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
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.
Inner layers are not plated so the copper thickness is that of the copper foil used. However, some copper is lost during the cleaning processes. The IPC A 600 Class 2 standard gives the following values for the minimum acceptable copper foil thickness after processing:
Minimum thickness after processing
This image show the copper thickness after processing for an inner layer with 35 µm start copper:
Outer layers are plated up when we plate through the holes, so that the final copper thickness is the start copper foil less any copper lost during cleaning plus the plated copper. IPC A 600 Class 2 gives the minimum acceptable finished copper thickness after processing as:
Minimum thickness after processing
Microsectioning shows the thickness of an outer layer track with 18 µm start copper:
We can also measure separately the thickness of the base copper and of the plated copper. The base copper foil in this case was 12 microns.
The minimum solder mask thickness over the copper conductors should be 8 µm.
We can use microsectioning to measure the thickness of Leadfree Hot Air Levelling (HAL). For electroless gold over nickel (ENIG or Che Ni/Au) we can only use microsectioning to measure the nickel thickness (as shown in the picture) as the gold thickness is under 0.1 µm. For measuring the thickness of the gold and for Immersion silver we use non-destructive X-ray measurement.