Silicon Die Package Form Factors and Board Land Pattern Tolerances
Precision land pattern design balances component manufacturing tolerances against board assembly capability to eliminate thermomechanical stress on silicon die output registers.
Solder Voiding Limits
Area Percentage
Acceptance criteria for x-ray inspection define the maximum allowable volume of air pockets within a completed solder interconnect. Establishing solder voiding limits is necessary to ensure the electrical conductivity and mechanical strength of the joint. These thresholds are typically expressed as a percentage of the total joint area as seen in a two dimensional x-ray image.
Location Sensitivity
A pocket of air located at the interface between the solder and the pad is more critical than one in the center of the joint. Voids at the interface significantly increase the risk of brittle fracture and electrical failure. Inspection protocols often require higher scrutiny for these interfacial defects.
Process Influence
The chemistry of the solder paste and the reflow temperature profile are the primary factors that control gas formation. Outgassing of flux volatiles during the liquid phase can get trapped if the cooling rate is too fast. Adjusting the soak time allows these gases to escape before the solder solidifies.
Structural Integrity
IPC-A-610 specifies that a void should generally not exceed twenty five percent of the total joint area for ball grid arrays. Large voids can reduce the cross sectional area available for current flow and create points of stress concentration. Smaller voids are often considered acceptable and may even help to stop crack propagation.
Verification occurs during the final post reflow inspection stage.