Hundreds of thousands of pounds are being wasted needlessly in PCB production due to contaminants entering the production line. Unless boards are pre-cleaned to remove all dust, loose surface particles and static it can severely affect production, create waste, reduce yields and ultimately impact on the bottom line.
Many manufacturers believe that they are working with substrates which are 100percent clean and that a ‘clean’ environment is completely free of contaminants. It is believed that 80percent of contaminants are introduced into a clean area by people and products, 15percent is generated by equipment and 5percent is created by the room and filtration system. SMT lines are generally outside the clean room environment and are therefore vulnerable to contamination. This can come from a variety of sources such as hair, lint, fibres, dust and even skin particles (an average person sheds one layer of skin every 24hours). The SMT process itself can contribute to the problem through:
u Epoxy dust: during PCB manufacture contamination removal systems are used at every step in the process, however the edges can be rough and have loose particles. During packing and transportation the boards rub together freeing more particles and distributing them between boards.
u Glass splinters: PCBs are made from a fibrous material.
u Solder paste: misprinted boards
are often wiped ‘clean,’ generating static which attracts any airborne particles.
u Packaging material: boards can be individually wrapped in paper resulting in small tears attached to the board by static.
u Laser etching of ID tags: generates dust which are held on the board with static.
If PCBs are cleaned and static is eliminated before screenprinting it will ensure a cleaner surface for printing, improve solder joint integrity and provide better stencil to board gasketing. Moreover, it can reduce the incidence of blocked adhesive nozzles, reduce rework, improve end of line yields and dramatically cut the frequency of underside stencil cleaning.
An effective method of cleaning the boards is to pass them through special elastomer rollers. These rollers can lift particles, dust and other debris down to one micron in size. The contaminants are then transferred to an adhesive roll for disposal.
Static is a major problem. PCBs are insulators by nature therefore they will hold a static charge and during handling a small static charge is easily generated. Also if packed in plastic film this will create static. Boards with a static charge will attract dust and other particles so it is essential that they are passed through a static neutralisation unit. This unit will reduce static to less than 50V to prevent the clean PCB attracting further particles and eliminates any static problems.
Equipment such as the contact cleaning and static neutralisation units are relatively inexpensive and easy to install. Generally they can be retrofitted ‘in-line’ to an existing PCB production system with the minimum of downtime. Choosing the correct design also means that access to change the elastomer rollers and adhesive rolls should also be quick and straightforward.
The latest models of contact cleaner have an integrated control unit with a graphical touchscreen making it easy for the operator to monitor and control the functions of the unit including speed of board throughput, display of edge width and various alarms.
One company had dust/contamination issues which it put down to static in its process. It was using a vacuum-based system which it thought effective but was not certain. Following a visit of an engineer from StaticClean International (MicroClean’s USA representative) it was agreed to run a trial on one of its production lines and compare results against the current set up.
A full specification was defined which included a PLC controlled MicroClean CL-SMT400 machine with the addition of a static monitoring unit with two probes which would provide feedback to the PLC of the cleaning unit.
Static charge was identified as critical on the board as a floating charge may damage critical components while they were later being placed. The line was run for a three month period with the results showing a clear downturn in rejected/reworked boards by 30–40percent and the adhesive rolls showed positive proof of the levels of contamination being introduced to the screenprinter.
The concern over the level of static voltage on the board was interesting with one probe monitoring the output of the CL-SMT unit and the secondary probe monitoring the output of the screenprinter. The
CL-SMT unit incorporates static bars on its exit side and the levels monitored were as expected below 50V. On the exit of the screenprinter the voltage monitored was in excess of 1KV which was surprising as this may be high enough to affect critical components.
The company has now removed the vacuum system from its production lines and installed CL-SMT units and also installed an extra anti-static control bar on the exit side of the screenprinting process.
John Penman is managing director of Microclean Technologies, Renfrewshire, Scotland. www.microcleantech.com"