Cleansing and Reliability of Smoke-Contaminated Electronics

Lennart Cider, Ph.D. Cider, Institute of Production Engineering Research, Gilteborg, Sweden.

Abstract

This work deals with problems that arise when modern surface-mounted electronics are to be reconditioned after smoke contamination.  In a fire, hydrogen chloride, which is formed when polyvinylchloride is present, is deposited on various materials.  Electronic equipment is especially sensitive since malfunction may occur after a longer or shorter time of operation due to the chloride contamination.

Earlier work has shown that through-hole electronics can be reconditioned, with good results, after deposition of up to 100µg chloride/cm² in the surrounding area.  The lower limit when cleaning is needed is often specified to 10 µg chloride/cm².

In this work, therefore, surface-mounted electronics have been exposed to smoke containing hydrogen chloride, which has contaminated the test boards with 45 to 75 µg chloride/cm².  Three different methods for cleaning smoke-contaminated electronics were investigated: manual, automatic spray, and ultrasonic.  Each method was able to clean to a contamination level lower than 1.5 µg sodium chloride equivalents/cm³.  The automatic spray method could not remove all contaminants beneath the components.  All three methods were capable of improving the surface insulation resistance to a satisfactory level.

Conformed coating can, to a large extent, protect the electronics against corrosive smoke.  However, decontamination of boards conformally coated acrylic coatings may be complicated since smoke products are partly absorbed into the conformal coating.  The large difference between chloride contamination in a fire and during the manufacture of electronic equipment is the nature of deposition.  Experiments have shown that the hydrogen chloride reacts with lead in the solder to form lead chloride.  During manufacture, chloride salts are deposited all over the test board.

Migration between conductors of different potential has not occurred.  This is due to the fact that chloride is localized on the conductors as lead chloride.  Galvanic corrosion, on the other hand, has occurred between metals within one conductor.

Introduction

Smoke-contaminated electronics are cleaned to restore the function of the equipment.  Hitherto, electronics that were cleaned were produced using the through-hole technology.  Modern electronics are predominantly surface-mounted electronics, which involve a fiber pitch and smaller cavities.  The possibility of getting undesired migration between two conductors is increased.  Hence, the need for cleaning is increased.

Electrochemical migration means that a conductor with a positive potential dissolves into positively charged metal ions, which then move in the electrical field towards the negative conductor.  The ions again turn into metal at the negative conductor, forming long, tree-shaped dendrites, which will eventually short-circuit the electronics.  Three factors are necessary for electrochemical migration:  conductors of different potential, humidity, and contamination.

A U.S. military specification, MIL-STD-2000A, for measuring cleanliness on electronics specifies that the contamination in sodium chloride equivalents (e.g., NaC1) should be lower than 1.5 µg/cm² on electronics to be conformally coated.  An equivalent amount of NaC1 means, in this case, an amount of ionic contamination having the same conductivity as NaC1 in an isopropanol/water mixture (ratio 75/25 by volume).

Cleaning smoke contaminated electronics is not currently considered until there are 10 µg/cm² of chlorides in the surroundings.  It has also been shown that through-hole electronics can be reconditioned, with good results, after deposition of up to 100 µg/cm² of chlorides in the surrounding area.  This is in accordance with research done at BELLCORE, where 600 µg/in. (600 micrograms per square inch = 93 µg/cm²) of chlorides are reported as the upper limit for reconditioning.

This work deals with the problems that arise when restoring smoke-contaminated surface-mounted electronics.  The problems are determining the state of contamination, the most effective method of removing it, and a way of ascertaining that it has been removed.

This work is a part of a series of investigations.  Earlier work reviews cleaning methods, reliability measurements, and smoke contamination.

Experimental Design and Procedure

The test board (see Figure 1) is an FR-4 laminate assembled with one 68-terminal, 50 mil pitch, J-leaded plastic chip carrier (PLCC68) on Segment 4; one leadless ceramic chip carrier (LCCC68) on Segment 3; two small outline components with 28 terminals (S028) on Segment 6; and 12 Type 2220 (CC2220) chip capacitors on Segment 5.  The test board is also equipped with comb patterns (Segment 1 and Segment 3 through 6) for measuring the surface insulation resistance (SIR), with conductor width and distance of 250 µm.

Some of the components chosen were shown in earlier experimental work to be very (PLCC68) and extremely (LCCC68 and CC2220) difficult to clean underneath due to their size and low stand-off height.  These different types of components were therefore used to evaluate the effect on the cleaning method.

The treatment and analysis of the test boards are indicated in Table 1. The right-hand side of the table shows the analysis of each board (1 to 100), and the left-hand side shows the treatment.

Table 1

Experimental Design

Table 2

Location of Each Test Board and Steel Board During Smoke Deposition