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Introduction
The vast majority of modern electronic components come in SMT--surface-mount technology-- packages. These require a very different set of tools and techniques for assembly than traditional through-hole components.
This tutorial presents the bill of materials ($300-$800) and the steps necessary for the ideal assembly.
Materials Needed
| Picture | Description | Brand | Part Number | Price | Notes |
|---|---|---|---|---|---|
| 1 |
Hot Air Station with Pre-heat plate
|
Aoyue |
Aoyue 866
|
$199 | Having a pre-heat plate is essential |
| 2 |
Straight Tip Tweezers
|
Excelta Cobaltima type 2-CO
|
352TW221
|
$109 |
Cobaltima tweezers are the absolute best of the best
|
| 2 |
Straight Tip Tweezers
|
Erem type TE3
|
TE3SA-ND | $37 |
|
| 2 | Curved Tip Tweezers |
Techni-Tool type 7
|
758TW639
|
$63 |
|
| 2 | Curved Tip Tweezers |
Wiha type 7abb
|
431-1026-ND | $42 |
|
| 3 | Solder Paste (Leaded) |
Kester R276
|
KE1507-ND
|
$31 |
AVOID: CHIPQUIK paste
|
| 3 | 10cc syringe plunger |
Apex 10LL4
|
10LL4-ND
|
$1 |
AVOID: CHIPQUIK SMDSG10CCR syringe gun
|
| 4 |
Dispensing needle tip, 22GA, plastic
|
Apex KDS22TN25
|
KDS22TN25-ND
|
$10 |
Good for 0805 and bigger pads
|
| 4 |
Dispensing needle tip, 22GA, metal
|
Loctite 98402
|
98402
|
$12 |
Good for 0605, 0402 and most IC packages
|
| 5 | Tack flux |
Chemtronics CW8500
|
CW8500-ND
|
$23 |
AVOID: CHIPQUIK tack flux
|
| 5 | Flux pen |
Chemtronics CW8300
|
CW8300-ND
|
$14 |
|
| 6 | Flux remover |
3M Novec Flux Remover
|
3M155811-ND
|
$22 |
Generally excellent
|
| 6 | Flux remover |
Chemtronics ES835B
|
ES835B-ND
|
$22 |
Occasionally complements the 3M Novec
|
| 7 | Solder wick, 30ml |
Chemtronics 80-1-5
|
80-1-5-ND
|
$5 |
|
| 8 | Kapton tape |
3M 5413
|
3M10235-ND
|
$19 |
Used to mask heat-sensitive components from hot-air jet
|
| 9 | Lint-free wipes |
Kimtech Kimwipes
|
7367T56
|
$12 |
|
| 10 |
Cotton tipped applicators, Mini Tip
|
Puritan 826-WC
|
HW303
|
$14 |
Great for wiping off paste
|
| 11 |
Acetone 99%
|
|
3190K16
|
$18 |
Used to clean PCB
|
| 11 |
Isopropyl Alcohol 99%
|
|
3190K809
|
$12 |
Used to rinse-off acetone
|
| 12 |
Thermometer, dual sensor
|
Amprobe TMD-56
|
08783.W | $160 |
Used to monitor PCB and air jet temperatures
|
STEP 1: Prepare PCB
- Wipe the PCB with Acetone.
- Rinse with Alcohol.
- The solder pads should all look shiny and flat and free of fingerprint marks.
- If the pads have excess solder, remove it with solder wick and repeat the Acetone + Alcohol wash.
STEP 2: Apply Paste
- Resist the urge to apply more paste on each pad than what’s shown below. A surprisingly small amount of paste is all that is needed.
- Aim for consistency in the amount dispensed. The more consistent, the more the board will look like it was manufactured on an automated process.
- For IC packages size SO, apply paste to each pad individually.
- For IC packages smaller than SO, apply a very thin continuous bead with the metal 22GA syringe tip.
If unsatisfied, wipe away paste with the fine cotton-tip applicators and start again. Careful not to wipe the pads you’ve already done. Make sure not to leave any paste on the solder mask – during reflow, it will pool together into very problematic solder balls which can cause shorts or intermittent behavior.
STEP 3: Place the parts
- Manually pick and place all the parts.
- Invest as much effort as possible to align the parts with the PCB. Part alignment will be the primary criteria by which one distinguishes a board assembled on the bench or on an automated process.
- During reflow, capillarity and surface tension properties of molten solder will help align the larger parts such as the ICs or the MOSFET.
Temperatures
For the Aoyue 866 station:
- Set preheat plate to 140-150 deg C (in the picture, the board is still warming up)
- Set hot-air jet to 230-250 deg C (in the picture, the hot-air jet stream is too hot)
STEP 4: Preheat
- Turn on the preheat plate on the Aoyue 866 station.
- After 2-4 mins you will notice a change in the paste consistency and lack of shininess. This indicated that the flux has activated.
- The board is now ready for solder reflow with the hot-air jet.
STEP 5: Reflow
- Turn on the Aoyue hot-air jet and verify that the output temperature is around 220-230 deg C.
- One by one, position the hot-air jet over each component for 5-10 sec.
- DO NOT EXCEED 15 sec per part.
- Turn the jet away, wait 20 sec and start again.
For most parts, I tend to use the smallest hot-air tip and a 10% air flow setting.
For the largest parts such as the power MOSFET, I also use the smallest tip and an air flow at 25%. If it is still not ‘taking’, I recommend switching to a larger tip rather than continuing to increase the airflow. This is because more than 25% air flow setting tends to blow the component off alignment.
If solder bridges form between IC pins, touch the pads with solder wick while under the hot-air jet. Be careful not to heat the part for longer than 15sec under the hot-air jet!
If you need to re-work a part while the board is hot:
- Don’t use paste, this will melt the flux and plug your syringe tip.
- Each pad should have a small bump of solder. If too much, remove with wick, if too little, touch the pad with some fine gauge solder wire, all this while under the hot-air jet.
- Apply a dab of tack flux and position the part. The tack flux will hold it in place. Reflow with hot-air jet.
STEP 6: Clean the flux residue
- Notice the pools of flux on the previous picture. All the flux will need to be cleaned off.
- First, spray the board with flux remover. Be generous here. Use an acid brush to speed up the process. My favorite is the 3M Novec Flux Remover (Chemtronics Flux-off works well for old-school rosin-based fluxes).
- Next, rinse the board with Alcohol in order to remove the white flux residue that the flux remover created. Be generous with the Alcohol.
Rolf Niemand
2017/1/26 17:02:27
I really appreciate your content.Good resource for new beginners.