Solder copper dissolution

S. Chada, W. Laub, R. Foumelle, and D. Shangguan, Microstructural Evolution of Sn-Ag Solder Joints Resulting from Substrate Copper Dissolution, Proceedings of SMTA Conference, Sept 1999, p 412-418... 

Boards subjected to a Sn-Ag solder wave pass exhibited a significantly higher copper consumption rate than eutectic Sn-Pb solder. It was demonstrated that no copper remained after 6 passes of a PCB laminate coated with 1-ounce copper through a Sn-Ag wave. Figure 48 shows the consumption rate of copper by Sn-Ag-Cu was significantly lower compared to Sn-Ag. Sn-Cu solder yielded the lowest copper dissolution rate of the lead-free alloys tested. 

FIG. 48 Copper dissolution into two lead-free solders compared to eutectic Sn-Pb at 260°C. 

Thin ( 2 /mi) layers of nickel and tin are electroplated over the silver termination, the former to act as a barrier preventing dissolution of the silver during the subsequent wave-soldering operation when the chips are surface-mounted onto a substrate. The tin layer ensures good wetting of the termina tion by the solder. In the case of the BME capacitor (see below) the termination is copper fired on under reducing conditions and covered with an electroplated layer of nickel. 

Immersion silver dissolves into the tin phase of molten solder. The silver does not melt, but rather forms a solid solution. The dissolution rate of silver is in the range of 0.5 to 1.5 jum per sec. at standard soldering temperatures. Once the silver is dissolved, the underlying copper forms intermetallics with tin as discussed previously. Flux has little effect on silver, but can help clean trace contaminates and reduce surface tension. 

Sn drift Excessive dross on the surface of the solder can disrupt normal wave dynamics. In the case of Sn/Pb solder, Sn oxidizes more easily than Pb. The solder can become Sn depleted over the long term. This is known as Sn drift. The same is true of Sn-based Pb-free solder alloys, but since some alloys are nearly all Sn (SAC305 with 96.5 percent Sn by weight), this effect will be less dramatic. However, other Pb-free alloy perturbations such as dissolution of Cu become mnch more important. For example, in the case of the eutectic Sn-Cu alloy, the copper content is only 0.7 wt percent. Small changes in the copper content by means of copper dissolntion of copper pads on the board can have a dramatic effect on the melting temperatnre of the solder. 

Schaefer s method was improved, taking into consideration the dissolution of the q-layer into the molten solder until the copper concentration in the solder reaches the saturation limit, as well as the additional growth due to the reprecipitation of Cu as Tj after the Cu reaches it saturation limit (Ref 7). In consideration of Cu dissolution into the molten solder to reach the Cu solubility limit, they assumed that Cu dissolution into the molten solder is entirely from the dissolved q IMC. They also assumed that during cooling from the reflow temperature, Cu becomes incrementally supersaturated. The excess Cu atoms in the solder react with readily available Sn to form ri in the bulk solder, or migrate to the interface and precipitate on the interfacial rj layer. The portion that precipitated at the interface is calculated based on the growth controlled by diffusion, and the diffusion coefficient of Cu in liquid Sn was used (Ref 56). 

Minimize the problem of copper pad/trace dissolution caused by excessive solder-contact times... 

FIG. 7 Dissolution rate of copper for several lead-free solder alloys relative to 60Sn-40Pb solders. (From Ref. 20.)... 

Similar to traditional Sn-Pb wave soldering, metals dissolve relatively quickly during lead-free wave soldering operations. The rate of alloy or metal dissolution of exposed pads on PCBs or component leads depends upon their composition, solder composition, bath temperature, and the flow velocity of the wave solder. The rate of dissolution of a specific metal is lower if this metal is already present in the lead-free solder bath. Of particular concern is the dissolution of copper from circuit boards when exposed to lead-free solder waves. 

Dissolution rates for copper from PCB surfaces into the wave are influenced by several factors including wave temperature, the percentage of Sn in the solder alloy, and the amount of copper already in solution. The copper primarily bonds with Sn to form CusSn or CueSns intermetallic compounds. High-Sn alloys dissolve copper more rapidly than low-Sn alloys. 

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