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1. IPC-B-52 Coupon Preparation
Ten IPC-B-52 coupons were processed for each of the two selected solder pastes. These coupons were used as delivered from the board fabricator; no further cleaning was done. The pastes were printed using a 0.125 mm (5 mil) stencil. Positions 2, 3, 5, 7, 8 and 10, as shown in Figure 1, were populated with dummy components. The coupons were then reflowed using an OmniFlo 7 oven with a 1.1 °C/s straight ramp to a peak temperature of 243°C with a time above liquidus of 53 s using a nitrogen atmosphere (600–800ppm O2), as shown in Figure 3.
Figure 3: Applied reflow profile comprising a straight ramp to 243°C under nitrogen (1.1 °C/s, 53 s TAL).
2. SIR Measurements
The SIR portion of twenty coupons and two unprocessed control coupons were mounted in a temperature-humidity chamber. Teflon-insulated leads were hand soldered to the coupons. The chamber was programmed to run at 40°C +-1°C and 90–93% RH, and a GEN3 AutoSIR was programmed to apply 12V bias and to measure SIR at bias voltage every 20 minutes for seven days.
SIR data were recorded for pattern No. 5, 6, 7 and 8 (Figure 1). The boards were mounted in the temperature/humidity chamber and connected to a “Gen3 AutoSIR” instrument for measuring the SIR. The chamber was programmed to record SIR readings at 12 V every 20 minutes. No edge card connectors were used. Teflon-insulated wires, were soldered with ROL0 solder cored wire.
3. Ionic Contamination Measurements of IPC-B52-Coupons
Ten IPC B 52 ionics break away with three populated patterns were measured for each of the two pastes. An Ionograph 500M SMD II was used to measure the ionic contamination of each sub coupon. A 75% isopropanol 25% water extraction solution heated to 45°C was used. A dwell time of 15 minutes in the ionograph was used. This time was a balance between complete removal of ionic contamination, versus CO2 absorption increasing the apparent conductivity of the solution from a source other than the test vehicle. A PCB surface area of 65 cm2 was used in the calculations.
4. IPC-B-24 Coupon Preparation
A. Pre-cleaning
Modified IPC-B-24 test coupons, with bare copper FR4 were immersed in a 75% isopropanol/25% water solution in an Ionograph 500M SMD II. The solution was heated and circulated. The boards remained in the solution until a solution resistivity of >300 ohm-cm was achieved. The boards were then baked at 50°C for one hour.
B. Coupon Assembly
The solder paste was printed on three test coupons per paste using a 150 µm (6 mil) stencil. The coupons were then exposed the temperature profile similar to that shown in Figure 3 below using a OmniFlo 7 oven in N2 (600-800ppm O2).
Results and Discussion
Log SIR results for solder paste A and solder paste B measured on the leads around the small QFP on the IPC-B-52 coupons are depicted in Figures 4 and 5 respectively. Figure 6 shows the mean Log SIR values for all six patterns measured for both solder pastes. As can be seen from the 7-day test, paste A has a consistent SIR reading of three orders of magnitude greater than paste B. Paste B is a marginal fail if 100MΩ is used as the pass/fail standard.
Figure 4: IPC-B-52 SIR test results of paste A at small QFP leads.
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