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Introduction

This page contains an extract from the Buffer Amplifier Assembly and Operating Manual describing my approach to surface mount work. I'm a relative beginner with surface mount components and this page describes what I've found to work for me. If you have a different methodology that works for you, don't feel that you need to change it to mach my preferences.

3.2 Working with Surface Mount Components

If you are unfamiliar with installing surface mount components, you should review this section of the Assembly Manual. The surface mount components in this kit are considered “large” by industry standards, and should not represent difficulty if you have prior kit assembly experience, either surface mount or through-hole.

3.2.1 Tools

I use the following tools in assembling surface mount printed circuit boards:

Tool

Comments

Approximate Cost

Source

I use curved nose forceps to hold parts while soldering. I prefer the pattern 7-SA forceps with gently curved tips, but the angular pattern 6-SA may be preferred by some. These are by Technik and are non-magnetic stainless steel.

$9.00 each

MSC part number:
7-SA pattern:
88348099

6-SA pattern: 88348081

I find clip-on magnifiers essential to view small parts. I keep both 1.75 x and 3.5 x magnification types at hand.

 The 3.5 x magnifiers have a working distance of 4”, so it gets you “up close and personal” with the parts! (I’m not a fan of the headband style magnifiers, but personal tastes differ in this regard.)

Of course, if you don’t wear glasses, a different style magnifier will be in order.

$20 each

MSC part numbers:

1.75X: 06533202

3.5X: 06533236

Small diameter solder is essential. I use either 0.015” diameter or 0.022” diameter solder. If you can find it, I find solder with a slight amount of silver (3% typically) produces a better appearing joint.

 My 0.015” solder is Kester “44 rosin, 63/37 eutectic mixture. My 0.022” solder is (believe it or not) Radio Shack 62/36/2, with 2% silver.

$15 for ½ pound Kester

Mouser part number 533-23-6337-07

If the pad being soldered to is at all tarnished, a touch of solder flux will clean it up. Some like to use flux regardless of pad condition. I prefer a pen-type dispenser.

$4

Mouser part number: 533-0951

Solder wick can’t be beat for cleaning up unwanted solder, or for removing solder from a pad after a part is changed. For small surface mount parts, I like narrow solder wick, 0.025 to 0.030.”

$2.50

Mouser part number: 5878-80-1-5

 

 

Finally, you need something to hold the PCB in place whilst you work on it.

 I use a Panavise #301 (with nylon jaws) for small boards and a Panavise #324 board holder for larger projects.

$85 for the PCB holder

$45 for the standard Panavise

#324 PCB Holder is Mouser PN: 591-324

#301 Standard Panavise is Mouser PN: 591-301

 

Mouser: http://www.mouser.com/ (excellent line of electronic parts and no minimum order size)

MSC: http://www1.mscdirect.com/ (Tools of variety almost unimaginable and it’s all in stock. No minimum order, prices may be a bit on the high side, but I’ve never bought a tool from them that I’ve been disappointed in. Their customer service is among the best I’ve seen.)

 
The tool not to use—self-closing forceps. These tools are quite useful for many purposes around the electronics workbench, but when it comes to holding surface mount parts, I’ve found them to excel at being “parts launchers” with the ability to shoot a small surface mount component across the room in an instant.

 

This tools list assumes you have a suitable soldering iron. I use an old Weller TC202 soldering station with a TC201 42 watt iron. For integrated circuits such as U801/U901, I use a 0.032” diameter long taper conical tip. For larger surface mount and through-hole parts, I use an 1/8” chisel tip. My experience is that a lower power soldering iron isn’t adequate, and one of much greater power is too much. Whether this is a true Goldilocks optimum or just my personal preference remains to be determined.

 

3.2.2 Installing Surface Mount Components

3.2.2.1 Integrated Circuits

U801/U901 is a reasonably easy to install surface mount IC, if you have the correct tools and follow a few simple steps.

  • Before starting, remember to apply proper anti static procedures. Study the IC’s marking and the PCB layout so that you know the correct orientation.

 

  

  • The idea is to hold the IC in place by soldering two diagonal corners in place and then solder the remaining pins. Start by applying a sparing amount of solder on one of the corner pads. Then apply a similar amount of solder to the diagonal corner pad. Place the IC on the PCB, correctly oriented, with pins centered on the pads. While holding the IC in place, touch the soldering iron to the first tinned pad, tacking the IC’s pin in place. The small figure shows an AD8007 after the diagonal pins have been tack soldered in place. Note how the pins are centered over the PCB pads.

 

  • Check the orientation of the IC, verifying that it is properly centered over the pads. If you don’t get this right now, it will be far more difficult to correct when the second pin is soldered in place. When you are satisfied that the IC is centered on its pads, touch the other diagonal tinned pad with your soldering iron.

 

  • Double check that the IC has not shifted during the earlier steps and that the IC’s pins are centered on their respective pads. Then solder the remaining pins to their pads. Depending on how the PCB was constructed, it may have sufficient solder plating so that additional solder is unnecessary—simply touching the soldering iron to the pin and pad flows enough solder to form a reliable joint. If this isn’t possible with your PCB, then apply a small amount of solder to form the joint.

 

  

  • After each time you touch the solder to the soldering iron, check the end of the solder for a blob or ball. If you find the solder has formed a ball, cut it off at an angle to form a pointed end. Otherwise, it will be difficult to control the amount of solder deposited.

 

  • After soldering, check for bridged pins or shorts to ground. Clean up any problems with solder wick.

 

  

3.2.2.2 Passive Components

To install a passive component, use a similar approach:

  • Lightly tin one pad.

  • With angled forceps (or your favorite tool) hold the part in place and solder the part to the tinned pad with a light touch of the soldering iron. This will anchor the part in place. Notice how nicely Technik pattern 7A forceps hold the part in place without obscuring vision or blocking the soldering iron.

  • Solder the remaining pad(s). If necessary rework the first pad connection.

It does not take a lot of solder to properly attach a surface mount component. A correctly installed part will be flush against the board and have a small filet at the end caps.

If the part is to be installed on one grounded pad and one ungrounded pad, it is easiest to solder it to the ungrounded pad first.

 

3.2.3 Removing Defective Parts

In almost every instance, trying to salvage a surface mount part installed wrong is a waste of time. A 1% 1206 size surface mount resistor, for example, costs about 2 cents in 100 lots. It’s much easer to use a pair of dykes and cut the part in half. Then use solder wick to remove the two halves. Clean up the pads with additional applications of solder wick.

To remove a multi-pin IC, a similar approach can be used—carefully cut the part into pieces with dykes, exercising care to avoid damaging traces or the PCB. A hot air gun can also be used to simultaneously liquefy the solder at all IC pins. The IC can then be lifted free of the PCB. Care must be taken, of course, to avoid damaging nearby parts with the heat gun.

An alternative IC removal approach is ChipQuik (DigiKey PN SMD1-ND, approximate price $15). ChipQuik’s kit includes special flux and a low temperature melting point solder that mixes with the normal solder and allows the chip to be removed without extensive heating.