Latent Assembly Defect Screening Standards for High Density Interconnects
Screening latent microvia and fine pitch HDI assembly defects combines thermal shock cycling with continuous surface insulation resistance measurement.
Knee Defect
Plating Irregularity
A localized structural weakness in plated through-holes occurs when the copper deposited at the shoulder of the drilled hole is thinner than the specified thickness. This condition, referred to as a knee defect, reduces the current-carrying capacity and mechanical strength of the via. High current densities running through the restricted area can cause localized heating and eventual open circuits.
Inspection Method
Cross-sectional analysis of sample circuit boards remains the primary technique for detecting this plating deficiency. Microscopic evaluation of polished board cross-sections reveals the thickness profile of the copper layer along the entire hole wall. Automated optical inspection tools struggle to identify these issues because the defects are hidden within the board layers.
Destructive thermal stress testing forces board samples to undergo multiple solder float cycles to see if the plated knees break under stress. The resulting microsection images allow quality control technicians to measure the exact remaining copper barrier.
Prevention Strategy
Adjusting the chemistry of the electroplating bath ensures uniform copper distribution across both the board surface and the hole walls. Organic additives such as brighteners and levelers control the local plating rate to prevent excess copper build-up on the surface while ensuring sufficient shoulder coverage. Proper mechanical agitation and fluid flow through the holes also improve the plating uniformity.
These processing controls ensure high reliability in multilayer board fabrication.