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PCB | November 22, 2007

Time for the electronics industry<br>to clean up its act?

As track and gap widths become smaller and layer counts become higher the problem of contamination in the electronics industry has become more acute.
Increasing miniaturisation and smaller form factors mean that the risks of dust and other particulates affecting the production process have become that much greater. A single speck of dust or a fibre is enough to halt an entire production line, costing significant sums in product wastage and manufacturing downtime.

You may think your PCB production environment is clean but you would be wrong. PCBs are rarely assembled in a clean room environment with the possible exception of fineline circuitry. This means that there is ample opportunity for contaminants to enter the production process. Research has shown that 80% of contaminants are introduced in to a "clean area" by people and products, 15% is generated by the products themselves and 5% is actually produced by the room and filtration system.

A key driver in the electronics industry is the increasing use of "greener" materials. This has changed the melting points, flow characteristics have changed and the material is stickier. As a result, the solder paste process is more open to corruption from other sources of contamination. In addition, copper has a tendency to dissolve into tin rich lead-free solder alloy which can happen during dipping, wave soldering and rework processes. This is a key reason assemblers are moving "zero tolerance" on rework. So getting it right first time is a key objective. Any possible sources of contamination which might cause rejects or rework have to be removed.

Sources of contamination
Most manufacturers assume that the bare boards they receive are 100% clean but this assumption is wrong. Contamination can come from a number of sources including:

* Hair - a person typically loses 50 hairs per day.
* Lint - cloth material used in production for wiping purposes. Most wipes designed for use on production lines are referred to as being "lint" free. This actually means free of loose surface lint but as these are wiped across pad areas or stencils these cloths can snag and generate lint. Using higher quality clean room cloths can eliminate this.
* Fibres - fibres from clothing worn by operators can enter the production process.
* Dust - much of the airborne dust is made up of human skin flakes. The average person breaths 700,000 skin flakes per day and sheds one layer of skin every 24-hours.
* Plant sources - ceilings, floors, shelving and packaging materials are sources of particles.
* Epoxy Dust - during PCB manufacture contamination removal systems are used at every step in the process, however after routing only the top surface can be cleaned. The edges still have loose dust attached. During packing and transport this dust is disturbed.
* Glass Splinters - as PCBs are made from a fibrous material there is the risk of loose glass splinters from routing or snap outs.
* Solder paste - misprinted boards are often wiped "clean" and sent down the line again.
* Solder resist - can flake away.
* Packing materials - boards individually wrapped in paper can be affected by paper snaps which attach themselves to the board by static.

Static
The key problem is that loose surface contamination caused by any of the above can attach itself to new boards due to static. PCBs are insulators by nature therefore they will hold a static charge. During handling it is easy for the boards to pick up a static charge from a variety of sources from the plastic wrapping to wiping with a cloth.

Now that we have established that PCBs arriving from the manufacturer are not pristine what strategies can be put in place to reduce wastage and increase yields?

The best way to achieve clean PCBs is to use specialist contact clean equipment to pre-clean the boards before they enter the production line. Such equipment uses a series of special elastomer rollers which make contact with the substrate to be cleaned. These rollers can attract contaminant particles down to one micron in size. The particles are then transferred on to a special pre-sheeted adhesive roll for examination and disposal. Typically such equipment also incorporates powerful anti-static facilities to eliminate the possibility of re-contamination as the boards move through the production line. However, ensure that the elastomer rollers employed are of the highest technical specification. All rollers are not equal. An inferior roller will fail to remove all traces of contamination which will lead to more defective boards, decrease production yield and increase downtime.

Contact Cleaning Technology
Using an effective contact cleaning technology has been proven to improve yields by 90%-plus and improve reject rates by over 50%. 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. All this can help reduce rework on boards and in the worst case scenario having to scrap boards altogether.

In summary, as the electronics industry strives for ever greater miniturisation and higher standards of quality it is essential that the sector adopts efficient contact cleaning technologies to eliminate the impact of contaminants on production yields and wastage.

The article was written by Hunter Paterson, Product Manager, Teknek

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