Optical Inspection

The inspection of Lead-Free solder joints is not fundamentally different from the inspection of joints with tin-lead solder. However, Lead-Free solder joints can have a different visual appearance to those formed with lead-based solders. This could lead them to being rejected if they are judged by the same inspection criteria used for lead-based solders.

Lead-Free solder joints are typically rougher and more granular in appearance than conventional leaded joints and may appear slightly duller and uneven. Also, the higher surface tensions than tin-lead based solders can cause a slightly different-shaped solder fillet to form and because Lead-Free alloys do not wet as well there may often be peripheral areas of exposed copper haloes around the edges of solder pads For optical inspection, it is important to educate operators about the different wetting characteristics of Lead-Free solders and the different nature of both the solid solders and the joints.

The visual differences between Lead-Free and tin-lead alloys may also require the recalibration of Automated Optical Inspection (AOI) equipment and software. It has been found that, most AOI systems could be used to inspect Lead-Free surface mount assemblies. AOI can also be used to evaluate solder pastes before soldering i.e. directly after printing. Solder paste inspection (SPI) can be very useful because as many as 80% of solder joint related problems can be attributed to problems with the solder paste printing process.

In its simplest form, SPI can be carried out two dimensionally giving indications of problems such as solder bridging. A more sophisticated approach uses three dimensional measurements to give information about paste height and volume. SPI could become increasingly important with the move to Lead-Free because of the different characteristics of some Lead-Free solder pastes and because of the increasing popularity of 0201 devices, chip-scale packages (CSPs), ball grid arrays (BGAs), and ceramic column grid arrays (CCGAs), all of which require specific quantities of solder paste if successful joints are to be produced.

The main advantage of solder paste inspection is that it can identify potential defects and process indicators which will lead to fewer problems and faults.

For some types of devices, such as Ball Grid Arrays (BGAs), where most of the solder joints are directly under the package optical inspection may be limited. Joints around the periphery may be inspected using an endoscopic device, although there are some limitations. In these cases X-ray inspection may be a useful alternative inspection method.