Reflow Soldering

Nitrogen helps, but is best only in reflow areas.

Copper organic solderable protection surface finish provides a temporary layer of protection from oxidation of the copper surfaces of a PCB. During soldering, the organic coating is penetrated and dissolved by flux. Then the flux removes oxides from copper surfaces beneath, as well as from the alloy and components. Nevertheless, during multiple soldering processes, such as those that may be required to assemble double-sided boards (e.g., double reflow and wave processes), cross-linking of the coating can occur, making it more difficult for fluxes to penetrate.

Oxidation occurs in the presence of air, and any oxidized coating will grow thicker with reflow temperatures above 200°C. Pb-free systems – in which alloys are high in tin – and copper surfaces result in the formation of SnO (mainly), SnO₂ and CuO₂ oxides. The surface tension of these oxides is lower than that of the corresponding metals. This affects wetting behavior. When wetting is jeopardized, new flux chemistries may help. Nevertheless, if this is not an option, the use of nitrogen in the soldering process will help by reducing surface finish degradation and system oxidation.

To measure the effect of a reflow environment on the solderability of a copper OSP surface finish, a small experiment was carried out. BGA coupons of 0.030" in diameter (Figure 1) and non-solder-mask-defined pads were coated with a commercial high temperature OSP finish. The coating thickness was approximately 0.30 µm. The coupons were subjected to two reflows, one profile (peak temperature: 245°C and TAL: 50 sec.), two nitrogen supply methods: full tunnel (FT) and reflow only (RO) configurations, and two oxygen content levels: <100 ppm and air (210,000 ppm). For the FT configuration, nitrogen was applied throughout the entire tunnel, while for the RO configuration, nitrogen was applied only in the reflow areas of the oven.

Fifteen coupons with 256 pads each were tested for each reflow condition. The coupons were dipped in a mildly activated rosin-based flux (ROL1) for 1 min. This flux is designed for solderability testing of leads and PCBs. Excess flux was drained by holding the coupons over the flux container. The coupons were individually vertically depressed into molten Pb-free solder (SnAg3AgCu0.5) for 5 sec. After the elapsed time, coupons were placed in an alcohol bath and the number of unsoldered pads was counted.

Figure 2 shows a box plot with the results. The data show poor solderability when coupons were reflowed in air. The total number of opportunities per condition was 3,840. The air results showed 554 non-solderable pads versus 183 in nitrogen.

On average, 36 defects per coupon were observed in air versus six defects per board in nitrogen. At alpha level = 0.05, there was not a significant difference between FT and RO oven configurations.

Copper surface solderability is affected by reflow environment, and a copper OSP surface finish degrades more in the presence of air. When solderability is causing defects, the use of nitrogen in reflow may be beneficial. To make nitrogen use cost-effective, it should be applied only in the reflow areas.