One of the difficulties I have with baking – unlike with cooking – is how it’s impossible to see/sample within the dish and determine its doneness. Sure, there are other ways to check without peering inside, but the dish preparation and the oven settings (both requiring significant advance time) prevent on-the-fly-tinkering. One of the important things I try to remember during this process is how temperature flow depends on the cross-sectional area. It’s not just cooking where this point comes in handy – PCB design processes also rely on cross-sectional areas; fortunately, microsectioning affords analysis of the board’s internal structure. Inspection can then use the inspection results as feedback to improve quality control and, ultimately, the performance and reliability of the board.
The Different Levels of PCB DFM
| Type | Definition | Examples |
|---|---|---|
| Destructive | Uses destructive techniques to analyze where nondestructive methods are unable to operate. The destructive analysis does not necessarily have to affect yield: intentional coupons on otherwise unused panel sections and some testing that degrade the service life are considered essential despite the damage. | Microsectioning, burn-in, insulation resistance, general stress testing |
| Nondestructive | This method has no lingering effects on the device under test. | Microscopy, x-ray inspection, contaminant testing, automated optical inspection |
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Microsectioning: An Investigative Procedure
Microsectioning is an invaluable evaluation technique for PCBs whose importance continues growing as components and board features shrink due to HDI pressures. Inspection can diagnose production defects or abnormalities but is limited to the surface (i.e., outer layers); microsectioning allows manufacturers to peer inside the board to confirm that assembly process outcomes meet standard requirements. Microsectioning is a destructive analysis, and unlike the more specific couponing that only accounts for panel fabrication, its location is not a fixed position. In this sense, microsectioning is more disruptive since its coupon does not come from an off-board panel area reserved for destructive analysis. However, this does allow technicians the flexibility of a more targeted assessment.
Soldering defects can be numerous due to various quality control factors, and visual inspection can only ascertain the most immediately apparent issues. Because soldering contributes the most to a board’s long-term reliability, manufacturers must fully characterize solder joint exteriors and interiors. Inspection will often use microsectioning to check post-assembly for
- Solder porosity – Trapped moisture in the solder flux and residue from improperly dried boards between manufacturing steps may result in solder outgassing events. These pores can prove incredibly detrimental to a joint’s mechanical reliability in aggregate.
- Solder meniscus – While melting, deposited solder takes on a rounded shape due to surface tension, total volume, and the area of the land pattern. This shape is necessary to form an ideal solder joint. Microsectioning doesn’t check for the appearance of the meniscus per se, but rather the shape of the joint to intuit backward.
The Stages of Sample Preparation
Removal
Significant consideration for the sample is necessary to ensure sample integrity throughout the viewing process. As such, removal and preparation are of the utmost importance. Technicians will want to use equipment that removes the sample with minimal damage to the PCB structures undergoing observation – sawing tools can be too large or imprecise and undermine the removal process. A precise table-cutting tool offers optimal precision and cost-convenience (sharper cutting instruments will likely have worse longevity before sharpening or replacement is necessary).
Mounting
An often overlooked mounting step between cutting and inspection can protect the sample before polishing and grinding the cut plane. Mounting helps correct the general trend of miniaturization in electronics – as the spacing between components and board features tends to shrink, it becomes a greater challenge for a prepared sample to exhibit the spacing and gaps between these PCBA elements. Further compounding the issue, modern component package styles may obfuscate viewing angles; technicians must make cross-sectional cuts that capture the solder bond between lead and pad.
Cleaning
Just like with the manufacturing itself, any residual contaminants from processing or handling require removal before inspection. Gentle cleaning and agitation can help remove trapped contaminants from the gaps or recesses in the sample before resin casting to prevent the concealment of board features or degradation due to abrasion, reactivity, etc. Vacuum impregnation of the resin will fully penetrate the cavities of the sample; the encapsulation creates a long-lasting barrier that permits handling while offering rigid support to the sample.
Polishing
Polishing and grinding is the final stage before a sample is ready for viewing. Selection of the abrasive is key – depending on the distance of the cut to the desired cross-sectional plane, the grit of the abrasive may increase or decrease (the lowest possible grit is preferable to avoid damaging the sample). The first step of grinding will remove any material up to this desired cross-section, with subsequent phases of polishing to correct for any abrasive damage present at this location. Polishing will also establish the planarity of the sample at the cross-section, which is vital for microscopic examination. The final polish will not abrade any material but will correct surface scratches, gouges, and other mechanical deformations; the final polish must remain a quick touch-up of the surface as extended contact can warp and round the viewing surface.
Finally, the microsection is ready for viewing. Careful adherence to best practices is vital to ensure the preparation is not the cause of any visible defects. Documentation of the changes in the sample through microsectioning will also allow technicians to discern any superficial imperfections arising from production versus sample preparation. More often, the initial removal of the sample from the board is likely to result in appreciable damage; take caution during this stage to minimize the amount of later corrective work.
Your Contract Manufacturer Commits to Quality, Big And Small
Microsectioning gives inspection a direct view of sub-surface PCB structures, typically related to the solder joint formation during assembly. While nondestructive techniques like ultrasound or x-ray inspection are valuable for preserving boards, there are limits to the resolution of those methods that microsectioning can solve. However, proper quality control is never this-or-that: the best manufacturing outcomes arrive from a dedication to PCB analysis that leverages all available tools. At VSE, we’re a team of engineers committed to building electronics for our customers, no matter how challenging or demanding the design. Along with our manufacturing partners, we’ve been producing life-changing and life-saving devices for over 40 years.
