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

As originally published in the SMTA International Conference Proceedings.EVALUATION OF STENCIL FOIL MATERIALS,SUPPLIERS AND COATINGSChrys SheaShea Engineering ServicesBurlington, NJ, USAchrys@sheaengineering.comRay WhittierVicor Corporation – VI Chip DivisionAndover, MA, USAABSTRACTThe past few years have brought PCB assemblers amultitude of choices for SMT stencil materials and coatings.In addition to the traditional laser-cut stainless steel (SS) orelectroformed nickel, choices now include SS that has beenoptimized for laser cutting, SS with smaller grain structures,and laser cut nickel. Available post-cutting processesinclude electrpolishing and nano-coating.Each option touts advantages over the others. To identifythe best options for the real-world application of a highlyminiaturized, very densely populated SMT product, anexperiment was devised. It included different materials,manufacturing methods and suppliers. Stencils were testedin pairs in order to capture the effects of a new hydrophobiccoating. The surface treatment was applied to one stencil ofeach pair, allowing for direct comparison of printperformance with and without the coating.Output variables included print yields, transfer efficiencieson 0.5mm BGAs and 0201s, volume repeatabilities onBGAs and 0201s, and dimensional accuracy of the stencils.INTRODUCTIONThe goal of stencil printing is to get the right amount ofpaste in the right location, every time. To support that goal,a number of analytical techniques are available tocharacterize, quantify, and monitor the inputs and outputs ofthe process. They are all based on the ability to accuratelymeasure the volumes of individual solder paste deposits.Paste deposit volumes can be measured by a variety ofmethods; the currently available best-in-class method usesstructured white light in a process known as Moire, phaseshift, or white light interferometry. Paste volume readingscan then be manipulated in a variety of ways to analyze theprocess from different perspectives.Basic statistics are calculated: Average (mean) volume Standard deviation of volumeVariability is examined: Coefficient of Variation (CV%), is the standarddeviation expressed as a percent of the meanvolume. Generally speaking, a CV of less than10% indicates a repeatable process. Cpk, the process capability index, compares theprocess output to its control limits. Typicalbenchmarks include 1.33, 1.67 and 2.0, indicating4, 5 and 6-sigma process quality, respectively.The paste-stencil relationship is characterized: Aperture Area Ratio (AR), is calculated as the areaof the aperture’s PCB-side opening divided by thearea of the aperture walls, and is an indicator of therelative adhesive forces on the solder paste depositduring separation from the stencil. As area ratiosdecrease, so does the amount of paste transferred.The minimum acceptable area ratio is oftenconsidered to be 0.66 for typical SMT purposes. Transfer Efficiency (TE), is the percentage of pastethat is actually transferred to the PCB, as opposedto that left inside the stencil aperture. 1 It iscalculated as the average paste deposit volumedivided by the aperture’s volume, and expressed asa percent. A common benchmark is 80% TE.ARs and TE’s may be either theoretical or actual.Theoretical ARs and TEs are calculated from the stencilspecification, whereas actual ARs and TEs are based onactual measurements.In addition to derived indices, production yields, whenavailable, are the ultimate indicator of process capabilityand fitness for use. Print test yields are measured at the PCB level, notthe per-deposit level. In the case of 10,000deposits per print, all 10,000 must fall within theircontrol limits. An output of 1 bad deposit and 9,999 good ones ona PCB would not be considered a 100 ppm process;it would be considered a zero yield process.

As originally published in the SMTA International Conference Proceedings.Each of these metrics can be applied to the stencil printingprocess to characterize the relationship between processinputs and outputs. In the following study, they are used toselect the best stencil options for a high volume, productionoperation.EXPERIMENTAL SETUPTest VehicleTest VehicleFigure 1. Test Vehicle (non-BGA circuitry on closeup isintentionally blurred)The PCB shown in fig 1 is a typical high-volume productionproduct. Each 32-up array measures approximately 3x7inches, and has nearly 15,000 SMT pads. Of the 14,468pads, roughly 8500 are mask-defined (SMD) BGA pads and1900 are metal-defined (NSMD) 0201 pads. The same setof 10 PCBs were used for all tests.Suppliers A & D applied the coating at their sites, prior toshipping the stencils. The same coating product was appliedto stencils provided by suppliers B & C after arriving at theVicor facility.Test MatrixFour suppliers, coded A-D, submitted stencils in a variety ofconfigurations. Materials, coded 1-5, included: Electroformed stencils (#1) Electroformed nickel foils that were laser cut (#2) Standard 301SS (#5) 304SS designed for laser cutting (#3) 301SS with modified grain size (#4)Thicknesses of the foils included 0.0045” and 0.004”. Thecurrent production standard is 0.0045” laser cut nickel foils.0.004” is under consideration because the preferred 0.0045”is not available in rolled steel.Electropolished stencils were not tested in this evaluation,because not all suppliers provide electropolishing capability,and while electropolised apertures have been reported torelease higher volumes of paste due to their roundedcorners, 2 they have also reported to produce higher rates ofvariation in volume consistency. 3For each stencil, 10 prints were taken, providing roughly85,000 BGA paste deposit measurements and 19,000 0201deposit measurements. The test prints were producedsequentially on a well maintained and calibrated 2009 DEKhorizon stencil printer using, both front-to-back and backto-frontsqueegee strokes, with an automatic dry wipe aftereach print. Print parameters were: Print speed: 15 mm/sec Print pressure: 5 kg (250mm blades) Separation speed: 20mm/secThe solder paste used in all tests was Indium 3.2 HF Type 3,water soluble, lead-free, halogen-free, lot # 37310. Freshpaste was used on each stencil. The paste was not kneaded;2 dummy prints were produced before measurements weretaken. The 27 stencils were print tested in a climatecontrolled NPI manufacturing area over 5 different runs.During the tests the climate ranged from 23.0 to 25.5 o C, andrelative humidity ranged from 32.9 to 46.9%.The PCB was supported with a flat, non-vacuum toolingplate and edge clamps. Deposit volume measurements weretaken with a Koh Young 3030VAL.StencilsEach supplier was invited to submit stencils in pairs. Onestencil was printed in the as-received condition; the otherhad a hydrophobic nanocoating applied before printing.

As originally published in the SMTA International Conference Proceedings.Table 1. Experimental MatrixNo. Supplier Material Nano Coat Thickness1 A 4 N 4.02 B 2 N 4.03 B 2 Y 4.04 C 1 Y 4.55 A 4 Y 4.06 A 3 Y 4.07 A 3 N 4.08 B 1 Y 4.59 B 1 N 4.510 B 1 Y 4.011 B 1 N 4.012 C 2 N 4.513 C 2 Y 4.514 C 1 N 4.515 B 2 Y 4.516 B 2 N 4.517 D 1 Y 4.518 D 2 N 4.519 D 2 Y 4.520 D 3 N 4.021 D 3 Y 4.022 D 4 N 4.023 D 4 Y 4.024 D 5 N 4.025 D 5 Y 4.026 D 1 N 4.527 D 1 N 4.5Not all suppliers provided all combinations of materials andthicknesses. The matrix of submitted and tested stencils isshown in table 1. The single unpaired stencil, labelednumber 26, was an experimental run by one of the suppliersto investigate the effects of a process change.RESULTSAperture MeasurementsTable 2. Average Aperture measurementMaterial No. Supplier BGA Dia0201 0201Width Length4 C 10.1 11.0 13.18 B 9.9 11.0 13.09 B 10.0 11.1 13.110 B 10.5 11.6 13.51 11 B 10.4 11.4 13.314 C 10.0 11.0 13.217 D 9.5 10.7 12.726 D 9.5 10.7 12.627 D 9.4 10.6 12.52 B 10.2 11.1 13.13 B 10.2 11.1 13.012 C 9.9 10.9 12.9213 C 9.9 10.9 12.815 B 10.1 11.0 13.016 B 10.1 11.0 12.918 D 10.4 11.3 13.219 D 10.4 11.3 13.36 A 10.5 11.4 13.437 A 10.5 11.4 13.320 D 10.5 11.5 13.421 D 10.5 11.5 13.41 A 10.5 11.5 13.545 A 10.5 11.6 13.522 D 10.5 11.5 13.423 D 10.5 11.5 13.4524 D 10.5 11.4 13.325 D 10.4 11.4 13.3SPEC 10.8 11.8 13.8average 10.2 11.2 13.1

As originally published in the SMTA International Conference Proceedings.Thickness MeasurementsTable 3. Foil Thickness MeasurementsMaterial No. SupplierThcknss ThcknssSpec Avg% Diff4 C 4.5 5.5 23%8 B 4.5 4.3 6%9 B 4.5 4.5 0%10 B 4.0 4.4 9%1 11 B 4.0 3.9 2%14 C 4.5 5.6 24%17 D 4.5 4.4 3%26 D 4.5 4.4 2%27 D 4.5 4.4 3%2 B 4.0 4.7 16%3 B 4.0 4.6 16%12 C 4.5 3.7 19%213 C 4.5 4.3 4%15 B 4.5 4.7 5%16 B 4.5 5.0 11%18 D 4.5 4.5 0%19 D 4.5 4.5 0%6 A 4.0 4.0 0%37 A 4.0 4.0 0%20 D 4.0 4.1 1%21 D 4.0 4.0 0%1 A 4.0 4.0 0%45 A 4.0 4.0 0%22 D 4.0 4.1 1%23 D 4.0 4.0 0%524 D 4.0 4.0 0%25 D 4.0 4.1 2%KEY: 0‐3% 4‐10% >10%To calculate actual transfer efficiencies and area ratios, thestencils’ apertures and thicknesses were measured. Theapertures were measured on the PCB side with a Microvueautomated vision system; 20 of each aperture size weremeasured per stencil and the average is reported in table 2.The foil thicknesses were measured at all four corners of theprint area with a Mitotoyo 12” throat micrometer; theiraverages are reported in table 3. The average figuresreported in the tables are used to calculate the apertures’actual volumes and area ratios.Paste VolumesTable 4. Paste volumes in cubic milsBGA PasteMaterial No. SupplierVolume123450201 PasteVolume4 C 281 6268 B 306 6679 B 255 57110 B 241 58811 B 267 59914 C 290 61917 D 308 66526 D 312 69127 D 315 6892 B 251 5763 B 267 60812 C 200 48713 C 185 45415 B 260 66516 B 293 64218 D 296 64719 D 263 6356 A 352 7417 A 320 66520 D 347 72421 D 293 6221 A 306 6705 A 282 59822 D 339 71123 D 337 71124 D 313 75025 D 321 635The measured solder paste volumes, shown in table 4, arethe averages of the individual measurements for eachfeature. Standard deviations and coefficients of variationwere also calculated but not shown. Most CVs for theBGAs were less than 10%; the highest CVs were 16%.

As originally published in the SMTA International Conference Proceedings.Transfer EfficienciesTable 5. Theoretical and actual transfer efficienciesBGA Transfer Efficiency 0201 Transfer EfficiencyMaterial No. Supplier um Theo Act Diff um Theo2 Act2 Diff24 C 68% 55% ‐13% 85% 91% 6%8 B 74% 96% 22% 91% 121% 30%9 B 62% 90% 28% 78% 113% 35%10 B 66% 67% 1% 90% 95% 5%1 11 B 73% 81% 8% 92% 109% 17%14 C 70% 59% ‐11% 84% 91% 6%17 D 75% 85% 11% 91% 125% 34%26 D 76% 101% 25% 94% 124% 30%27 D 77% 106% 29% 94% 127% 33%2 B 68% 81% 13% 88% 97% 8%3 B 73% 68% ‐5% 93% 98% 5%12 C 49% 72% 23% 66% 143% 76%213 C 45% 56% 11% 62% 122% 60%15 B 63% 77% 14% 91% 109% 18%16 B 71% 75% 4% 88% 104% 16%18 D 72% 93% 21% 88% 109% 21%19 D 64% 84% 20% 87% 108% 22%6 A 96% 83% ‐13% 114% 106% ‐7%37 A 87% 89% 2% 102% 107% 5%20 D 95% 98% 4% 111% 105% ‐6%21 D 80% 84% 4% 95% 106% 11%1 A 84% 81% ‐2% 103% 105% 3%45 A 77% 77% 0% 92% 104% 13%22 D 93% 87% ‐5% 109% 105% ‐4%23 D 92% 81% ‐11% 109% 106% ‐3%524 D 85% 98% 13% 115% 108% ‐7%25 D 88% 96% 8% 97% 104% 7%Actual TEs were calculated. The aperture volumes used inthe TE calculations are based on the averages of themeasured aperture sizes and foil thickness. The use of theactual sizes versus theoretical sizes was essential to thisanalysis, which compares different stencils. Print studiesthat use the same stencil throughout, i.e. those that examinepastes or print parameters, can usually use theoretical arearatios and transfer efficiencies, because the stencil remainsconstant and any deviation in the stencil will apply equallyto all measurements. When different stencils with varyingdimensions are used, however, measured values arenecessary to properly characterize their behavior. Table 5shows the differences between theoretical and actualtransfer efficiencies for the stencils used in this study, andillustrates the necessity of using measured values to getaccurate results.Transfer Efficiencies, Cpks and YieldsTable 6 shows the ARs, TEs and Cpks for the BGA and0201 components, and the overall print yields.The Cpks are based on the theoretical aperture volumes andthe following control limits: BGA: 20 to 139% of theoretical volume 0201: 50 – 200% of theoretical volumeTable 6. Transfer efficiencies, Cpks, and YieldsStencilNo.123456789101112131415161718192021222324252627StencilType3 ‐ Anot coated2 ‐ Bnot coated2 ‐ Bcoated1 ‐ Ccoated3 ‐ Acoated4 ‐ Acoated4 ‐ Anot coated1 ‐ Bcoated1 ‐ Bnot coated1 ‐ Bcoated1 ‐ Bnot coated2 ‐ Cnot coated2 ‐ Ccoated1 ‐ Cnot coated2 ‐ Bcoated2 ‐ Bnot coated1 ‐ Dcoated2 ‐ Dnot coated2 ‐ Dcoated4 ‐ Dnot coated4 ‐ Dcoated3 ‐ Dnot coated3 ‐ Dcoated5 ‐ Dnot coated5 ‐ Dcoated1 ‐ D*not coated1 ‐ Dnot coatedComponent AR TEBGA 0.66 81%0201 0.77 106%BGA 0.55 81%0201 0.64 97%BGA 0.55 68%0201 0.65 98%BGA 0.46 55%0201 0.54 91%BGA 0.66 77%0201 0.78 105%BGA 0.66 83%0201 0.77 106%BGA 0.65 89%0201 0.77 107%BGA 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 0.68 72%0201 0.81 143%BGA 0.58 56%0201 0.69 122%BGA 0.45 59%0201 0.54 91%BGA 0.54 77%0201 0.63 109%BGA 0.51 75%0201 0.59 104%BGA 0.55 85%0201 0.67 125%BGA 0.58 93%0201 0.68 109%BGA 0.58 84%0201 0.68 108%BGA 0.65 98%0201 0.76 105%BGA 0.66 84%0201 0.77 106%BGA 0.65 87%0201 0.76 105%BGA 0.66 81%0201 0.77 106%BGA 0.66 98%0201 0.77 107%BGA 0.64 96%0201 0.75 104%BGA 0.54 101%0201 0.66 124%BGA 0.54 106%0201 0.66 127%BGACpk0201CpkYIELD3.15 2.13 1003.34 2.18 802.94 1.7 801.94 1.71 03.01 2.03 1003.44 2.06 1003.7 2.3 803.85 2.55 1003.63 2.24 703.8 1.68 1002.75 1.85 302.26 0.97 602.04 0.79 1002.27 1.88 03.25 2.3 403.25 2.23 202.88 1.92 102.75 2.59 02.04 2.37 603.02 2.36 603.11 1.91 1003.21 2.04 302.97 1.76 1003.17 2.32 803.27 2.28 903.17 2.29 103.34 2.25 20Yields are based on the ten print tests used to gather thevolume data. Each print counts as 10% of the yield.

As originally published in the SMTA International Conference Proceedings.OBSERVATIONSDimensional AccuracyThe measurements shown in tables 2 and 3 are grouped bymaterial type. The electroformed stencils exhibited thegreatest amount of variation in aperture size, with a range ofapproximately 0.001”; the laser cut nickel foils showedabout half that at 0.0005”, and the laser cut SS foils showedabout one-tenth the size variation of the electroformedapertures, with a 0.0001” spread from smallest to largestmeasured sizes.Transfer Efficiency110%100%90%80%70%60%Effect of Coating on SS30.004" foil3 ‐ A uncoated3 ‐ A coated3 ‐ D uncoated3 ‐ D coatedThickness variation also trended with material type. Theelectroformed foils showed more thickness variation thanthe rolled foils. Of the electroformed stencils, supplier C’sfoils showed the greatest deviation from its specification,measuring almost 25% thicker than desired. Of theelectroformed foils that were laser cut, both supplier B’s andC’s submissions showed considerable deviation from thespecification (4–19%). Supplier D’s stencils did notdemonstrate as much thickness variation in theelectroformed materials as the other electroformed samples.Supplier A did not submit any electroformed samples. AllSS foils showed extremely low thickness variation.Positional accuracy was not measured on the stencils, butpaste print offsets were measured and recorded as part of thesolder paste inspection routine.Transfer Efficiencies and Area RatiosPlotting TE against Area Ratio (AR) is an industry-acceptedmethod of measuring the release characteristics of a stencil.For all stencils, the two data points generated by the BGAand 0201 measurements form the endpoints of the trend lineand the basis for the comparison. The BGA ARs aredesigned to be in the 0.60 to 0.66 range, depending on foilthickness; the 0201 ARs are designed to be in the 0.71 to0.80 range, again depending on foil thickness.All the data was plotted and reviewed. The more notablecomparisons include: Comparisons of release performance with andwithout surface coatings Comparisons of two specialized stainless steelalloys Comparison of electroformed and laser cut nickelstencils50%0.64 0.66 0.68 0.70 0.72 0.74 0.76 0.78Area RatioFigure 2. Comparison of print performance of SS #3stencils from two suppliers with and without coatingTransfer Efficiency110%100%90%80%70%60%50%Effect of Coating on SS40.004" foil0.64 0.66 0.68 0.70 0.72 0.74 0.76 0.78 0.80Area Ratio4 ‐ A uncoated4 ‐ A coated4 ‐ D uncoated4 ‐ D coatedFigure 3. Comparison of print performance of SS #4stencils from two suppliers with and without coatingWhen comparing the release characteristics of each stencil,performance differentiation is noted for the low area ratiosassociated with the BGA, but the release properties allappear to converge at the higher area ratios associated withthe 0201s. This trend was seen in all data sets.Also seen in all datasets were the slightly lower transferefficiencies of the coated stencils on the low AR (BGA)deposits, regardless of the material type, as seen in figures 2and 3. This trend appears to counter popular beliefs aboutthe coating’s ability to improve transfer efficiency, but isconsistent on all 13 pairs of stencil tests.

As originally published in the SMTA International Conference Proceedings.110%Supplier A SS3 and SS40.004" foil110%Laser Cut Nickel0.0045" foilTransfer Efficiency100%90%80%70%60%50%0.64 0.66 0.68 0.70 0.72 0.74 0.76 0.78 0.80Area Ratio4 ‐ A uncoated4 ‐ A coated3 ‐ A uncoated3 ‐ A coatedFigure 4. Comparison of print performance of SS #3 andSS #4 from same supplierTransfer Efficiency110%100%90%80%70%60%50%Supplier D SS3, SS4 and SS50.004" foil0.64 0.66 0.68 0.70 0.72 0.74 0.76 0.78 0.80Area Ratio4‐ D uncoated4 ‐ D coated3 ‐ D uncoated3 ‐ D (coated5 ‐ D uncoated5 ‐ D coatedFigure 5. Comparison of three types of SS from the samesupplierOf the two specialized stainless steels, the one with thesmaller grain size did not appear to release as much materialas the one with the coarser grain size. Replotting the databy supplier (figures 4 and 5) shows the trend more clearly.Regardless of the stencil provider, the foils with the largergrain size released approximately 10% more solder pastethan the stencil with the smaller grain size, and stencilswithout coatings released 8-10% more than stencils withcoatings. Supplier D also submitted a pair of stencilsproduced with non-specialized SS alloy. Its performance isplotted with the specialized foil alloys in figure X. Itappears to perform as well as one of the specialized alloys,regardless of coating.Due to their relatively larger AR differences, theelectroformed foils cannot be compared as directly as thesteel foils, but provide interesting observations whenplotted.Transfer Efficiency100%90%80%70%60%50%0.50 0.55 0.60 0.65 0.70 0.75 0.80Area RatioLaser Ni ‐ B uncoatedLaser Ni ‐ B coatedLaser Ni ‐ C uncoatedLaser Ni ‐ C coatedLaser Ni ‐ D uncoatedLaser Ni ‐ D coatedFigure 6. Comparison of print performance of laser-cutnickel foils from three different suppliersThickness variation in pairs of stencils is the primary driverfor differing ARs on submissions from suppliers B and C, asseen in figure 6. Supplier C’s 0.0045” foils measured0.0047” and 0.0050”; supplier B’s measured 0.0037” and0.0043”. Consistent thickness on supplier D’s stencilsmaintained very close AR’s between the two foils. Again,at similar area ratios, the uncoated stencil appears to stencilrelease more solder paste than the coated one.Transfer Efficiency130%120%110%100%90%80%70%60%50%Electroformed Stencils0.0045" foil0.40 0.45 0.50 0.55 0.60 0.65 0.70Area RatioEform ‐ B uncoatedEform ‐ B coatedEform ‐ C uncoatedEform ‐ C coatedEform ‐ D uncoatedEform ‐ D coatedFigure 7. Comparison of print performance ofelectroformed stencils from three different suppliers.As with the laser-cut nickel stencils, varying foil thicknessesdrove varying AR’s. Both of supplier C’s stencils measuredabout 0.001” too thick, and their apertures measured nearly0.001” too small, driving area ratios down to the 0.45 range,which is considered unacceptable. Supplier B’s stencilthicknesses also varied; one measured 0.0002” thicker thanthe other, creating the AR offset seen in figure 7. A similaroffset due to a 0.0005” thickness difference was alsoobserved on the same supplier’s 0.004” electroformedstencils. Again, supplier D’s foils showed very littlevariation, and followed trends similar to the SS foils withrespect to transfer efficiency differences between coated anduncoated foils on BGA ARs.The electroformed foils, despite having low area ratios,appeared to deposit more volume than expected, exhibiting100% or better transfer efficiency for a BGA with an AR of

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