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Ideal Guides for SMT Assembly Reply 2017-01-24 17:02:27
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Mikel Rodriguez

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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.
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Statement: This post is only the personal view of the author and does not represent the opinions of ALLPCB.com.

Rolf Niemand

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I really appreciate your content.Good resource for new beginners.
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