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

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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
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