Electromigration and Flux Residues

Electromigration and its subcategory electrochemical migration is a serious problem in electronic industry working with printed circuit boards (PCB). Smaller equipment with high density of interconnection (HDI) is assembled with surface mounted devices (SMD) and through hole components (THC) Assembly techniques are realised mainly by soldering process with no clean fluxes. Result is not only a reliable solder joint, but also flux residues. The first part of the article after short theory is focused on gatering basic knowledge about fluxes and surface finishes by using cyclic voltammetry (CV) and electrochemical impedance spectrometry (EIS). The second part of the experiments is oriented on practical test with different fluxes for wave and reflow soldering. These tests are associated with the reduction of surface insulation resistance, corrosion, dendrite/fiber growth and the formation of subsequent short circuits. The acceleration of these electrochemical reactions is helped by higher working temperatures, higher humidity, and magnitude \ and frequency of electrical voltage between the conductors.

Solderability and Reliability Evolution of No-Clean Solder Fluxes For Selective Soldering

Flux consumption for wave soldering tends to decrease, mainly due to its gradual replacement by reflow soldering methods (i.e. pin-in-paste) in many electronics applications. However, in several cases, wave soldering still remains a must, with an increasing share of “selective” soldering processes, either using wave frames with dedicated apertures or solder fountains. Such processes are more challenging for the fluxes in terms of reliability under operation, since some chemistries remaining on the printed circuit boards after soldering may promote corrosion. Thus, flux manufacturers had to adapt their formulations to minimize such issues while keeping an efficient activation level, with several types of alloys (tin-lead, tin-silver-copper and low/no-silver) and associated with the numerous types of finishes encountered. The paper will cover the types of flux used in the electronic industry according to their chemistry and activation level (rosin-based, halides, alcohol-based or water-based flux…), and their characteristics with reference to standards. The limits of current standards will be discussed in regards to the last generation solder fluxes. Then, the development of two low-residue new generation fluxes, an alcohol-based flux and a true VOC-free flux, will be described, according to requirements: the lab tests results (surface tension, spread tests, wettability tests…) will be presented and discussed. Reliability will be especially investigated through surface insulation resistance, electro-chemical migration test, ionic contamination as well as Bono tests to determine the candidates able to provide high processability combined with chemical inertness of residues. Finally, the performance of flux will be assessed through customer tests, involving several types of boards, finishes and different solder alloys and wave equipment.

Analysis of surface insulation resistance related failures in electronics by circuit simulation

Purpose The purpose of this study is to show that the humidity levels for surface insulation resistance (SIR)-related failures are dependent on the type of activators used in no-clean flux systems and to demonstrate the possibility of simulating the effects of humidity and contamination on printed circuit board components and sensitive parts if typical SIR data connected to a particular climatic condition are available. This is shown on representative components and typical circuits. Design/methodology/approach A range of SIR values obtained on SIR patterns with 1,476 squares was used as input data for the circuit analysis. The SIR data were compared to the surface resistance values observable on a real device printed circuit board assembly. SIR issues at the component and circuit levels were analysed on the basis of parasitic circuit effects owing to the formation of a water layer as an electrical conduction medium. Findings This paper provides a summary of the effects of contamination with various weak organic acids representing the active components in no-clean solder flux residue, and demonstrates the effect of humidity and contamination on the possible malfunctions and errors in electronic circuits. The effect of contamination and humidity is expressed as drift from the nominal resistance values of the resistors, self-discharge of the capacitors and the errors in the circuits due to parasitic leakage currents (reduction of SIR). Practical/implications The methodology of the analysis of the circuits using a range of empirical leakage resistance values combined with the knowledge of the humidity and contamination profile of the electronics can be used for the robust design of a device, which is also important for electronic products relying on low current consumption for long battery lifetime. Originality/value Examples provide a basic link between the combined effect of humidity and contamination and the performance of electronic circuits. The methodology shown provides the possibility of addressing the climatic reliability of an electronic device at the early stage of device design by using typical SIR data representing the possible climate exposure.

Solder Paste Corrosivity Assesment: Bono Test

Lead free soldering with no clean solder pastes represent nowadays the most common process in electronic assembly. A solder paste is usually considered as no-clean if it passes all IPC J-STD-004 corrosion tests: copper mirror, copper panel corrosion test, Surface Insulation Resistance (SIR) and Elecrochemical Migration (ECM). Other SIR and ECM tests are described in Bellcore GR-78-CORE and JIS Z3197 standards. Although SIR and ECM tests are recognized by all standards authorities to evaluate the solder paste residue corrosivity after reflow, a more selective method, the Bono test, has been developed and implemented in some French companies as a qualification criterion. It has been proven that compared to common corrosion tests, the Bono test better differentiates the nature of solder paste residues.

Reliability of High Temperature Laminates

Printed circuit boards have been reported to have limited lifetime at 200 to 250°C. Characterization of high temperature laminates for application at 200 to 250°C was conducted to better quantify their lifetime using accelerated testing of key functional parameters. Eight high temperature laminates consisting of 3 material types was evaluated. Life testing was applied for via cyclic life, weight loss, peel strength, and surface insulation resistance. Via lifetime was characterization using Interconnect Stress Test (IST) coupons. Weight loss was measured at intervals during the life of the tests. Peel strength was tested using IPC IPC-TM-650 method 2.4.8c. Weight loss was characterized using isothermal aging. Comparison of lifetime is made between the laminate samples. The non-polyimide laminates exhibited the longer life times than polyimide laminates in most tests except peel strength. Peel strength is the life limiting parameter for the laminates. Parylene HT was found to improve stability in peel strength and weight loss of one PTFE laminate tested.

High Temperature Laminate Characterization

Characterization and modeling of high-temperature laminates for application at 200–250°C is reported to compare and better quantify the mean lifetime based on key functional parameters of via cycling, weight loss, peel strength, and surface insulation resistance (SIR). Life testing and models are applied for via cyclic life, peel strength, and weight loss. Five high-temperature laminates, consisting of three polyimides and two nonpolyimides, were evaluated. The polyimide laminates behaved similarly in via, weight loss, peel strength, and SIR testing with small variances. The nonpolyimides performed longer in via and weight loss. They degraded more rapidly than the polyimides in peel strength and failed in SIR, due to its flame retardant. A comparison of lifetime among the five laminates is presented.

Design of Insulation Resistance Measurement System for Electrical Steel Sheet Surface Coating

The surface coating of insulation resistance is one of the key technical indexes of electrical steel sheet, and one of the important parameters of the products of electrical steel sheet. Nowadays the surface coating of insulation resistance device is relatively heavy, measurement error is relatively large, and measurement repeatability is not good. To solve these problems, we design an insulation resistance measurement system based on Infineon XE166 microcontroller. This article analyzes the measuring principle of silicon steel coating surface insulation resistance and includes the three aspects of mechanical, circuit and software. The experimental results validate the feasibility of this system. The measurement system is stable and reliable, high repetition rate, and has a good application prospect.