Spraying Systems. - World's leading designer and manufacturer of industrial spray nozzles

NEW APPROACH TO SPRAY FLUXING OF PCBs

Spray fluxing as opposed to foam fluxing in the manufacture of printed circuit boards has many important and well-known advantages. But in converting its battery of small-scale flow soldering machines to spray operation, Brighton-based instrumentation manufacturer West Instruments has broken new ground. Moreover, by using precision air atomising guns, rather than the hydraulic nozzle types normally used on this application, the benefits in terms of product quality, material savings and production efficiency have been intensified.

The new flux application system was devised in-house in conjunction with spray technology specialists Spraying Systems Ltd. - resulting in conversion costs significantly lower than those proposed by suppliers of 'bolt-on' spray fluxing systems.

While converting its fourteenth soldering machine to spray fluxing, West Instruments reported that investment costs on the first eight conversions had been recovered in less than a year, and was able to project material savings of some Ł1000 each month continuing into the future.

A member company of the Danaher Corporation, West Instruments has been developed as the parent group's worldwide centre for the design and manufacture of temperature and process controllers. Production is organised on a flexible basis, using 'lab-sized' soldering machines upgraded in-house to cope with a wide variety of product and special customer needs.

As original equipment, each machine incorporated a flux foam bath. But suspicion fell on this method of fluxing when unwanted dendritic growth occurred on certain products exposed to conditions of very high humidity in the far-east market region. Tasked to research this problem, and with the help of sample testing carried out by the National Physical Laboratory, project engineer Nigel Coombs established that the primary cause was excess residual deposits of flux on PCBs following soldering.

As an initial counter measure West Instruments had installed additional sophisticated post-production cleaning machinery. Meanwhile the company investigated alternative flux application methods and the possibilities of converting to spray fluxing.

"What we looked for," said Nigel, "was a solution that was physically adaptable to the scale of our soldering machines and could deliver flux more precisely, to reduce any risk of ionic contamination of the PCB. Cost was also a factor, bearing in mind that we operate 16 individual machines." Ready-made retrofit spray options were considered, but for one reason or another rejected.

They were physically too big for most of the soldering machines in the plant, and they were relatively expensive - to convert just one machine would have cost at least Ł4500.

At this point West Instruments decided to build its own system, calling in the specialised help of Spraying Systems as advisers and suppliers. The first new installation was on a machine handling a particularly sensitive product. A successful 3-month trial production run was backed with NPL tests, showing that spray fluxed boards trapped 77% less ionic residue than their foam fluxed equivalents. These results combined with other major benefits convinced West Instruments to put in hand progressive conversion of all the plant's flow soldering machines.

The new system works at low pressure (under 3 bar). It incorporates a single Spraying Systems automatic air atomising gun in the 1/8JJAU series. Compressed air joins the liquid stream at the orifice to give finer flux atomisation and more economical material use than would be possible with a hydraulic nozzle. Droplet size can be controlled by adjusting air pressure in relation to liquid flow.

A second air stream acts on a spring-loaded shut-off cylinder inside the gun to control on/off cycling, synchronised with conveyor speed to stop spraying the instant the PCB moves clear.

Flux is held in a stainless steel pressure vessel, with low-level warning switch, and is moved to the spray gun by air pressure. There is no pump in the system.

The new spray process, said West Instruments, was easy to retrofit and is easy to control. It delivers repeatably accurate, consistent fluxing performance to a standard that makes post-production cleaning almost redundant. 'First-time pass' percentages for completed PCBs are higher than ever.

Flux consumption was halved and the use of thinners eliminated. Machine set-up time and costs were greatly reduced and other unproductive time (e.g. for drain down at day's end) was cut out. At current rates, cumulative returns on the investment after allowing for the costs of conversion will top Ł25,000 during 2005.