PCBs
How clean are your boards?
David Westwood, sales & marketing manager, KSM Superclean talks about the effects of not cleaning bare board PCBs
which are not clean. It is wrong to assume that bare boards arrive in pristine condition free of any contaminants. They are rarely packed in cleanroom conditions and during transportation there is ample scope for the boards to become contaminated. When the boards are unpacked for the production line it is unlikely that it is in a cleanroom so all kinds of debris and particles can land on the boards from dust and hair to threads from operator clothing. Left unchecked these particles can affect the solder paste print quality and reliability. If the defect goes unnoticed the populated board may need to be scrapped at the end of the production line or worse still, the board is shipped to the customer and is returned because of reliability problems. When margins have never been tighter and customers are demanding zero defects such problems can become intolerable. The main sources of contamination are
David Westwood M
ost electronics manufacturers will readily admit that their key priorities today are to improve yields, eliminate defects, cut waste and boost efficiency to achieve unprecedented demands on zero defect excellence in a world of intense competition. There is one simple action which can achieve these objectives but how many companies have adopted it? I am talking about the removal of loose contaminants from bare board PCBs.
If ionisable contaminants are not
effectively removed it can play havoc with production leading to defects, wasted product, downtime and operational inefficiencies, creating the potential for competitors to attract your customers. Removal of loose contaminants should start at the bare board level as the potential for contamination is substantial. A high percentage of faults are introduced at the solder paste print stage and many of these errors can be attributed to boards
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already in the manufacturing environment and include dust, human hair, lint, clothing fibres, epoxy dust and glass splinters. Misprinted boards are often wiped clean and put through the production line again giving rise to loose solder particles remaining on the board. The materials which bare boards are packed in such as cardboard or plastic can also produce microscopic particles which remain on the board’s surface.
Added to this is the key issue of
electrostatic charge. As boards are insulators a static charge is created when the boards are handled, for example, being placed into a feeder or cassette. This charge attracts particles to the board and holds them there. The main problems particles cause can include the following: 1. Dry or unreliable solder joints 2. Poor solder wetting 3. Fibres trapped between pads can cause short circuits
4. Chip components are “tombstoned” – the chip component stands on end; only one end is soldered
Contact cleaning machines use a series of special elastomer rollers which make direct contact with the substrate. The rollers attract any loose particles down to one micron in size; these particles are then transferred on a special adhesive roll. Once the sheet on the roll becomes saturated it is simply peeled off leaving a fresh sheet underneath. The contact cleaning process should be carried out at the start of the production
contact cleaning equipment and static neutralised have proven to result in less failures and rejected boards. In summary, contact cleaning offers the
most effective method of removing contamination from bare board PCBs. When used in conjunction with anti-static equipment, it can help manufacturers dramatically increase yields, eliminate rejects and reduce waste.
www.rainbow-technology.com/solutions/ksm-superclean/ Components in Electronics April 2018 31
5. Contamination in the solder can cause a crater during reflow
6. Stencil holes can become blocked causing the print to be incomplete. Cleaning of the boards should start
before they enter the production line so that all surface contamination is removed. A number of methods have been employed including brushes, vacuums and blowers – these can remove some contamination but not all. By far the most effective method of cleaning bare boards is contact cleaning.
line so that boards are 100 per cent clean before the solder and assembly stages. It is also important to incorporate anti-static equipment so that the boards do not become re-contaminated as they leave the contact cleaning unit. How can you measure the effectiveness of the cleaning process? A number of manufacturers now use Automatic Optical Inspection or Solder Paste Inspection equipment which can give a visual indication of why boards have failed. Bare boards which have been pre-cleaned using
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