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SMT Assembly (SMA) is the term given to the process by which electronic components of various types, shapes, sizes and values are soldered to the surface of a bare printed circuit board (PCB) usually by an automated process. This process is also known as Onsertion and the general processes surrounding surface mount technology are referred to as SMT.

There are basically three steps to the onsertion process:-

  • Screen Printing
  • Component Placement
  • Reflow Soldering

Screen Printing

A bare PCB has first to be printed up with solder paste. This involves printing solder paste through a solder stencil within a screen print machine onto a substrate or PCB.

The stencil is usually made from stainless steel and has laser cut apertures in it which match exactly the component termination locations on the bare PCB. The solder Gerber files are used by stencil manufacturers to create the stencil.

Within the screen print machine the PCB is mechanically clamped and brought into contact with the underside of the stencil forming a 'gasket' seal. At this point the apertures in the stencil should exactly match the pad locations on the bare PCB.

Solder Paste is then applied to the topside of the stencil.

A squeegee blade is then 'driven' across the topside of the stencil squeezing solder paste through apertures of a solder stencil and on to the pads on the PCB beneath.

Once printed, the PCB is snapped away from the stencil leaving a solder paste deposit on the solder pads only. Theses solder paste deposits are also known as solder paste islands.

Component Placement

The printed PCB is then transferred to a SMT placement machine. These are available in all shapes and sizes ranging from semi-automatic bench mounted stations (where operators manually deposit components on to the solder paste islands with vacuum pencils and tweezers) through to full automated systems which can place tens of thousands of components per hour and also inspect/electrically test some parts prior to their placement.

Reflow Soldering

Once all component parts are placed on to the PCB, it then needs to go through a reflow oven to melt the solder paste which as it cools will form a solder joint between the component terminations and the PCB. This is known as the reflow process.

The reflow process is critical to product reliability and quality. It is very important that the PCB and the components are gradually taken up to the temperature at which the solder will become liquid (known as the liquidus point) to minimise thermal damage (known as thermal shock) to the components and the PCB itself. As a result, a reflow oven has a series of zones set at different temperatures through which the PCB assembly is gradually exposed to increasing temperature as it is transported by an automatic conveyor or mesh.

The PCB sees the liquidus temperature for a very short time period and is quickly cooled to minimise the time spent at these elevated temperatures. (Typically around 183 deg C).

The correct temperature profile is monitored using tools such as thermocouples and 'moles' which accurately detect the temperatures on various different parts of the PCB assembly.

Reflow ovens are available using different technologies including:-

  • Infra Red Reflow
  • Forced Air Convection
  • Condensation Reflow - Vapour Phase

This is a typical process/service that might be carried out by a contract electronics manufacturer (CEM).

PCB Assembly Topics




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Guide Written by Paul Wilson

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