Apply Solder Paste
To start the assembly process, solder paste needs to be added to those areas of the bare,
unpopulated PCB board. Solder paste only goes where a component must be in electrically conductive contact with a
metal landing pad on the board itself. Typically, this is achieved with a Solder Stencil. Using the solder screen placed directly onto the board and registered in the correct position, a runner is moved across the screen squeezing a
small amount of solder paste through the holes in the screen and onto the board.
As the solder screen has been generated
from the printed circuit board files, it has holes on the positions of the solder pads, and in this way solder is deposited only on the solder pads.
There are two basic types of solder stencils: framed stencils for use with solder paste machinery, and prototype stencils for hand‐application of solder paste.
Pick‐and‐Place can occur either by hand, or by machine. With hand pick‐and‐place, a
technician uses tweezers to arrange each component on top of the appropriate bits of solder paste.
As suggested by its name, with machine pick‐and‐place, a machine loaded with components picks the components and
places them onto the board. The tension of the solder paste is usually enough to keep the components in place.
The pick‐and place machine uses location and orientation information (commonly held in a Centroid file)
as the reference for placing the components onto the board correctly Centroid files can usually be derived from the CAD design data for the board design.
Once the components have been added to the board, the next stage of the assembly process is to
pass it through the soldering machine. Especially for prototypes, reflow soldering techniques are the most common
technique for SMT soldering, though wave soldering machines are sometimes employed.
Test: Functional testing to ensure a working board.
As we can see from this flow, the assembly process starts with the application of the solder paste.
This photograph of a board shows how surface mount components can be not only close together
but also prone to misalignment or shorting if not precisely soldered.
It’s one thing to be able to solder a through‐hole component, filling the hole with a bead of solder.
It’s another thing entirely to tease solder under a chip type component such as the typical SMT package,
without having solder or components slide around or solder create unintended short circuits.