Mike Bixenman & Mark McMeen
A significant contributor to electrochemical related “no-fault found” customer returns are leakage current failures. Leakage current failures depend on the humidity levels, presence of ionic contaminants, and potential bias between metal interconnects on an electronic circuit. This type of failure is difficult to isolate as the fault may occur due to an initial interruption to functionality without further recurrence. Isolating the root cause of leakage current failures is gained by understanding proper design rules for low clearance components.
One of the significant factors for controlling the water film formation and subsequent corrosion failure is the process-related contamination resulting from the reflow soldering process. The effect of flux residue on humidity related failures depends on the amount and chemistry of the residue, especially the ionic activator component in the flux component. The standoff height, from the assembly surface to the bottom of the component, factors into the level and activity of the flux trapped under the components termination.
The purpose of this paper is to research the activity of flux residues as a function of the standoff height using insulation resistance. A second factor that will be part of this research is design options for outgassing flux residues.
The ability to quantify and qualify the multiple variables that impact cleanliness and reliability with a SIR tool allows both designers and process engineers to quantify any harmful effects that might arise from solder flux, component designs, or other process residues left on external surfaces after soldering.
Different industries and products require different levels of reliability and warranty expectations. The ability to determine the level of insulation resistance using high-resolution testing allows one to determine which material sets, processes and process parameters are needed to meet the system design engineers' reliability and warranty objectives.
Cleanliness is becoming a significant concern for reliability due to the miniaturization and reduction in form factors of today’s electronics. The ability to measure and understand surface contamination and corrosivity underneath our electronic components are essential to our future. SIR testing is the tool that paves the way for future miniaturization by allowing engineers to know and measure surface insulation resistance underneath high I/O devices, bottom terminated components and leadless devices.
The above three studies showed us the importance of finding all the variables that influence SIR values underneath components and that the flux cubic volume of residue plays just as big a role as standoff height, flux type and they're ionic makeup. Our challenge of tomorrow is to continue to design test tools and test vehicles that continue to allow us to gather real actionable data for these small and miniature locations where ionic contamination can exist and activate into leakage currents and electrochemical migration paths.
Initially Published in the SMTA Proceedings