGA 0.51 75%0201 0.59 104%BGA 0.58 56%0201 0.69 122%BGA 0.68 72%0201 0.81 143%BGA 0.58 84%0201 0.68 108%BGA 0.58 93%0201 0.68 109%BGA 0.66 77%0201 0.78 105%BGA 0.66 81%0201 0.77 106%BGA 0.66 81%0201 0.77 106%BGA 0.65 87%0201 0.76 105%BGA 0.66 83%0201 0.77 106%BGA 0.65 89%0201 0.77 107%BGA 0.66 84%0201 0.77 106%BGA 0.65 98%0201 0.76 105%BGA 0.64 96%0201 0.75 104%BGA 0.66 98%0201 0.77 107%BGACpk0201CpkYIELD3.85 2.55 1003.63 2.24 703.8 1.68 1002.75 1.85 301.94 1.71 02.27 1.88 02.88 1.92 103.34 2.25 202.94 1.73.25 2.32.26 0.97 602.04 2.37803.34 2.18 803.25402.23 202.04 0.79 100602.75 2.59 03.01 2.03 1003.15 2.13 1002.97 1.76 1003.21 2.04 303.44 2.06 1003.7 2.3 803.11 1.91 1003.02 2.36 603.27 2.28 903.17 2.32 80Table 8 orders the stencils to allow for easy comparison oflike pairs. Of the 13 pairs of stencils that were compared, 7of the coated ones produced 100% yields, while only 1 ofthe uncoated ones produced the same.In 11 of 13 cases, the coated stencils produced higher yieldsthan uncoated stencils. The only situations where thecoating did not improve yields were on poorly formedstencils with ARs below 0.55 and yields at 20% or lower.
As originally published in the SMTA International Conference Proceedings.DISCUSSION AND CONCLUSIONSThe stencil technology selected for this production operationis stainless steel with two-part nanocoating applied. Onlysmall differences were noted between types of SS andsuppliers in terms of print volumes and transfer efficiencies,but substantial yield improvements were observed onstencils with the surface treatment.The SS foils offered the best dimensional accuracy.Electroformed nickel foils and stencils varied considerablymore than SS in both thickness and aperture size. Thepositional accuracy of the electroformed stencils alsoappears poorer than that of the SS stencils, introducing morealignment error into the printing process.REFERENCES[1] Shea, C., et al, “Characterizing Transfer Efficiencies andthe Fine Feature Stencil Printing Process,” Proceedings ofSMTA International, 2005[2] Mohanty, R., et al, “Effect of Nano Coated Stencil on01005 Printing,” Proceedings of IPC-APEX 2011[3] Shea, C. et al, “Quantitative Evaluation of New StencilMaterials,” Proceedings of IPC-APEX 2011The overall print performance of the SS foils were betterthan that of the electroformed ones. The actual differencesbetween the optimized SS with different grain sizes need tobe further quantified, as the experimental results from themare very close.Nanocoatings did not improve the transfer efficiency ofsmall apertures with area ratios in the 0.6 to 0.66 range. Infact, all the stencils with the coatings released less paste atthis AR than their uncoated counterparts. The paste releasefor ARs in the 0.70 - 0.80 range were similar with andwithout the coatings. Nanocoatings improved yieldsdramatically. The improvement in yields afforded by thecoated stencils equates to an undeniable boost inproductivity.The slightly lower transfer efficiencies of coated stencils,and of specialized stainless steel has not been investigated.It is speculated that crisper print definition may account forthe small differentials, but no formal analysis has beenperformed to date.Concerns of depositing adequate solder volume with athinner stencil were addressed. Laser-cut nickel stencilswith 0.0045” foil thicknesses deposited an average of 250cubic mils, whereas the SS stencils with 0.004” foilthicknesses deposited an average of 322 cubic mils.Furthermore, the 0.004” SS stencils showed less variation inthe volumes than the laser-cut nickel stencils. 0.004” SSfoils with modified grain size and surface coating are nowused in production for assembly of the test vehicle PCB andmany similar products.ACKNOWLEDGEMENTSThe authors would like to thank the four stencil providersfor their participation in this study. They would also like torecognize their colleagues who assisted in executing thetests: Karan Barabde John Zalaket
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