Effect Of Silicone Contamination On Assembly Processes

sufficient to detect silicone well below the threshold of human visibility. Without quantification and correspondinginformation on the impact of silicone at varying concentration levels, this high degree of measurement sensitivity introducesa dilemma as to whether or not to clean hardware if the presence of silicone is detected.ATR is so effective at detecting silicone because the sample comes into intimate contact with an evanescent IR wave, thusallowing a stronger IR response for the same volume of analyte (Figure 4). In order to quantify the silicone IR response on asurface using the ATR sampling accessory, several conditions must be satisfied: (1) The layers of silicone stack together toform a film that does not exceed the penetration depth of the ATR crystal, (2) The measuring surface permits IR reflectanceback to the ATR crystal, (3) The silicone does not migrate sufficiently to spread the layer, (4) The pressure on the sample issufficient to make good contact with the ATR crystal, and (5) Silicone is not pushed away by the force necessary to keepgood contact with the ATR crystal.Sample in Contact with Evanescent WaveATR CrystalTo DetectorInfrared BeamFigure 4. Diagram showing the sample interface of the ATR sampling accessoryATR CrystalAs previously mentioned, the IR wave penetrates into the supporting substrate so that other factors affecting ATR analysisare refractive index and reflectivity of the measured surface. Aluminum and gold were chosen to represent typical metalsurfaces used in microelectronic manufacturing. Aluminum is widely used in structural applications because of its lightweight with respect to strength. Gold is widely employed as a plating to protect metallic surfaces from corrosion. Gold andaluminum oxide (native oxide coating exists on all aluminum surfaces) have differing refractive and reflective properties asshown in Figure 5[6] so that the FT-IR response to silicone concentration is expected to differ for each material.Al2O3 (Aluminum Oxide)Gold (Au)Figure 5. Refractive index and reflection coefficients of aluminum oxide and gold[6]For these experiments, DC-200 (polydimethylsiloxane oil) was selected as the source of non-curable silicone contamination.This oil has a somewhat low viscosity of approximately 20 Centistokes, can be used as a silicone rubber thinning agent, and

is fully terminated (non-polymerizing); thus it is a suitable representative of uncured or non-curable silicone. A series ofcontamination standards were prepared by first dissolving 1 gram of DC-200 into 100 mL of heptane and serially diluting thisstandard by 10 each time.For quantification of silicone on the aluminum surface, a non-corrugated aluminum weighing pan was filled with at leastenough silicone standard solution to completely cover the bottom of the pan, and the heptane was allowed to evaporate(Figure 6). The amount of standard to be added to the pan was calculated from the surface area of the bottom of the pan andwas weighed using an analytical balance.Figure 6. Diagram of method used to contaminate aluminum surface for silicone quantification by FT-IR/ATRanalysisThe quantification measurements on a gold surface were performed on microscope slides that were sputter deposited with1000 Å gold over a 50 Å sputter deposited chromium adhesion layer. Since these slides are without edges to contain thestandard silicone solution, a micropipette was used to administer the standard silicone infused heptane onto the surface,allowing the heptane to evaporate (Figure 7). The volume of standard to be administered was calculated for a 0.5” x 1”surface area scribed into the gold surface. For aluminum, the measured silicone concentration levels were 0.5, 1, 2, 4, 8, 16,32 and 64 µg/cm2 and for the gold sputtered microscope slide, 0.25 µg/cm2 was measured in addition to the aforementionedconcentrations.Figure 7. Diagram of method used to contaminate gold surface for silicone quantification by FT-IR/ATR analysisOnce prepared, samples were pressed onto a single reflectance diamond ATR crystal attached to a FT-IR spectrometer. TheFT-IR spectrometer uses mid-IR radiation with a range of 4000 cm-1 (2.5 µm) to 550 cm-1 (18 µm), and the single reflectiondiamond crystal plate ATR sampling accessory (45 degree nominal angle of incidence) has approximately a 2 µm penetrationdepth. A torque limited press attached to the ATR accessory applied a consistent pressure to the selected test point. Spectrawere collected for 60 seconds using a resolution of 4 cm -1. A transect of points (minimum of three) was measured acrossboth the aluminum pan and the gold sputtered microscope slide, and the average FT-IR/ATR absorbance response area(corrected) at 1259-1263 cm-1 (silicon-methyl vibration) was calculated. The average FT-IR/ATR response area was thenplotted as a function of silicone concentration to construct a calibration curve in accordance with the Beer-Lambert Law. TheBeer-Lambert Law, A λbc, where A is absorbance, λ is the wavelength dependent absorptivity coefficient, b isconcentration, and c is the path length states that IR absorbance is directly proportional to concentration.Determining Impact of Silicone Contamination on Adhesive BondingA widely used adhesive bonding performance test is ASTM D1002 “Standard Test Method for Apparent Shear Strength ofSingle-Lap-Joint Adhesively Bonded Metal Specimens by Tension Loading (metal-to-metal)”. The single-lap specimen testis practical for evaluating manufacturing control of bonded products, including adherend surface preparation. For this study,two types of single-lap specimens were tested: (1) 2024-T3 aluminum treated with FPL etch (chromic/sulfuric acid solution)and then spray coated with Cytec BR-127 corrosion inhibiting epoxy primer to a nominal 0.0002” thickness and (2) 2024-T3aluminum electroplated with gold over electroplated nickel. The primed aluminum surface was chosen because most

aluminum structures at Harris utilize this primer to improve adhesive bond strength and bonding directly to untreatedaluminum is atypical.Silicone contamination of the single-lap specimen surface was performed using a micropipette to apply a precise volume ofsilicone/heptane standard solution to the critical 0.5” x 1” (0.5 in2) overlap region on each single-lap specimen test panel.Figure 8 indicates the overlap region on each test panel that was contaminated and subsequently bonded. This contaminationmethod was very similar to the method described earlier (depicted in Figure 7) which was used to apply silicone to the goldsputtered microscope slides.Apply silicone and adhesive to this 0.5” x 1” overlap regionFigure 8. Single-lap specimen test panelThe opposing single-lap specimen panel was left uncontaminated to serve as a comparison for the silicone contaminatedpanel. In addition, the silicone contaminated electroplated gold panel was bonded to an uncontaminated BR-127 primedpanel to induce failure at the gold interface. Single-lap specimen test panels were bonded immediately with Epon 828 resinmixed 1:1 by weight with Versamid 140 hardener, and cured for 1 hour at 93 C. After cure, the single-lap specimen panelwas cut into five individual single-lap coupons for mechanical testing. Five test coupons were tested for each siliconecontamination level. The contamination levels on each single-lap specimen type are shown in Table 1. Note that prior tobonding, it was noted on the gold plated single-lap specimen panel that the silicone contamination is visible to the naked eyeeven at the lowest concentration of 0.5 µg/cm2 (Figure 9).Table 1. Silicone Contamination Levels for Single-Lap SpecimensSampleSilicone Contamination ( g/cm2 )BR-127 PrimedGold Plated1020.500.534441632532-----664-----

No silicone0.5 g/cm4 g/cm232 g/cm2Silicone oil somewhat visible at0.5 g/cm2 and becomes morepronounced as concentration isincreased to 32 g/cm2.2Figure 9: Silicone contaminated gold plated single-lap specimen panelsFT-IR/ATR ResultsThe FT-IR/ATR responses for silicone contamination on an aluminum surface are shown in Figure 10. The response islinear up to at least 64 µg/cm2 with absorbance increasing approximately 0.03 peak area units under the 1263 cm-1 peak forevery one µg/cm2 of silicone. Each calibration point represents an average value of at least three points along the pandiameter, with the coefficients of variation ranging from 9% to as high as 71%. The resulting calibration curve of theaverages is well correlated with silicone concentration (R2 0.997) so that on this surface, the FT-IR/ATR technique satisfiesthe necessary conditions for the Beer-Lambert Law. The variation in absorbance response along the transect was elevatedwhich suggests that the evaporation method tended to concentrate the silicone more on one side of the pan over the other andthat an improvement in the sample preparation method is desirable.FT-IR/ATR Response at 1262 1/cmATR Response to Silicone Contamination on Aluminum2.141.791.431.070.710.36Linear Fit: y a bxa 0.019700.0 0.1b 0.0306Correlation Coefficient: 0.997311.823.535.246.958.670.4Silicone µg/cm2Figure 10. FT-IR/ATR response to silicone contamination on aluminumThe gold sputtered microscope slide samples exhibited a stronger FT-IR/ATR response than aluminum for the same siliconecontamination. The absorbance increased approximately 0.1 peak area units under the 1263 cm-1 peak per one µg/cm2 ofsilicone, which is nearly 350% higher than for an aluminum surface. Unlike the aluminum surface, the linear response ofabsorbance area as a function of silicone concentration begins to falter after 32 µg/cm2 (Figure 11a). With the highestconcentration level included (64 µg/cm2), an exponential relationship of absorbance to silicone concentration was a muchbetter fit for the calibration (Figure 11b).The method detection limit for silicone on an aluminum surface is estimated to be 0.036 g/cm2 or 0.006 g/cm2 on a goldsputtered microscope slide surface. This limit was calculated by measuring the area of the baseline at 1780-1810 cm-1,multiplying the area by three[7

Effect of Silicone Contamination on Assembly Processes John Meyer and Carlyn A. Smith, Ph.D. Harris Corporation Palm Bay, FL Abstract Silicone contamination is known to have a negative impact on assembly processes such as soldering, adhesive bonding,