Thermal Gradient Reduction in Multi Layer Circuit Board Assembly
Reducing thermal gradients across multi-layer assemblies requires balancing inner copper mass, profiling zone dwell times, and applying rigid carrier support.
Voiding Ratio
Porosity Measurement
Percentage of the total area of a solder joint that is occupied by empty spaces or gas bubbles. The voiding ratio acts as a primary metric for evaluating the quality of bottom-terminated components like QFNs or BGAs. High levels of internal gas entrapment can reduce the thermal and electrical conductivity of the connection.
Inspection Analysis
X-ray systems calculate this value by comparing the dark pixels of the solder alloy with the light pixels of the voids. Calculating the voiding ratio involves dividing the sum of all individual void areas by the total area of the solder pad. Outgassing of flux volatiles during the reflow stage is the most common cause of these defects.
Surface finishes like OSP or immersion silver can influence the frequency of bubbles depending on the storage conditions. Process engineers adjust the soak time in the reflow profile to allow these gases to escape before the solder solidifies.
Acceptance Boundary
Standards like IPC-A-610 define the maximum allowable percentage for different classes of electronics. A voiding ratio exceeding twenty-five percent typically triggers a failure for high-reliability applications. This limit ensures that the remaining metal is sufficient to withstand the mechanical stresses of thermal cycling.