Surface Mount Technology Lab Tutorial
Department of Engineering
25 May 2006
The equipment and procedures described in this document are suitable for rapid assembly of circuit boards using surface mount technology (SMT) parts. Very fine pitch high pin count ICs can be accurately placed, including packages with exposed pads on the bottom. It is assumed that no through hole parts are used, or at least that they will be mounted after the surface mount parts. The lab is located upstairs in Hicks Hall, in Professor Cheever’s research lab.
Equipment and Procedures
The general idea for mounting surface mount parts is to precisely apply a solder paste to the pads on the printed circuit board, position the components on the pads with the help of fine tweezers and a microscope, and to reflow the solder by heating the board on the hot plate. After the solder paste has reflowed, the joints can be cleaned up and any shorts removed using solder wick and a fine tip soldering iron. The solder paste is sticky enough that parts will not slide around unnecessarily once they are placed. The heating on the hot plate is not terribly localized and as a result parts that have previously been mounted might reflow. This is not a problem if the board is handled with care and the parts are not jolted around. If more localized heating is necessary, a small block of metal can be placed on the hot plate underneath the part of the circuit board needing the heating. To remove parts, simply reheat the board, and once the solder is melted, simply remove the desired part with normal tweezers. For rapid removal of resistors and capacitors, the tweezer iron can be used as well.
- Reflow Hot Plate: This is low profile hot plate mounted on a board with arm rests and handles that is used to reflow the solder paste by heating the circuit board from the bottom. The temperature is set usually at 270 degrees Celsius using the integrated thermostat. A piece of aluminum foil is used to protect the surface of the hot plate from unwanted solder reflowing onto the plate.
- Microscope: This stereo zoom microscope allows for precise application of solder paste, placement of components, as well as post-reflow inspection of connections. The microscope has a homemade illumination ring made of bright white leds, and is powered by the small regulated power supply located behind the microscope.
- Fine Tip Soldering Iron: Used to rework parts and to clean up excess solder from the pins. The temperature is digitally controlled, and is usually set around 700 degrees for general purpose work. After a period of inactivity, the iron cools itself to 300 degrees, and after an even longer period shuts off. To set the temperature or to remove the iron from ‘sleep mode’, simply used the up/down arrows.
- Solder Paste Injection System: This unit uses compressed air (from an air compressor located next to the table) to apply very controlled amounts of solder paste from a syringe. The unit is controlled by a foot pedal, and can be configured in manual mode, or a timed shot-meter mode. In manual mode, solder is ejected while the foot pedal is pressed, whereas in timed mode, each press of the foot pedal ejects solder for a configurable length of time. The control unit also has a pressure regulator, as well as a vacuum system to keep solder from continuing to flow after the foot pedal is released or the shot times out. The air supply is filtered to remove particles and moisture. The solder paste is stored in its syringe in the refrigerator located in the back corner of the Hicks computer lab to extend its lifetime.
- Tweezer Iron: Used for quick removal of SMT resistors and capacitors.
A drawer full of various useful tools is on the left side of the desk. Various fine pitch tweezers, fine solder wire, desoldering braid, and an Xacto knife are the most frequently used.
Example Board Assembly
These were the steps taken to mount a simple surface mount IC and a few resistors. The sequence is probably in more detail than necessary.
- Turn off and unplug the hot plate.
- Turn off the soldering iron, microscope illumination ring, solder injector control unit, tweezers.
- Disconnect the solder paste syringe, replace the protective needle cover, and return the paste to the refrigerator.
- Return tweezers and other hand tools to the appropriate drawers.
- Turn off the power strip on the right.
- Ensure that the air compressor is off.
Feel free to contact me at any time with questions on my cell phone or by email.