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For reliable telecommunication systems, Bellcore recommends that Surface Insulation Resistance (SIR) be monitored at key points in printed wiring board and circuit pack manufacturing. The Bellcore SIR criteria are based on the old ‘Bell System’ test pattern having 0·025 inch conductor line widths, and 0·050 inch conductor spacings. Since divestiture, many equipment suppliers have suggested using different test patterns, or even conductors on actual product for SIR testing. Also, with the trend to high density packaging and smaller conductor spacings, the Bellcore pattern now represents old technology. This work confirms and advances prior work suggesting pattern translation based on the SIR per square concept. Essentially exact SIR per square correlation has been found over an order of magnitude of pattern conductor space widths. Critical experimental techniques to modify the FR‐4 epoxy surface appropriately and an important theoretical hypothesis involving shadowing (proven experimentally) are developed in this work.

Electrical leakage beyond control levels did not occur in high temperature and humidity stressing of surface insulation resistance patterns when the relatively high contamination levels of 5 and 10 µg LiBr/cm were laminated into a simple test multilayer board structure. When sputtered layers of LiF, LiBr and NaCl were covered by a thin 0·002 in. lamination layer, generally similar results were obtained at 35°C/90% relative humidity and even at 85°C/85% relative humidity. Biasing of some samples at 85°C/85% relative humidity out to 400 hours did cause leakages which vary from one to three decades above the controls, but drifting with time beyond 96 hours towards a shorting condition, or to the level of unlaminated samples, on a leakage per square basis was not observed. Because the surface insulation resistance per square concept did not hold in these experiments, the surface leakage mechanism is apparently overridden by bulk leakages which occur in parallel in the laminations.

Bellcore has two generic physical design criteria for telecommunications product cleanliness. Residual insoluble contamination levels are monitored by surface insulation testing, and soluble ionic species by solvent extract conductivity testing. The previously undefined relationship between the two has been elucidated and is reported in this paper. Contamination of insulation resistance test pattern surfaces with LiBr at the criterion limit of 1 μg NaCl equivalent per cm2 results in surface insulation resistance levels in the 109 to 1010 ohm range, which is the criterion level. A discussion correlates the Bellcore cleanliness criteria with satisfactory performance of today's leakage sensitive ICs. Contamination levels at or above 5 μg NaCl equivalent per cm2 result in significant circuit corrosion and migration at 85°C/85% relative humidity stress, clearly indicating unsatisfactory field performance for a product with such contamination levels.

‘No clean’ fluxes (NCFs) have been in existence for a few years now. The initial claimed advantages of these fluxes were that post flow soldering cleaning would not be required, therefore a substantial cost‐reduction could be obtained in terms of no cleaning plant or cleaning solvent being necessary, and a consequent reduction in floor space requirements. Latterly, the restrictions to be placed on the manufacture of CFCs (the major flux cleaning solvent) via the Montreal Protocol have given these NCFs a much higher level of prominence. The advantages claimed for NCFs are very attractive; however, the fluxes represent a considerable technology shift from the conventional high solids rosin type fluxes which have been successfully used for many years. Probably the most important questions to be raised when considering their use are: ‘Will any remaining residue be corrosive and will the long‐term reliability of the printed circuit boards be affected?’ This paper sets out to address the following issues: (a) A definition of corrosion and long‐term reliability and what it means in practical terms, (b) an understanding of the basic formulation of NCFs and (c) evaluation and selection of test methods to establish confidence that corrosion and reduction in long‐term reliability, as described in (a), will not occur.

This study examines solvent extract conductivity (SEC) testing, e.g., Ionograph or Omega Meter testing, which measures ionic cleanliness of printed wiring boards (PWBs). SEC has been a quality control (QC) monitor to assure product electrical reliability. Typical SEC measurements occur after wave soldered products have been solvent‐cleaned. This study concerns SEC testing on new wave soldering processes that involve no solvent cleaning, i.e., inert gas soldering with ‘no clean’ fluxes. Results show ionic residues from ‘no clean’ fluxes may have other characteristics that make QC testing for ionic cleanliness inappropriate. However, SEC may be appropriate as a process control monitor after soldering with these fluxes. An Ionograph measured SEC response for the following chemicals: NaCl, NaF, NaBr, KCl, MgCl2, CaCl2, HCl, succinic acid, malic acid, glutaric acid, adipic acid and ethylene glycol. The list includes inorganic salts, strong electrolytes, which may arise from manufacturing or PWB materials. The list also includes weak organic acids (WOAs) common to ‘no clean’ fluxes. One non‐ionic hygroscopic chemical, ethylene glycol, was studied. Ionograph response was measured via (i) direct injection of aqueous solutions and (ii) immersion of PWBs with individual chemicals as surface deposits. All ionisable compounds, including all WOAs, produced substantial SEC response. Surface conductivity was measured at 35°C/90% relative humidity (RH) with controlled amounts of the above chemicals deposited on clean PWB test circuits. Surface loadings corresponded to the molar‐ionic equivalent of 2.0 ?g/cm2 NaCl. In addition, NaCl, adipic acid and polyethylene glycol (PEG 400) were examined as a function of concentration. Several ionisable chemicals including all WOAs produced no measurable effect, i.e., surface conductivities were indistinguishable on clean and deposited specimens. Surface conductivity increased for ionic contaminants with critical RH below ∼80% and for the non‐ionic hygroscopic glycol. SEC measurements and surface conductivities were compared. The latter is more directly related to electrical reliability. Although all ionic compounds including the WOAs showed a SEC response, not all enhanced surface conductivity. Achievement of critical RH appears to be the important factor. Adipic acid required the presence of hygroscopic glycol to enhance surface conductivity. Therefore, SEC can be a misleading QC test for electrical reliability when WOA flux residues are present.

Alpha Metals has announced the appointment of Stan Renais to the position of General Manager, Alpha Singapore.

Modern electronics is characterised by the increasing level of integration in printed circuit board (PCB) technology and the reduced insulation spacing between adjacent conductors. Surface insulation resistance (SIR) measurement has often been used alone to determine the cleanliness of PCB assembly; however, when proper SIR measurement is used in conjunction with surface leakage current (SLC) measurement, the result can reveal the dynamic nature of surface electrochemical migration (SECM) processes at the microscopic level, and the effect of such processes on product quality and reliability. This paper presents a newly developed measurement methodology, which measures SLC per square unit area at a sampling rate that is orders of magnitude higher than that of conventional SIR measurement methods. It is aimed to capture the transient surge of SLC which is detrimental to the functionality of product.