Failures on PCBA
The inspection by optical microscopy is used, at first, for sample documentation and failure localization. Typical failures includes: poor solder joints, cracks in components or in solder joints, solder balls, contaminations or mistakes in the marking. These failures are then investigated more closely using other microscopic techniques (cross-section analysis) to determine the exact failure and their root causes (Balogh et al., 2008a).
Examples of nonconformities
Uncovered area on solder mask
The solder mask is a heat-resisting coating material applied to selected areas to prevent the deposition of solder upon those areas during subsequent soldering. The cured solder mask shall be uniform in appearance and free of any surface anomaly that would interfere with the assembly or operation of the printed board. In the Fig. 1a, the solder mask is missing from the copper conductor. The lack of metallic luster during the evaluation of the specimen indicates a too small thickness of the solder mask. The evaluation of the cross-section (Fig. 1b) shows a solder mask thickness of 15 μm in the position with acceptable solder mask thickness (position 2 in Fig. 1a) and a thickness of 4 μm in the position where the solder mask is missing (position 1 in Fig. 1a).
White residue on PCBA surface
In the most cases, the appearance of the white residues indicates the presence of flux traces. Figure 2a shows a contaminated area direct on the PCB surface. In order to confirm that the observed contamination correspond to flux residues, a FTIR-microscopy analysis using the ATR-technique (attenuated total reflection) was performed directly at the contaminated area on the PCBA surface. The founded spectral data (Fig. 2b) clearly show the presence of adipic acid, the active part of the flux system. The recorded spectra (red line) fully corresponds to the library spectrum (blue line, adipic acid, Zollner own library) seen by the measured absorption bands: ca. 3000 cm− 1 (−O-H stretch and –C-H stretch), 1700 cm− 1 (−C=O stretch), 1430 cm− 1 (−O-H bend), 1280 cm− 1 (−C-O-stretch), 930 cm− 1 (−O-H bend). The peak near 1100 cm− 1 in the spectra marked with red line is attributed to solvent residues of the flux remained in the white residue. Any flux residues on electrical contact surfaces is not accepted by the standard (Huang et al. 2009; IPC-A-610 Revision G 2017).
Component damage
The used components should be in a perfect condition and fulfill the technical specifications. Minor deviations on surface damages can be accepted if the functionality of the component is given. Major component damages such as crack in the component body (Fig. 3a) or metallization loss on termination side (Fig. 3b) are not accepted (IPC-A-610 Revision G,2017).
Failures in the solder connections
Examples of nonconformities
Voids and cavities in through hole technology (THT) solder joint
In this example (Fig. 4a and b) voids and cavities are visible in the solder joints for the through holes components. They are due to outgassing from the organic base material contained in flux. Possible causes for the outgassing occurrence are the humidity in the base material, organic contamination on the leads, pins and holes. The large cavities increase the susceptibility for voids of solder joints and therefore, impair their mechanical robustness. These voids and cavities are not visible from outside; an inspection is not possible excepting an X-Ray analysis. Only the cross-cut analysis can detect the size of the cavities and pores. The vertical fill with solder paste is < 75% (measured value: 32,1%) as specified in the standard, so that this solder joints are not acceptable (IPC-A-610 Revision G 2017). These solder joints should undergo rework (possible reflow).
Copper leaching
In Fig. 5 the phenomena of copper dissolution can be observed. The copper wall of the plated through hole is disappeared because the copper is solved in the solder paste (Fig. 5 phenomena was marked with white arrows). Not only disappearance but also thinning of copper layers may induce to disconnection problems, which are not desirable from the viewpoint of the joint reliability. Possible cause can be a too high soldering temperature or a too long soldering time. The Sn-Ag-Cu alloy is reactive to metals (for example copper or iron), the dissolution or/and disappearance of copper layers is a problem on wave soldering where PCBs are dipped in molten solder paste (Izuta et al. 2007; Snugovsky et al. 2009).
Crack in the solder joint and black pad phenomena
At the presented component soldered on a FBA an electric malfunction was found. To determine the root-cause of the defect, a microscopic verification of the prepared cross-section was done. The microscopic examination approve the presence of a crack in the solder joint (Fig. 6a). The crack is formed between the intermetallic layer and the Nickel-layer of the pad on a PCB with ENIG surface (electroless nickel/immersion gold). Figure 6b shows a corroded Black Pad structure: corrosion spikes are visible in the nickel layer. Black Pad failures are a nickel corrosion process. The main features of the phenomenon are partial wetting by solder of certain ENIG areas, the presence of Ni surface morphology defects (mud cracks, spikes) and black spots. The Black Pad phenomena in the corroded nickel surface results in the loss of solderability and that has caused poorly formed solder joints at the interface between the solder and the nickel interface. When such weakened solder joints are subjected to mechanical stress, they can be easily fractured (Hui Lee 2011; Ramanauskas et al. 2013; Snugovsky et al. 2001; Snugovsky, 2002; Susan et al. 2004).
Damaged solder joint and fallen component
In the present case a missing component from the assembled printed circuit board was observed (Fig. 7a). The component was separated from the solder joint where the original form of them can be observed. The microscopic verification of the components pin at the maximum magnification factor of the metallographic microscope (1000x) confirms the presence of solder paste (Fig. 7b). The use of a scanning electron microscope which allows a higher magnification (picture was taken at a magnification factor of 4200x) indicates the presence of solder paste on the pin (Fig. 7d). The EDS-analysis performed at this point shows the presence of tin and copper, which are the mainly elements in the composition of the solder paste and of the intermetallic layer (Fig. 7c). The evidence of tin on the components pin shows that the pin was soldered to the PCB. Probably due to a mechanical stress the strength of the solder joint became weaker and the component was separated from the PCB.
BGA (ball grid array) defect
A ball grid array (BGA) is a type of surface-mounted devices used for integrated circuits. BGAs use solder bump interconnections instead of pins. Once the package is soldered into place, it is difficult to find soldering faults. X-ray analysis, industrial computed tomography scanning and endoscopy are non-destructive analysis methods which can be used to identify the soldering faults. To find the real root cause of the soldering fault the cross-section analysis offers good possibilities to reach this goal. Figure 8 shows possible soldering failures on BGA components:
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➡ BGA ball with macrovoid (Fig. 8c). Macrovoids are caused by volatile compounds that evolve during the soldering processes. These macrovoids generally do not affect the solder joint reliability unless they are present at interfacial regions in the solder joints where cracks typically propagate.
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➡ Head-in-pillow (Fig. 8b) is a solder joint defect where the solder paste wets the pad, but does not fully wet the ball. For example, in the case of a ball grid array, the predeposited solder balls on the component and the solder paste applied to the circuit board may both melt, but the melted solder does not join. The cross-section through the failed joint shows a distinct boundary between the solder ball on the component and the solder paste on the circuit board. This solder joint fulfills the conditions for a connection to have electrical integrity, but is lacking sufficient mechanical strength. Due to the lack of solder joint strength, this component may fail with very little mechanical or thermal stress. The defect was probably caused by surface oxidation or poor wetting of the solder.
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➡ Deformed solder ball (Fig. 8a, d). The dynamic warpage of the PCB and/or BGA can lead to varying shapes of solder joints. The left solder joint in the upper row (Fig. 8a) shows a convex solder joint. The warpage can lead also to solder joint being stretched into a columnar shape (figure left in the lower row Fig. 8d). Both solder joints are acceptable from a quality standpoint (IPC-7095D 2018).
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➡ Crack in solder joint (Fig. 8e, f). A cracked or fractured solder ball is not accepted. This failure can be caused by mechanical stress, for example the BGA was not aligned parallel to the PCB surface (Champaign and Wiggins 2007; Hofmeister et al. 2008).