SMT Engineers are realizing the impact of lead-free alloys on the soldering process. How does the elimination of lead from solder impact X-ray inspection?
It is true that the characteristics of lead make for a great, high-contrast X-ray image. Lead has a very high density and, therefore, absorbs X-ray radiation very efficiently. That is why it is used to insulate X-ray cabinets. So it stands to reason that removal of lead from solder will make X-ray inspection more challenging.
But when one compares the contrast of a typical tin-lead alloy to lead-free alloys, the difference is not all that great. Tin-lead eutectic solder is 63% tin and 37% lead. Lead-free SAC alloy (396 for instance) is 95.5 % tin, 3.9% silver and 0.6% copper. Tin is the major constituent in both alloys.
The very dense lead component (atomic weight 207.2) is replaced by another relatively dense material, silver (atomic weight 107.9). Though significant, this reduction is small when compared to the other elements that make up a PCB assembly. For example, copper (atomic weight 63.5) used for pads and traces is significantly less dense than silver. Organic materials used in substrates, packages, etc. are far less dense than copper (i.e., carbon: 12.0, hydrogen: 1.0, oxygen: 16.0, nitrogen: 14.0).
The SMT industry already has many years of experience with X-ray inspection of lead-free assemblies. X-ray remains an excellent tool for analyzing mechanical integrity of lead-free solder joints, especially for BGAs.
What should a user look for in an X-ray system for the new lead-free solders?
The change to lead-free solders has not, as yet, resulted in any gross changes in solder volumes or component geometries. Therefore, system characteristics such as magnification, field of view, kV and power are not impacted.
There is, however, some concern over the potential for tiny voids. To resolve a small void the system must have adequate resolution. For instance, to identify a 50 µm void, the X-ray source must have a focal spot of 25 µm or less (almost any microfocus system will meet this criterion). However, the real challenge is not resolution, but contrast.
Assuming a uniform void area, the relative density difference between a 25 µm void and a 250 µm thick solder joint will be 10%. When you factor in the other layers of the circuit and non-uniformity of the void, the contrast may be reduced below 5%. In this case, the user will achieve the best results using low kV and high power to maximize the image contrast. Other system features such as contrast enhancement or automatic defect enhancement will greatly improve the ability to detect small defects.
Voiding is only one aspect of the solder process that can be observed through X-ray inspection. The greatest value will come out of the process control feedback X-ray inspection can provide. An X-ray system should be able to automatically collect data that will indicate process consistency. At VJE, we look for consistent solder joint size and shape, as well as perform accurate void measurements. Significant variations from board to board, or lot to lot may indicate solder paste printing problems, poor wetting, or even incorrect component placement. Variations across an individual component may indicate reflow profile problems.
The actual requirements will depend on the goals of the user. But there is no doubt that the benefits of X-ray inspection will continue to grow as the transition to lead-free continues.
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