Special Seminar III

This seminar is dedicated to students taking senior design project or attempting to take it very soon

The focus is on practical aspects of engineering necessary to complete senior design project successfully and professionally

SMT in Practice

Dr. Peter W. PachowiczDepartment of Electrical and Computer Engineering

Volgenau School of Engineering

Goals

To use modern technology available only as SMT components

To understand SMT

To learn SMT soldering and rework techniques

To handle SMT devices with ease

To use senior design lab resources properly and efficiently

Topics

ECE SMT fabrication capabilities

SMT components

SMT tips

SMT soldering techniques

SMT soldering tips

SMT soldering tools

Rework for SMT

Note: Please listen carefully and keep notes. There will be a lot of information provided that is not on these slides.

Additional Resources

Books:

“Mastering Surface Mount Technology,” by V. Himpe

“Surface Mount Technology for PC Boards,” by G. Blackwell & J. Hollmon

On-Line articles:

Intro to SMThttp://www.ece.msstate.edu/~reese/senior_design/pcb/Intro_to_SMT.pdf

Search youtube.com for soldering tips

ECE SMT Fabrication Capabilities

PCB fabrication

Solder paste disposer

IR soldering and rework

Hot air rework, hot tweezers

Quality inspection

Why SMT ?

THT is phased out (!!!)

Reduction of size when compared with THT

Weight reduction

Availability of advanced multi-pin chips

Ability to populate a PCB on both sides

Reduction on parasitic inductance, capacitance and resistance

Better noise immunity (due to shorter interconnections)

Easier to repair/rework (but when using proper tools)

Lower manufacturing cost (less floor space, small machines)

Lower overall cost (However, components can be more expensive, but

there is no need to drill so many holes)

SMT in Senior Design Projects

Limited availability of THT chips! This causes significant problems for teams

Use of advanced Off-The-Shelf modules is frequently necessary Consider GPS, sensors, power management, etc.

Many components are only available as SMDs

New components are only SMDs (!!!)

You need to avoid certain packages (BGA)

More flexibility during the design stage Advice: Use a multi-pin MCU to speedup

your design effort

Less space required

Easier to rework than THT components

SMT Dimensioning

Imperial units are still in use for PCB design

US is the only country left in the world using Imperial units

Think about implications and confusions

Metric units are used most of the time in SMT

Migration to metric is accelerated due to miniaturization

Virtually all new SMDs are metric-dimensioned (!!!)

Plan accordingly when using PCB design software1 mil = 0.0254 mm

Divide your PCB design process into stages and work w/ single dimension units (do not mix them up); e.g.,

1. Sketch board contour inch (or metric) units

2. Assign THT footprints inch units

3. Assign SMD footprints metric (or inch) units

4. Make connections metric (or inch) units

SMT Component Dimensions

Units have implications on device dimension type and indirectly on pad/footprint size

Standardization; e.g.,

Inch notation 1206 implies 0.12”x0.06” (3.048x2.032 mm)

Metric notation 3216 implies 3.2x1.6 mm (0.126”x0.063”)

However, inch 1812 is very similar in size to metric 4532

Plan to work with chips at least 3mm long

Good luck if you decide to work with ultra-small chips

Plan to work with multi-pin chips of pitch >0.5mm

Manual work with pitch 0.5mm or lower is very difficult

If so, allocate a lot of time, be very patient, and learn soldering on a separate breakout board first (!)

Plan to work with larger pads – for easier soldering

SMD Resistor Technology

Standard packages:

0805 (in); 2012 (mm): 0.125W, 150V

1206 (in); 3216 (mm): 0.25W, 200V

1210 (in); 3225 (mm): 0.50W, 200V

1812 (in); 4532 (mm): 0.75W, 200V

Bigger chip higher the cost

Larger chip ratings are really high but the size will make you comfortable to place and solder them

Resistor technology:

Thin film – are of better quality, better tolerances, lower induced noise (E24 and E48 series)

Thick film – low cost, better for very high resistance values >5 MΩ (E12 series)

SMD Resistor Values

No longer color coded

Worse, certain manufactures avoid stamping values

Use a 3 or 4 digit stamped code – it’s a bit of a madhouse

Coding:101 10 followed by 1 zero = 100 Ω

100 10 and 0 zeros = 10 Ω

103 = 10 kΩ

2212 = 22.1 kΩ

10R Most common notation = 10 Ω

8R2 = 8.2 Ω

R100 or R10 = 0.1 Ω

General denomination code

3 digit indicates a 5% part

4 digit indicates a 1% part or better

SMD Capacitor Technology (1)

Standard packages – the same as for SMT resistors

Capacitors have a third dimension

(their thickness) depending on the cap value

Capacitor values are not stamped – don’t mix chips

Ceramic capacitors (general use) come in different grade

NPO, X5R, X7R, Y5V, etc. depending on dielectric material

Highly depend on an application and have tolerances >=10%

Film capacitors

Better quality with tolerances within 5%

Mostly for signal coupling applications in analog electronics

SMD Capacitor Technology (2)

Tantalum Capacitors

Needed, for example, to cut noise in a MCU package

May have different footprints

They have internal fuse and self extinguishing case material

Because, they can malfunction, explode, and/or set fire

They are polarized. It’s important not to reverse voltage on them.

Reverse voltage of 1.5V is enough to cause a catastrophic failure

Electrolytic capacitors

Generally more expensive than ones in THT when ordered in small quantities. Still price is low if you need few of them.

Check footprint spec for one you need (typically: CAN-SMD)

SMD Diodes and Transistors

Diode packages:

SOD-xxx Small Outline Diode

DO-xxx Outline diode

Can be two-, three- or more pin packages (SOT-xxx)

Transistor packages:

SOT-xxx Small Outline Transistor

DPAK and D2PAK

Have minimum three pins (can have more, even to eight pins)

Avoid sizes below 3mm (or <2.5mm)

Attention: diode size can be below 1mm

SMT Integrated Circuits

Contains by far the largest variety of packages

Footprints vary

Dual row style packages

Four sided packages

Complex BGA and QFN style packages

Some of these packages cannot be handled without specialized equipment !

However, there are tricks to solder them properly (ask before working on your own)

I will highlight selected packages which

1. You can solder relatively easily

2. You will need to be very patient

3. You should try to avoid, if possible

SMT IC – Easy Category

SOIC package (Small Outline IC)

A step forward from DIP

SOIC-14 (specifies number of pins)

Pitch 50 mil (1.27mm)

TSOP type II Pins on the longer side, pitch 1.27mm

(For larger pin#: pitch 0.8mm or 0.65mm !)

QFP/TQFP package (Thin Quad Flat Package)

For low pin# package: pitch 0.8mm to 0.65mm

All of above can be soldered by hand (!)

SMT IC – BeVeryPatient Category

SSOP with pitch 0.65 mm

QFP/TQFP package with larger number of pins (>=64 pins)

Pitch 0.5 mm (you need to be extremely patient)

This is an unavoidable package for many high-pin# MCUs

Practice soldering these chips before the final job

Practice using small breakout boards (cost $5-$7) rather than working directly with your PCB

They can be soldered by hand and traditional iron when: You have experience, work very slowly, and use right tools

HOWEVER:

Soldering should be done in the ECE fabrication lab using solder paste and IR soldering station (!)

SMT IC – TryToAvoid Category

mini-SOIC: pitch 0.5 mm and a very small chip

TSOP type I: pitch 0.5 mm and pins on the shorter side

PLCC: not a SMT package, solderable, but unreworkable

QFP/TQFP very high pin# (100 pins): pitch 0.4 mm

QFN: requires special soldering equipment or extra skills, pitch 0.5 mm and extremely difficult access

BGA: for extremely dense interconnections

All above can be soldered in the lab using solder paste and IR station – special care is needed (!!!)

SMT Component Tips

Size at least 3mm: 1206 (in) package If you are good at soldering then eventually use 0805 (in) or

2012 (mm) – they are popular and low cost

Quality storage boxes are a necessity for storage of SMT components Never store a SMT chip without/outside its own package/tape

Resistors: Buy 1% resistors no matter what (the cost difference is small)

Try to buy the E24 series resistors (common default) However, E48 series resistors are made with more values

Capacitors: Buy X7R type, if possible (caps of relatively stable parameters)

Avoid Y5V type (very unstable)

More SMT Tips

Never touch chips with your fingers – only with tweezers

Tweezers must have rounded tips

Otherwise, you may pierce through a chip

Open chip package right before soldering

Moisture absorption chips; e.g., sensors

Certain devices are sensitive to moisture – try to avoid them

They require soldering within a given time after opening a sealed package.

Usually within 24 hours

Check spec sheet before you buy

Read labels carefully

If time expires then they must be beaked first (!!!)

Zero Ohm Resistor

Use 0Ω resistors to jump traces

SMT resistor of a larger size will jump more than one trace

Be sensitive to price – large packages can be expensive

This resistor will help simplifying your PCB design

You can combine jump resistors but do it only in a series (no more than 45°angle between them)

Soldering Techniques for SMT

It’s difficult to solder SMT chips (!)

There are many methods – search the Internet and YouTube

I focus on methods that work for someone without extensive practice

You will need more than one soldering technique to use

You need a plan! (it depends on your preferences); e.g.,

First, solder most difficult parts (low pitch multi-pin SMDs)

Second, solder SMT resistors, caps, SOIC chips, etc.

Finally, solder connectors and THT chips (see Seminar I)

Have your plan written (!!!)

Each step should be approached with a single soldering technique – do not mix techniques at a single step

Soldering Tools Needed (1)

Iron for soldering larger items (THT, connectors)

Small tip iron for soldering SMT chips

Best if short in length for precision touch

Fine point tip vs. chisel tip

Solder paste disposer

IR soldering station

Soldering Tools Needed (2)

Cleaner: Isopropyl alcohol

Glue and toothpicks

Quality tweezers (ESD rated, stainless steel, rounded tips)

Dental picks

Flux pen – avoid flux dispensers (too messy)

Solder wire – as thin as possible

Solder paste and needles

Vise to hold your board; Fume extractor

Magnifying lamp or Visor Magnifier

Preparation Before SolderingClean milled PCB from dust, oil residues,

and oxidation

This will greatly improve solder wetting

Isopropyl alcohol and a toothbrush

Do not use acetone in the ECE lab

Remember, re-oxidation starts shortly – you only have about one hour time

Place your board in a vise

Understand how you can gain access from different directions

Understand how you can adjust vise to improve the access

Prepare the iron

Define temperature to work with (!!!)

Soldering Methods

We will discuss two classes of soldering methods, when applied to SMT devices:

1) Traditional manual hand-based methods

2) Modern semi-automated IR soldering methods

(with equipment available in the ECE Fabrication Lab.)

Manual SMT Soldering Technique 1

Using iron and solder in coordination

Process:

1) Apply flux to footprints only

2) Place (glue) chips to the PCB

3) Execute 1-2-3-4 soldering steps

Slide and point iron at pin-2-pad contact

Wait Touch with solder wire Release

Requires an extreme precision when used on small pitch multi-pin chips. It’s more suitable for SMT resistors, etc.

It’s easy for solder to spread around and join pins together

If so, do not panic and use solder-wick to remove excess solder

circuitwork.com

Manual SMD Soldering Technique 2

Using iron only – an effective technique

Process:

1) Apply flux to footprints

2) Apply solder to footprints (not too much, not too little)

Simply move iron with solder deposit through all pads

3) Inspect solder height across all pads – must be uniform

4) Clean and reapply flux

5) Place and hold/press a chip

6) Execute 1-2-3 soldering steps

Push iron down vertically on a single leg

Wait Release

YESNO

Manual Soldering Tips (1)

Do not use Technique 2 when soldering:

Chips of a block structure such as resistors, capacitors, etc.

You can easily damage copper trace and pads

Eventually, you can:

1. Apply solder on one pad only (!)

2. Solder a chip only on one side, so it will stay firm

3. Finally solder a chip on the other side

First, iron was applied here What is going to happen if you Push iron right here after the first step?

Manual Soldering Tips (2)

Use narrow solder-wick to remove any excess of solder which joins adjacent legs together

Run a sharp knife tip between pins to be sure there is no solder residue touching another pin

Using Hot Air Station (more about it later)

Will burn glue AND may blow chips out

Joint quality

??? ???

Modern Soldering Technique 1

Applicable to all SMDs

Process:

1) Apply flux to cleaned footprints

2) Glue (if needed) a chip to PCB (do not glue over flux residue)

3) Dispose solder paste at the edge of chip leg (dots or a line)

4) Solder using IR soldering station

5) Use a solder-wick to remove excess solderPaste line Paste dots Final resultGlue

Glue

Modern Soldering Technique 2

The same as the Technique 1, but

Dispose solder paste on footprints first, and next

Align and place a chip over

Difficult to do it precisely without spreading solder paste around

Solder paste disposed: line vs dots Chip placed and aligned

After IR soldering After excess solder removal

Modern Soldering Technique 3

Based on pre-soldering the footprints and following with IR soldering

Process:1) Apply flux to footprints

2) Apply solder to footprints (not too much, not too little)Simply wet the footprints by moving a larger iron with solder deposit through all pads

3) Inspect solder height across all pads – must be of equal height (!!!)

4) Clean and reapply flux

5) Use IR soldering station

(Optionally, push the chip down

using tweezers)

YESNO

Soldering QFN Chips

Soldering these chips is possible but requires a special technique

We soldered many of them already

Ask for help/instructions

Solder Paste Disposer

Zephyrtronics ZT-5100 instrument

Uses externally supplied compressed air (pressure up to 90psi)

Activation via a foot pedal

Parameters to work with:

Air pressure (do not exceed 70psi)

Disposal time (Use extremely small time for small diameter needles and dots)

Back suction

Test your skills first

Solder Paste Disposal Tips

Depending on your hand coordination – small dots are preferred rather than a line (!)

Small dots properly disposed give the best results

Remember: “Less is better”

Solder paste must be of room temperature

Remove from refrigerator minimum 2 hours earlier

Use small diameter needle

Practice on separate piece of cardboard

Experimentally test dispenser parameters

Touch the pad decisively – do not hold the needle above

If there is too much mess, clean the PCB and start over

Do not touch the solder paste with bare hands (!!!)

IR Soldering Station

Detail instructions and tips will be provided at the time you will use it

Safety first:

Bring your sunglasses (!) – the light is very intensive and you need extra protection in addition to the IR screen

Wait until the board cools down – this will take several minutes

More Soldering Tips (1)

Design your PCB is such a way that chip placement has a pattern suitable for easy soldering

Align all resistors, caps, etc. in one direction (maximum two) – horizontal or vertical

The same applies to multi-pin chips

In summary, have an order in chip placement on your PCB

This will speed up soldering and you will not miss joints

Watch out for placement errors (it happened to me)

Soldering should be done by one person (if you really need then second person can quietly assist)

Do not touch chips with your fingers !!!

They are already tinned

More Soldering Tips (2)

Do not overheat SMT components

Many are sensitive to heat (even resistors and capacitors)

Remove the iron immediately after smoke from the flux disappears

Approximately within 2sec after solder melts

Do not tin-plate PCB

Seriously, this is not needed for SMT PCBs

Prototype PCBs do not need tin plating anyway

Rework for SMT (1)

More difficult than soldering SMT components

Do not use excessive heat/force no matter what – remember

Copper traces are glued

Components are sensitive to excessive heat

First step: Remove solder using solder-wick (very narrow)

Second step: Use proper tool to remove entire chip

Hot Tweezers – for removal of:

Resistors, capacitors, etc.

Dual-row multi-pin chips

You need to select proper adapter,

if available

Rework for SMT (2)Hot air station – for removal of:

All-around multiple-pin chips

Dual-row multiple-pin chips

Other focused rework

Use proper adapter

IR soldering station – for removal of:

Larger chips

Non-typical chips, etc.

First, heat up PCB from bottom to 150°C

Next, use top heater to gradually increase temp

Remove a chip with thermally isolated tweezers

Rework Tools

Rework tools are available upon request or can be accessed in the ECE Fabrication Lab.

Contact Lab TA/Technician

Tools must be returned immediately

You need to babysit these tools – they are sensitive to abuse

Safety first !!!

NEVER work with hot air tool at temp >400° OR lowest air flow

You will damage this valuable station

It may melt in your hand (!)

Always use sunglasses in addition to IR screen when working with IR soldering

Questions