Electronic assembly warranties challenge the industry to improve risk mitigation test methods

Abstract

Circuit / System PCB designer's objective is to increase device functionality in a smaller form factor with a higher density electronic layout. Higher density and smaller form factor components are driving the industry toward a higher risk situation in intermittent electronic performance. Voltage leakage and even current leakage can lead to signal integrity loss and even flip bit issues or drops in processing commands. As the density increases and the miniaturization drops in size and spacing so does the increase in failure mode opportunities for signal integrity / signal loss. The spacing between conductors and pads in the past has been our friend and saving grace by giving us greater insulation between these conductors and pads. Even the standoff heights within the Z-Axis of miniaturized components are approaching one mil. This drop in z-axis means there is smaller cubic volume area to outgas volatiles or insure the volatization / boil off effect of flux additives such as inhibitors (against oxidation / corrosion), activators (promotes wetting) and thermal stabilizers (insures fluxing action at peak reflow temps) during reflow, flux residues can become entrapped under the component itself in a state that has not fully deactivated. Mobile ions within the flux residues form leakage currents and or voltages, especially when the device is operating within humid environments. Flux residue can contain halide / ionic materials which, when trapped under a part can lead to shorts across adjacent pads, or voltage/current leakage pathways. Companies who require devices to meet long term reliability / warranty expectations need an improved industry test specification that allows for an accurate risk assessment. The problem is that the risk assessment is a multi-variable issue influenced by flux type, flux make up (activators and inhibitors), activation temperature, component type and placement; the type and criticality of the circuit in which the component is operating in, wash characteristics if not a no-clean flux system, solder paste volume, PCB cleanliness and component contamination. Current measures of do not indicate if the product is clean enough in a critical area, which is what we hope to do. The goal is to design a test and test vehicle which encompasses the challenges of today so the System Design Engineer can define and characterize risk within his/her electronic assembly for long term reliability associated with cleanliness of the finished electronic assembly and final system warranty expectation. This is a tall order but there is a concern within critical industries such as Military, Medical, Automotive, Aerospace and safety critical industrial applications and industries to better define the risk and measure the risk associated with cleanliness. Cleanliness is a broad term for the mitigation and measurement of Electrochemical Migration (ECM) and pitting and creep corrosion, and general surface insulation breakdown due to free ionics. This paper will report ongoing research to study these multi-variant issues using a new test vehicle with sensors placed under and near bottom terminations such as QFN and LGA components. The test vehicles are designed to track impedance where it matters most. The goal of this research is to develop an improved method for studying multi-variables that may impact circuit / component reliability. This is not a single event / variable equation but a very complex multi — variable problem with varying degrees of interaction. Some of the main variables are processing equipment temperature controls / profiles; construction of the flux system and its chemical formulation; volume of flux residue remaining; and the type of component and its design as well as the plating finishes used on pads as well as the type of solder alloys used. This list was not all inclusive but gives an insight to the multi-variable problem statement. Data findings, inferences from the data findings, and recommendations will be documented and presented.