Previous work has shown that radiographic NDT techniques can detect connection defects such as missing solder, mis-registered solder spheres, bridging and mis-soldering as well as misplacement of components. It is proposed to develop these techniques further to provide reliable and automated detection of these defects and to improve the speed of examination. This and other improvements would provide the features necessary for this type of system to be installed in-line.

Background
For this project, the X-ray source is positioned at the bottom of a lead-lined chamber and a digital detector is positioned directly above. A PCB enters the chamber by means of edge conveyer belts and is positioned on a manipulator in-between the source and detector. The system can then carry out a complete inspection of various components on the PCB by manipulating the PCB's position accordingly. Magnification of a specific area of interest can also be achieved by moving the board closer to the source.

Specification
The X-ray source is fully computer controlled. It is housed within a vacuum continuously pumped by a turbo-pump backed by a two-stage rotary-pump and monitored by a Penning gauge. It contains a micro-focus electron gun with a magnetically focussed electron beam intercepting a 15µm air-cooled tungsten foil target behind a 0.5mm thick aluminium window. The X-rays are collimated at the source to a beam with a fan-angle of 14°. The cathode voltage is variable from 40 to 160 kV, and the maximum current is 500 µA, giving 20 watts target power at 40 kV.

The detection media system consists of a single field 6-inch aluminium window with an intensifier coupled via a large aperture lens to a digital camera. The camera produces images of 758 x 580 pixels and 12-bit depth with an adjustable frame rate of 25 fps and less, and is computer controlled. The nominal source to focus distance is 600mm, which allows magnifications of up to 1200 times depending on the sample position.



Mechanical Manipulation of the PCB
A five axis manipulator is used for panning the PCB in X,Y and Z planes of motion and for tilting the PCB. The manipulator can be directly user controlled via a joy-stick for manual inspection. Additionally, the manipulator can move according to a pre-programmed script and throughput of the inspection system can be increased by only looking at components of interest.



X-ray Safety
The X-ray inspection system is to be incorporated into a production line that will only allow less than 30 seconds for the inspection of a BGA component on a PCB. However, the system is housed inside a lead-lined chamber and allowing automatic access for the boards without leaking any radiation into the surrounding areas is a major issue. The source is required to be on constantly to prevent issues with 'warming up', and open access is also required for PCBs. Lead lined motorised trap doors were originally considered. However, the fact that the X-ray source would have to be switched on and off according to when the trap doors were open or closed would reduce system throughput.

Lead shielded ply tunnels comprising 6 mm thick lead panels are to be constructed, that are up to 1m long, on either side of the X-ray chamber to house the internal conveyer belts. The PCBs will enter and exit the chamber through a small gap, approximately 30mm high and as wide as the PCBs to be inspected, at the end of each tunnel.

X-ray Simulation



Using a known X-ray beam angle of 140° (i.e. 70° from the normal), the dimensions of the X-ray chamber, and the absorption of lead, simulations of the scattered and secondary radiation within the chamber can be calculated (3) as shown in figure 6. With this information the safe optimum length of the tunnels and size of the entrance gap can be determined to ensure that X-ray safety regulations are met.

Results
The digital radiographic technique is ideal for determination of flaws and defects that are characterised by local density and/or structure variation. Trials and experimentation with the system have already shown that micro focus radiography can identify the following range of defects: BGA tilting, voiding, shorting, misplaced components, foreign body entrapment, chip capacitor cracking and flip chip under fill (see defect catalogue)

BGA Void Inspection
Figure 7 shows images for a 180 ball lead free soldered BGA imaged at decreasing distance between the PCB and the X-ray source. From the 100 mm height it is already evident that there is some solder bridging between the balls occurring. With height just 10 mm above the X-ray source, voiding in the BGA balls is visible. Increased voiding in solder is a typical finding when changing from a lead to a lead free process. It results from an oven profile problem during solder reflow.

Generally, it can be said that each ball should be of the same size and circular in shape.