Quality assurance – Microsection analysis
Microsectioning or cross-section analysis is a destructive sample testing procedure, used throughout the PCB fabrication industry. We make regular microsections each day, as they allow us to see inside the PCB and make precise measurements to check our production processes and to confirm the quality of the finished PCBs.
We use microsectioning to check the quality of:
- PCB base material
- inner structure of multilayer boards
- plating in plated-through holes
- thickness and registration of external and internal conductors
- connection between the layers
- soldermask cover
- surface finish thickness
- Select the appropriate PCB or quality-control test coupon
- Cut out a piece for sample
- Embed the sample in resin
- Grind down to a flat surface
- Polish and etch back if needed
Multilayer build checking
We check the build-up of the multilayer PCB, the thickness of the cores, copper foils and prepregs, and the effectiveness of the bonding process. We also look for any defects in the laminate after thermal stress (delamination, blistering, voids or cracks etc.)
We inspect the registration of inner copper lands to the holes. The next picture shows the same PCB as the last one, but when we measure the registration of the hole to the inner layer copper we see that there is some misalignment (in this case within tolerance). We also use a special test coupon on all multilayer production panels to confirm the position of the drilled holes relative to the (already printed) inner layers.
There should be a robust connection between the wall of the plated through hole and the inner layer copper as shown in the next picture. A poor or broken connection points to process issues in drilling or hole-wall cleaning. A broken internal connection on a finished board would be caught by electrical test.
Through hole plating
We make 5 non-destructive measurements of the thickness of the plated copper in the holes on every production panel. We back this up with regular microsectioning to get more information about process quality. For this we use the test coupon which we put on every production panel.
The plating thickness is the average of six measurements taken three on each side of the hole roughly a quarter, a half and three-quarters down.
Our standard tolerance for component holes is +/- 0,1 mm. We measure the finished hole diameters at final inspection using a tapered gauge. Microsectioning backs this up and provides more detailed information on the quality of our processes. The next picture shows the actual width of a plated hole with nominal diameter of 250 microns.