- 07 February, 2012
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ElectroTestExpo
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Sheila Hamilton |
Contamination has always been a bugbear for the electronics production industry. Dust and debris entering the production process leads to lower yields, more rework and downtime not to mention wasted materials.
However advances in production techniques have made the issue even more critical when “right first time, every time” has become the industry’s mantra as the pressure to increase profit margins and improve efficiency has become intense. The drive to squeeze more performance into smaller boards has meant that track and gap widths have become much tighter and the number of layers has increased. Therefore very small particles (down to one micron) can have a much greater impact than before by, for example blocking holes in the PCB.
Moreover a move towards the use of more environmentally friendly materials has changed solder flow characteristics and solder melting points making the materials stickier and therefore more prone to contamination. The application of lead-free solder has also meant that copper has a tendency to dissolve into the tin-rich alloy during dipping, wave solder and rework.
Contamination sources
So how does contamination enter the production process and what are the key sources? On the whole electronics production does not take place in a highly controlled environment such as a cleanroom therefore there is enormous potential for airborne particles to interfere with the production process. These sources include:
· Human hair – on average, a person loses 50 hairs a day.
· Lint – whilst special cloths used in the production area are supposed to be “lint-free” they can still snag when used to clean pad areas and stencils creating loose fibres.
· Clothing fibres from the clothes of operators are a frequent source of contamination.
· Dust – the main source of dust is human skin. On average a person breathes 700,000 skin flakes a day and discards one layer of skin every day.
· Production plant – particles from ceilings, floors, shelving as well as packaging materials can create debris.
· Equipment – the internal surfaces of production equipment should be cleaned frequently to prevent residual contamination being redistributed to the substrate.
· Epoxy dust – during the manufacture of bare boards every effort is made to remove any contaminants. However, after the board is routed only the top surface can be cleaned therefore there may be loose dust left on the edges.
· Glass splinters – PCBs can produce loose glass splinters from snap outs or routing.
· Solder paste – particles of solder paste can remain on misprinted boards which are “wiped clean” and put back through the production line.
· Solder resist – this can flake off leading to contamination.
· Packaging materials – PCBs often come wrapped in paper and paper fragments can cling to the board due to static.
· Transport systems – totes and other sorts of transportation system are frequently used to move boards from one area to another and can be a potential source of contamination.
Preventing static
An added complication is the presence of static. Dust and debris is attracted to PCBs by static charge. PCBs are insulators so will generate a static charge whenever they are handled or removed from packaging. So any contamination control strategy must also incorporate static elimination processes into the production process. This can be achieved by installing static neutralisation equipment after the boards have been cleaned and before they are populated to prevent recontamination.
Contamination problems
Left unchecked contamination can lead to a number of serious problems in the production environment. Stencil holes can become blocked by particles so the print is incomplete and solder paste can become contaminated causing solder volatising during reflow leading to a crater in the solder. Tombstoning can also occur whereby only one end of the chip is attached to the PCB leaving the other end standing on end. If cloths are used to clean the stencils fibres can become trapped between pads leading to short-circuits. Other problems include poor solder wetting and unreliable solder joints.
Usually the underside of the printing stencil is wiped with a cloth and solvent to remove contamination. Frequently this causes fibres from the cloth to become loose and contaminate the board leading to downtime and lower yields. Due to solder fumes a substantial amount of particles are airborne. These particles can vary in size from 0.01 microns to 1,000 microns therefore if the pitch of the component leads is 20 microns these particles can become an issue. In general, the finer the component lead pitch the greater the risk of contamination causing errors.
Defects experienced during the screen printing process are a major contributor to overall levels of defects and so any reduction in contamination in this area can impact severely on overall yield levels. Cleaning and removing static from boards before they enter the production process is the best way of counteracting the effects of contamination.
Bare boards are usually purchased from a third-party supplier on a least cost per unit basis. Normally these boards have high levels of contamination when they arrive at the production plant.
Various methods have been employed by the industry over the years to clean the PCBs. These include blowers and vacuum systems. However, the main drawback with such systems is that they cannot break through the boundary layer of air just above the surface of the board caused by what is known as the Bernoulli Effect. As a board goes through the production line a boundary layer of air is created which traps debris and dust. For that reason, Contact Cleaning systems have become the most effective method of cleaning boards as they can penetrate this layer of air and remove contamination from the surface as it makes direct contact with the substrate. This equipment uses a special elastomer roller to lift loose surface contamination from the substrate. The debris is then transferred to a roll of special reverse-wound adhesive film for examination and disposal. The boards should then be passed through a static neutralisation unit to prevent contamination being re-attracted to the board.
This process has been further enhanced with the recent introduction of Nanocleen technology from Teknek. Nanocleen can remove much smaller particles (down to 25nm in size) as well as remove 25-50% more particles than other contact cleaners. In addition the special rollers have static dissipation qualities which can reduce static on boards by a factor of 10.
Employing contact cleaning equipment has proven to increase yields by 90% and reduce reject rates by over 50% offering the industry a highly effective method of combating the impact of contamination.
Sheila Hamilton is technical director at Teknek
