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Surface Mount Tutorial

July 1999, revised August 2002 by Ralph Tenny

These images are of various pieces and components used to conduct a tutorial on working with surface mount devices (SMD). Surface mount devices appear to be too tiny to be used by hobbyists, but this demonstration is designed to suggest how the average DPRG member can actually use SMD in personal robot controllers.

The discussion applies to a PCB that has just been etched, or one that has been treated with immersion tin after etching. Any other bare copper board must be scrubbed with steel wool to remove oxidation that would hinder proper solder flow.

Photo 1

Photo 1 shows sample SMD parts. To the left of the paper is shown a quarter-watt resistor. The SMD parts in order, top to bottom, are 1208 size resistor which happens to be a zero-ohm jumper used to allow conductors to cross without touching; a 0804 size 200 kOhm resistor, an SOT-23 transistor; an inductor; a 1 uF 16 volt tantalum capacitor, a 10 uF 6 volt tantalum capacitor; a 0.1 uF ceramic capacitor; and an SO8 size LM358 dual op-amp. Loose on the page is a standard LM-358 through-hole dual op-amp.

Photo 2

Photo 2 shows two Antex 15 watt soldering irons. This is the proper wattage soldering iron to use. The coin used for size comparison is a U.S. dime.

Photo 3

Photo 3 shows a holder for single-sided copper clad board material. It holds the board over the lamps during exposure. See discussion for Photo 8 for more details.

Photo 4

Photo 4 shows an etch tank with bubbler in operation. The bubbles keep the etchant agitated for more rapid and uniform etching.

Photo 5

Photo 5 shows interior view of the exposure box. Two florescent black light bulbs furnish the exposure light.

Photo 6

Photo 6 shows a closeup of the bulbs, starter ballasts and momentary pushbutton switches. In operation, line voltage is applied by plugging the box into the wall socket. The bulbs will not start with line voltage, but when the buttons are pushed, the ballasts generate a higher voltage that ignites the bulb. After the arc starts, it will maintain on line voltage.


Photo 7

Photo 7 shows the exposure box with an eight RPM motor that rotates the PCB in relation to the lamps. This ensures even illumination of the board during the exposure.

Photo 8

Photo 8 shows plastic suction cup that attaches to the shaft of the rotator motor and holds the board by suction.

Photo 9

Photo 9 shows a closeup view of the bubbler tube inserted through the side of a plastic beaker.

Photo 10

Photo 10 shows the plastic bubbler tube with numerous holes drilled through it. The plastic bolt and nut lock tight against the sides of the beaker to seal the hole against leakage. Note the plastic bolt that has been drilled full length to allow air from an aquarium pump to create the bubbles.

Photo 11

Photo 11 shows up the bubbler tube; it is heated and pinched on one end and turned to a smaller diameter for insertion to the plastic bolt.

Photo 12

Photo 12 shows a dual H-Bridge board. SO8 dual MOSFET devices go into the four pad layouts; the FET devices each contain an N-channel FET and a P-channel FET; two such devices make a complete H-bridge.

Photo 13

Photo 13 shows (top to bottom) a hypo syringe containing solder paste; a strip of SMD zero-ohm jumpers, sharp lab tweezers to handle parts, and a card of 1 uF 16 volts capacitors. The round can to the right of the tweezers is commercial tip cleaner, available from Radio Shack.

Photo 14

Photo 14 shows a workstation consisting of a magnifer on a Helping Hands stand, and a rubber mat to help hold the PCB in place during assembly.

Photo 15

Photo 15 shows a view of a PCB through the magnifier.

In an assembly operation, small amounts of solder paste are laid out on the pads associated with a a particular SMD. The solder paste tends to hold the part in place until soldering. If a set of pads and the SMD are heated with hot air at low velocity, the device tends to center exactly over the pads if all the solder is melted at the same time.

If you are using a 15 watt iron, hold the part down with the tweezers and heat one corner pad until the solder flows. Then, solder a pad on the opposite side of the part. With two opposite pins soldered, the remaining pins can then be soldered. Alternate sides so the part is heated more evenly.

Photo 12

Carefully examine Photo 12 again. The large capacitor at the left has some cold joints indicated by uneven, crystaline solder. Good joints are smooth and shiny, indicating the entire joint was molten at the same time. Cold joints must be reflowed for a clean joint.

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