Converting to lead free is not simply verification of long-term reliability, nor is it simply verification of critical temperatures. Most important, conversion to lead free requires verification of both manufacturability and testability. Understanding how lead-free conversion alters test performance and process control is key to successful implementation. One source of alteration is the differing mechanical characteristics of the alloys.

Mechanically, lead-free (PbF) materials are typically "harder" than lead alloy (PbR) materials. Solder physical properties quoted in the literature typically are based upon the bulk properties of a solid sample of some standard cubic area and mass of the solder before reflow. Unfortunately, the post reflow solder alloy is neither in a bulk state nor is the homogeneity the same as in the unreflowed alloy. For example, gold, copper, nickel, palladium, and other materials can all contaminate the alloy during reflow, changing its mechanical modulus. Only post reflow testing of the actual materials, after they are subjected to the actual reflow processes, should be used to determine lead free suitability.

We found that a careful examination of the solder bump contact area after probing reveals differences in hardness that will have a dramatic impact on socket design, electrical contact impedance and resistance, and overall yield. Not only are the PbF alloys typically harder than PbR alloys, but also the combination of surface oxide and residual flux coating after reflow can affect the electrical first contact and the contact resistance.

We tested two off-the-shelf BGA components having the same package, one PbR and the other PbF, to assess the impact of the solder conversion on bump probing. The 20 gram probe force chosen for this test is typical of electrical contact forces used on current SnPb alloys. The photographs of the bump contact areas show representative results.

Figures 1 and 2 show that an identical contact force results in a significantly different deformation of PbR and PbF bumps. As shown, the PbR device bump deforms by as much as 50% more than its PbF equivalent at this target load. The PbR bump also retains as much as 75% more deformation than the PbF bump after the load is removed. These differences result from the different mechanical properties of the solders.

Figure 1. SnPb bump after 20 grams of load.
Typical contact point (flat spot) for that load.

Figure 2. Pb-free bump after 20 grams of load.
Much smaller contact point (flat spot) on surface.

The degree of deformation directly correlates with the minimum force required on the bump to make an acceptable electrical contact. Our mechanical data suggest that the conversion from PbR to PbF is not a simple drop in replacement, electrically or mechanically, regardless of reflow temperature similarities. Material differences may be reflected at many points in the manufacturing process. Unless all effects are accounted for, the results may be unsatisfactory.

Contact CVInc for more information about probing lead-free bumps, including load vesus deflection curves. CVInc also provides single die and wafer bumping solutions and services.