Surface Mount


SMD PCBs - or surface-mount device printed circuit boards - are the predominant method of technology used in electronic manufacturing at the moment. Manufactured using surface mount technology, SMD PCBs present a challenge to circuit board fabricators such as PCB Train. How do we solve this problem? The surface mount pads themselves must be flat to allow the solder paste screen stencil to engage perfectly with the surface of the PC board under print.

Any lumps on the surface mount pads will cause a gap, and solder paste will bleed out from under the stencil placing solder paste. Such poor solder paste printing causes short circuits and solder balls, and is a major cause of assembly failure. So PCB assemblers seek perfectly flat surface mount pads on bare PCBs.

Traditional Hot Air Solder Levelling (HASL) techniques proved difficult to control and achieve good pad flatness. The process involves dipping SMD PCBs into a molten tank of solder, and withdrawing the PCB through high pressure hot air knives. Rather akin to a Dyson Airblade hand dryer, but cooling molten solder. The process has to be completely enclosed to contain the solder and flux residue which flies everywhere. The machines have to be constantly cleaned and adjusted to cope with the harsh environment. The high temperatures risk warping PCB panels. Only perfectly adjusted machines produce flat solder finish.

With the change to lead-free production, manufacturers had a choice to upgrade their HASL machines to lead-free solder, or to move to newer chemical based copper finishing systems. The HASL process was naturally unloved, so anything that did not involve molten solder was attractive. Immersion gold over electroless nickel was an obvious choice, but the process is also relatively expensive and difficult to control. However, it remains popular.

Process engineers realised that the natural copper finish of a surface mount pad was adequately flat. What was required was a finish with anti-tarnish properties which would also take solder, and permitted electrical contact with test probes without damage and residue being left on the probes.

Organic Solderable Preservatives (OSP) & flux varnishes, are very cheap to use. However, the PCBs copper pads retain their colour and it is not obvious if the entire surface of the circuit board has been treated. If any part of a surface mount pad has skipped treatment, then an assembly problem becomes possible. Shelf life and durability are also limited.

The Immersion Silver (IS) process solves many of these problems. A few microns of silver are deposited on the surface mount pads to provide a witness to the process, and a coating of OSP provides a long shelf life and solderability. The process is run at room temperature, little stressing the PCB panels. There is little effluent and the process is cheap to run. Very small amounts of silver are used per PCB. PCBs exhibit very flat surface mount pads and there is no risk of panels being warped. The surface is completely compatible with electrical test probes.

The is finish is a solderable finish. It is designed to be soldered. So designers should consider that any exposed copper (IS coated) on a finished PCB should be solder pasted and reflowed. If they are not, IS does provide an adequate long term corrosion-resistant surface coating for the PCB copper, but any exposed IS will in time turn grey in colour. This has no effect on the long term reliability of the circuits, but any exposed IS will in time turn grey in colour. This has no effect on the long term reliability of the circuits.

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