Contamination Investigation on PCBs

Introduction

PCBs (Printed Circuit Boards) are critical components at the heart of all electronic devices. The quality and reliability of PCBs have a major controlling effect on the quality and reliability of the overall piece of equipment. However, PCBs must be manufactured at a competitive price point and with significant industrial focus on mass production and cost reductions, many failures are reported during the production/testing of PCBs due to contamination. This makes finding and identifying contaminants an essential process for the determination of failure root cause.

This application note demonstrates a scanning electron microscope (SEM) based solution for the detection and characterisation of contamination on two PCB samples.

SEM-based Failure Analysis

The use of Scanning Electron Microscopy combined with Energy Dispersive X-Ray Spectroscopy (SEM + EDS) in the analysis of the failures of PCBs, printed circuit assemblies (PCAs) and electronic components is a well-established and accepted process. SEM is a powerful tool for imaging as it allows a higher magnification and depth of field than an optical microscope. By combining SEM and EDS, chemical information is additionally obtained to enable determination of whether the contaminant is organic or not, what elements are present and its quantified composition. This information is then used to identify the source of the contamination from the workflow.

By utilizing different electron beam scanning approaches, EDS can perform point, line and area analysis, to obtain information on element distributions. Elemental maps can be obtained for each element of interest with the variation in concentration of a specific element across an area represented visually by the variation in colour intensity.

Here, we show data which was acquired using a large area Ultim® Max silicon drift detector (SDD), which is able to collect high spectral resolution EDS data even at the very high throughput rates. All of the data which was acquired was automatically processed with AZtec's Tru-Q™ algorithms which are designed and tested to ensure that a quantification of the highest quality is consistently achieved.

Analysis

Two PCB samples were analysed with AZtecLive using an Ultim Max 100 EDS detector at 10kV. EDS was used for elemental analysis and for the determination of the thickness of the contamination on the surface of the PCB.

Contamination on a PCB

Figure 1 shows a contaminated area on the surface of a PCB. EDS mapping was performed for 5 minutes over the full field of view.

EDS element maps in Figure 2a show the elements that are present in the field of view and their distribution. The EDS layered image clearly indicates that the contamination area consists of a mixture of organic carbon and a range of embedded oxide particles. These include a Ti+Na combined oxide particle, an Al oxide particle, a Si oxide particle and dispersed small particles of Ca oxide. The sum spectrum together with the quant results from the area analysed is shown in Figure 2b. This level of PCB contamination is not acceptable to manufacturers and would result in the PCB failing by cleanliness standards. By performing this analysis, the manufacturer of the PCB is able to investigate which processes involve materials of the observed compositions and therefore design appropriate corrective actions.

Fig 1. 1 Secondary Electron (SE) image of a contaminated area on a PCB surface

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Fig 2. (a) EDS element maps and layered image; (b) Sum spectrum and quant results from the contaminant.

EDS data can also be used to non-destructively determine layer thicknesses (as well as composition) for layered structures of up to 2 μm thick using AZtec LayerProbe. LayerProbe complements the elemental and phase information gained from conventional EDS analysis, by determining the contributions of the substrate and overlying layers to the measured signal. By comparing a user generated model of a structure with the experimental data acquired via EDS, it is able to estimate layer thicknesses down to a nm resolution.

In this case, the thickness of the contamination area varies; two locations were measured, as shown in Figure 3, and their thicknesses were 1.857 μm and 1.803 μm respectively.

Fig 3. Thickness measurement from collected EDS data.

A Defective Area Around Bond Wires

A defective area was observed around bond wires in one part of the PCB – see the SE image in Figure 4a. EDS point analyses were acquired at 7 locations, an example spectrum is shown in Figure 4b and quant results from all 7 locations are listed in Table 1.

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Fig 4. (a) SE image of a contaminated area around a wire bond – point/small area analyses were acquired at the marked locations; (b) an example of spectrum from location #1

Table 1 Quant results from all 7 marked locations in Fig. 4a

In this case, it was important to observe the distribution of contaminants across the surface of the sample, particularly S and Cl. In order to do this, EDS mapping was performed in the region shown in Figure 5. The acquisition took 3 minutes and resulted in comprehensive elemental information being collected which highlighted the distribution of contaminants. It was evident from the maps that a contaminant AlO layer which also included S and Cl had formed on the bond pad surface.

Fig 5. EDS layered image and element maps collected from the marked region.

Conclusions

The use of SEM with a large area Ultim Max EDS enables quick and accurate characterisation of contaminations on PCBs. The information which is gathered on both the composition and morphology of contaminants provides users with a valuable tool to identify the root cause analysis of contamination related failures and therefore to identify potential remedial actions.