The inspection of fully assembled printed curcuit boards with X-ray photoelectron spectroscopy for quality control of automated assembly lines is not an easy task. First of all the sample needs to fit the sample plate, second the sample needs to be vacuum compatible, when using conventional XPS systems, third the sample needs to be properly grounded to prevent charging of the components or the coating.
In the following raw measurement data is presented that was taken on different position on a SO-DIMM PCB board. The board was taken with bare hands from its sales packaging and was placed on the EnviroESCA sample plate. The data has been taken on non sputtered surfaces the same way as it would be the case in a quality control application on a production line.
|1. Inspection of the Gold Contacts|
|The contacts of a printed circuit board modules are the connectors to the sourrounding electrical system and therefore play a major role in the functionality of electronic devices. The quality and purity of these gold coatings ensure the operability and durability of the module.|
The spectroscopic performance of EnviroESCA in XPS allows for a non-destructive quality control of these platings as the detection limit for surface impurities is extremely low. Even the lowest amount of metalic impurities can be detected.
||The picture on the left shows the view through the main sample positioning camera of the EnviroESCA. This camera is being used to navigate on the sample surface to select regions of interest where the XPS measurements will be performed. A pilot laser passing through the lens system of the electron energy analyzer indicates the measurement position and helps the user to address the desired region on the sample.|
The figure shows a detail spectrum of the gold 4f region. The FWHM of the Au 4f7/2 peak of only 0.55eV is not only a performance value for the energy resolution of EnviroESCA but also an indicator for the build quality of the SO-DIMM PCB itself.
With the high precision sample stage and the automated software user interface line scans across the PCB can easily be programmed. The following figure shows the result of a line scan over three gold contacts. Note that this is not a chemical contrast between two different metals on a surface but a real world sample with a pattern of metalic surfaces and insulators. In EnviroESCA the spot size of the X-ray source is matched to the acceptance area of the analyzer which is given by the diameter of the nozzle used for differential pumping. In the standard configuration this results in a spot size of below 200 micron. From the manufacturers specification sheet one knows that the width of the gold plated areas are 450+-0.03 micron and the spacing is 150 micron which can clearly be observed as shown in the line scan in the figure below.
|2. Inspection of a solidering joint on a SMD component on the PCB|
Another crucial part onthe PCB are the soldering joints between the PCB and the electrical components. XPS can be used to investigate the quality of soldering joints for process optimization.
|For the inspection of a soldering joint a capacitor on the PCB was chosen.|
The following figures show the XPS signals taken in the Tin and Oxygen region of the photoelectron spectrum.
|Two different components in the Tin peaks are easily visible in this high resolution scan. One belongs to metalic Tin and the other high energy peak belongs to a Tin oxide.|
|The O 1s region also show different components that can be related to the Tin oxide.|
|3. Inspection of the PCB coating|
|The solder resist covering the PCB ensures that solder is only taken by areas that needs to take solder and that no insulating areas are exposed to the sourrounding atmosphere to prevent corrosion of the contacts or vias and to prevent shortcircuits with other metalic parts, wires or cables.|
Because of the Environmental Charge Compensation of the EnviroESCA when working in higher pressures even large insulating surfaces can be analysed without any further masking, metallic coating or contacting.
|The following figure shows a XPS survey scan taken on a coated area of the PCB.|
|With EnviroESCA it is possible to investigate the manufactering quality even of fully assembled PCBs on an everyday basis. The resolution and sensitivity of the machine makes it a powerful tool in quality control processes. Additional components are availbale for EnviroESCA, as for example the AutoLoader that allows to take samples directly from production lines for automated sample analysis.|