Article by Lionel Smith first published in Archive Vol. 12 No.4
Archive Publications, 18 Mile End Road, Norwich, NR4 7QY
Tel: 01603-441777, Fax: 01603-460736
Placed here by arrangement with Paul Beverley
A solution looking for a problem?
In the newsgroups, I have noticed many people posting requests for help with hardware, and indeed software problems on their Acorn kit.
My guess is that many of these faults have their origins in poor contacts somewhere, between drive connectors and interfaces and drives, or between memory modules and sockets (this includes OS ROMs). This latter (with RAM and ROM, not forgetting those ROMs on expansion cards) is, probably, more prevalent and certainly much more invidious and pernicious.
Bad contacts in memory circuits can lead to intermittent program malfunction, often with data abort or address exception errors, data storage corruption because of file system malfunction, or complete system lock-ups.
It is probable that such memory related problems are now most common with SIMM modules, because it is here where we are most likely to see tin-flashed module connectors being fitted into gold-flashed sockets or vice versa. However, the most obvious case is the A3000, where many of the RAM expansion cards have tinned contacts and the motherboard has gold-flashed headers.
Another prime area for trouble is the tin-flashed links (i.e. those pairs of pins) which are connected by plastic jumpers containing gold-flashed clips. I have known cases of these links causing trouble with RISC OS 3.1 upgrades in A310s (LK6), and video sync on A4x0/1s (LK1), also with ROM select links, i.e. the ones moved when upgrading from RO2 to RO3 (e.g. LK17/18/19/20 on the A3000).
What is going on?
While a computer is working, vibration, temperature fluctuation and magnetic forces cause minute but repeated movement between the mating surfaces of connections. With tin (or solder) at one side of the connection and gold at the other, fretting (or frettage) corrosion occurs, whereby small amounts of tin oxide are transferred to the gold surface. This increases electrical resistance, the propensity for arcing (which exacerbates the corrosion problem) and hence the potential for data transfer errors. Of course, fretting corrosion can afflict any other point in a computer system where connections are made.
Once fretting has occurred, continued movement causes the hard oxide particles to act as an abrasive and work their way through the gold layer to the base metal or substrate below, eventually leading to delamination. Once this stage is reached, a satisfactory repair is difficult, if not impossible.
It might be thought that tin to tin contacts are not so good as gold to gold. However, the normal pressure, quite high, exerted on the connectors when SIMMs are locked into place is enough to disturb the oxides and make good electrical contact.
Gold has its own problems: being a soft metal, it can be prone to wear, so manufacturers add small amounts of cobalt or nickel to make the surface more durable. The thickness of the gold plating also has an impact on the number of cycles of fitting/removal which can occur before failure.
Also, gold is often porous and thus moisture and salts forming an electrolyte can seep under the coating and delaminate the gold plating from the nickel underlay. This delamination once started proceeds apace.
It is worth considering that contact surfaces, which to the unaided eye look shiny and smooth, under magnification look more like the surface of the moon, with peaks and troughs. Only through the numerous small points where the surfaces touch does electricity flow. As oxides form and break up into particles, these particles tend to fall into the troughs in the surface, wearing away these troughs and accentuating the peaks, further reducing the contact area. Also, under high humidity, arcing across the contacts can form molecules which, when combined with moisture, create corrosive nitrous acid.
Another contaminant that causes havoc when deposited within connections is silicone. Furniture and floor polishes are prime culprits in the spread of silicone oligomer which can contaminate connections a considerable distance from the source application. Fretting or arcing converts the oligomer into hard crystals of abrasive compounds, e.g. silicone carbide, which cause a loss of conductivity and exacerbate the wear in the contacts.
Many sprays sold for electrical use contain silicone oils which, although they may provide lubrication and repel water, do more harm than good in this context.
What can be done about it?
Scott Mueller in his, now legendary, Repairing and Upgrading PC’s, now in its tenth edition (and well worth the price of £51.49 to anybody involved in servicing computer equipment), characterizes the problem in practical detail and offers a solution.
This solution (no play on words intended) comes in the form of Stabilant. Stabilant, produced by D.W. Electrochemicals Ltd of Canada, is what is known as a contact enhancer. Stabilant is/was marketed in an even more diluted form by Sumiko Inc as Tweek to the home audio market.
For those who like to know such things, Stabilant is a “modified polyoxypropylene-polyoxyethylene block polymer of the polyglycol family”! More details can be found on D.W. Electrochemicals website at http://www.stabilant.com.
Stabilant comes in several forms. Stabilant 22 is the concentrated (and very expensive at about £58 to £72, ex VAT, per 15ml bottle) form which, for use, is diluted 4:1 with isopropyl alcohol (IPA), alternatively named isopropanol. This diluted form is known as Stabilant 22A. Fortunately, isopropanol (which can be bought in 500ml tins for about £6) is much cheaper than the Stabilant so 75 ml can be made up for between £64 and £78. A little goes a very long way, and a 15ml bottle of Stabilant 22A could last for a couple of years or more. I use a small artist’s paint brush, size 0 or 01, for application in most situations.
A good contact enhancer, such as Stabilant, performs several functions: corrosion digester and inhibitor, lubricator and semiconductor.
The application of Stabilant to old joints can work wonders, but improved and longer term contact will be achieved if heavy contamination is first removed using industrial alcohol.
Apart from reducing wear during normal system operation, the lubrication properties of Stabilant make the removal of treated memory or ROM I/Cs a dream. Before removing ROMs, on A30x0/4000 and RiscPCs, for example, a little Stabilant applied along the chip legs with a brush, allowing gravity and capillary action take their course for a short time, works wonders; the chips will then pull easily.
Stabilant’s action as a semiconductor is also important. By filling the pits in surfaces, Stabilant increases the contact area, thus improving the signal to noise ratio. Stabilant’s property as a semiconductor is derived from the fact that it “becomes electrically conductive in the presence of an electric field of suitable strength. The necessary field strength exists at the interface, generated by the potential across the contact resistance, and acting only over the very small distances involved” (Stabilant 22 Product Summary, Syence). In other words, Stabilant allows the effective propagation of electrical signals across the contact area but not between adjacent contacts where the distance is too great.
If you have ‘mixed metal’ contacts in the memory modules of your machine, expect trouble within six months to a year of use, according to Scott Mueller. A3000s seem to manage a bit longer than that. However, once trouble starts with any equipment, it can recur with monotonous regularity if only conventional mechanical means are used for cleaning, so be warned. You can actually do more harm than good by persisting with mechanical cleaning alone.
To reinforce his point about the problems caused by mixed metal contacts in memory installations, Scott Mueller cites an example of a computer system supplier who faced a law suit when refusing to replace the memory modules in a few hundred computers when dissimilar metals were shown to be the cause of the trouble.
Another voice that sings the praises of Stabilant is that of long time Byte correspondent Jerry Pournelle who gave it Byte’s ‘Product-of-the-Year’ award for 1984/5 and has mentioned the minor miracles it has performed on a number of occasions since.
Stabilant can be considered the best for professional use, but there are others on the market in quantities and at prices which may be more suitable for the occasional/home user. The most notable of these is Electrolube which is supplied in 5ml applicators (fluid pens) for about £2 each in quantities of 5-20 by RS. On the RS CD-ROM catalogue (also available from ElectroMail) can be found a data sheet on Electrical Contact Lubrication which also describes the problem. Similar Electrolube contact cleaner lubricant pens are available from Maplin, in quantities of one. I recall having an Electrolube fluid pen obtained from a model railway specialist which proved valuable in curing a fault on my Electron, some 10 years ago, when the fluid was applied to the five major expansion connectors on the system.
A good contact enhancer is useful for cleaning older connections, improving electrical conductivity, providing lubrication and corrosion inhibition.
Apart from the typical problems highlighted in the opening paragraphs, another major part of older Archimedes systems which could benefit from an application of contact enhancer are keyboards. Anybody who has ever opened up one of these, to clear out accumulated dust and fluff, will have seen all those tinned key connector pads often with arcing marks on the most used keys (Shift, Ctrl, Caps Lock, Return and Delete). Whilst open it is worth cleaning all these pads and applying a little contact enhancer to each.
Computers aside, there are many other uses for Stabilant. For example, it is very good for telephone connectors, and I have no doubt that model railway enthusiasts could benefit from its use.
Laboratory tests reveal that Stabilant’s effects include (reproduced from Stabilant 22 Product Summary, Syence): • Lower contact resistance • Reduced AC impedance • Reduced harmonic distortion • Lower noise floor • Improved transient rise/fall times.
- Stabilant is produced by D.W. Electrochemicals, 97 Newkirk Road North, Unit 3, Richmond Hill, Ontario, L4C 3G4, Canada. (00) +905 508-7500, (fax 7502), or visit their web site.
- Stabilant can be obtained in the UK from: Syence, 75 Rostover Road, Fulham, London, SW6 5AR. 0171-371-7771 (fax 7779). e-mail firstname.lastname@example.org, or visit their web site.
- RS/Electromail: Electromail, PO Box 33, Corby, NN17 9EL. Tel: 01536-204555. (fax 405555).
- Maplin have branches throughout the UK. HQ Sales: 01702-554000 (fax 554001). <recipient>@ maplin.co.uk, or visit their web site.
- Heller, Martin. - Byte, January 1992.
- Mueller, Scott (1998). - ‘Repairing and Upgrading PCs Tenth Anniversary Edition’, Que. ISBN 0-7897-1636-4. £51.49
- Pournelle, Jerry. - Byte, May 1985, August 1987, August 1991, October 1991, July 1993, February 1994, February 1997, May 1997.