In Circuit Fixture Spring Probe Force Balancing on Ultra Fine Pitch Assemblies
Balancing micro-probe forces with precision push-fingers prevents substrate strain over 500 microstrain, protecting fine-pitch BGAs and MLCCs from fixture damage.
Target Pad Diameter
Design Dimension
Layout dimensions for via-to-trace junctions specify the outer boundary of the copper region intended to receive a drilled hole for layer-to-layer connection. In printed circuit board design, the target pad diameter defines the size of the annular ring required to ensure electrical continuity after drilling and plating. This dimension must account for manufacturing tolerances and the positional accuracy of the drill.
If this dimension is too small, the drill may breakout of the pad area, resulting in an open circuit.
Registration Alignment
Drill registration tolerances combine the positional errors of the mechanical drill head with the dimensional stability of the laminate material during hot pressing. During lamination, substrates expand or contract, causing pad positions to shift slightly away from their theoretical coordinate positions. Using a larger diameter for the target pad provides a buffer that accommodates these shifts and prevents drill breakout.
This allowance ensures that the drill lands completely within the copper pad, maintaining a solid annular ring around the plated hole. Designers optimize this value based on the capability of the board fabricator to minimize the overall board size without compromising electrical yield. This design balance is particularly critical in dense multi-layer boards where spacing between traces and vias is extremely tight.
Manufacturing Defect
Inspecting trace-to-via connections involves optical testing to verify that the annular ring meets minimum width specifications. When the drilled hole falls partially outside the target copper region, it creates a breakout defect that can fail during thermal cycling. These failures are caused by stress concentration at the broken junction, which initiates cracks in the through-hole plating.
Ensuring the design meets standard specifications eliminates these micro-cracks.