X-ray inspection

X-ray inspection is a widely used method for assessing solder joint quality. However using it on Lead-Free alloys this requires greater control. It also offers a number of advantages over conventional optical inspection because it can identify problems such as voiding which would not normally be visible. A solder joint may contain a sufficient level of voids to make it likely to exhibit early failure and, although these voids would not be visible optically, they would show up using X-ray inspection as lighter spots in the joint's X-ray image.

If simple transmissive X-rays are used, the images obtained can be very cluttered because the image will show the internal structure of the PCB, as well as the components which may be on both sides of the board. Fortunately, some X-ray equipment now has the capability to display each board layer in turn whilst blurring the others.

Initial concerns that the absence of lead in Lead-Free solders would compromise equipment capabilities because of the lack of a high-density element in the solder have largely been dealt with. Bismuth has similar X-ray absorption properties to lead so X-ray inspection of solders containing this element should not be appreciably different to lead containing solders. And in the inspection of tin-silver-copper solder joints, the contrast achieved can be nearly the same as for tin-lead solder joints.

Modern X-ray systems use software to adjust the contrast of the captured X-ray images and can change the tube power and accelerating voltages to ensure an optimised image is obtained. Equipment manufacturers do not envisage any major problems and Lead-Free solders generally have sufficient density to allow cracks and voids to be detected if the equipment is set up for the specific type of solder being used.

X-ray inspection can be used for Lead-Free solders but as with optical techniques, inspection guidelines may need to be changed. With sufficient contrast and resolution, X-ray inspection allows clear fault identification to be performed with Lead-Free solder joints.