<=========================================================>
<= =>
<= "PCB ASSEMBLY TECHNIQUES" =>
<= =>
<= "TEXT ONLY" ARTICLE VERSION =>
<= =>
<= by =>
<= Harvey Twyman =>
<= =>
<= The relavent graphics files are =>
<= included within each topic =>
<= =>
<= A Web based version is available as an =>
<= E-mail Course and is described below:- =>
<= =>
<= www.Twyman.org.uk/PCB-Techniques =>
<= =>
<=========================================================>
<=========================================================>
<= =>
<= COPYRIGHT NOTICE =>
<= =>
<= I give my permission for this document to be copied =>
<= and distributed in any Internet media such as =>
<= Newsletters and Ezine Articles etc. =>
<= =>
<=========================================================>
<=========================================================>
<= =>
<= The Author: Harvey Twyman =>
<= E-Mail the Author: harvey_twyman@yahoo.com =>
<= About the Author: http://www.Twyman.org.uk/CV =>
<= =>
<=========================================================>
I'm sure you've noticed the rapid decline in the
availability of certain dual-in-libe components.
The devices might still be available but only in their
Surface Mount form.
So how do you construct PCBs using these *tiny* devices?
To enable you to keep up with the technology, this article
contains a series of special techniques that only require
Low Cost Hand Tools.
It's mainly intended for Prototype PCB designs where
manufacturing costly Plated-Thru-Hole PCBs for a "One-Off"
is uneconomic.
+----------------------------------------------------------+
+ => This article covers the following topics:-
+----------------------------------------------------------+
=> Working Practices;
=> Good and Bad Joints;
=> Soldering Rules
=> SMT Pads
=> VIA Holes
=> IC Sockets
=> Soldering SMTs
=> Fine Pitch SMTs
=> Handling SMTs
=> Removing SMTs
=> FAQs
+----------------------------------------------------------+
+ => Working Practices:-
+----------------------------------------------------------+
We know the term "Design for Manufacture", however here we
need to "Design for Prototype".
The Techniques described in this Article require certain
"Working Practices". The Topic Link below describes these
plus information on the Hand Tools required.
These Techniques described later can only be SUCCESSFUL
if the guidelines below are followed:-
=> CAD Considerations
=> Lighting
=> Cleanliness
=> Hand Tools
* CAD Considerations *
The PCB Design will need modification at the CAD Level
to enable these techniques to be WORKABLE:
=> VIA PADS need to be at least 55 to 60mil diameter
IF the drilling is to be done BY HAND.
=> SMT Rectangular Pads MUST be LONGER so that they
protrude out and are VISIBLE even when the device
is sitting on the pad. This is so that you can observe
and create a reliable joint when manual soldering.
<* Lighting *>
Use 2 standard 60W DESK LAMPS one either side of your
work area and placed low to the bench to give maximum
intensity. This will illuminate the work area adequately
and not produce SHADOW.
Visual Inspection is the KEY to success.
Most faults are VISIBLE with the lighting described above.
With these techniques you have to rely TOTALLY on
YOUR OWN EYES for quality control.
THE MORE LIGHT YOU HAVE, THE MORE YOU'LL SEE
<* Keeping Everything Clean *>
Before any soldering work is done on the pads they need
to be cleaned with IPA (SEE BELOW) using a soft toothbrush
style brush to remove oxides.
The PCB will re-oxidize within an hour, so clean JUST
BEFORE you intend to solder.
IPA (Isopropyl Alcohol) is used in AEROSOL SPRAYS as a
diluent and solvent and is a common component of such
products like MAGNETIC TAPE HEAD CLEANING SOLUTIONS.
It has the advantage that it EVAPORATES quickly.
<* Hand Tools *>
Standard tools are adequate for the job, accept for the
ones mentioned below:
=> Soldering Iron
Modern Temperature Controlled types are adequate.
The Solder Tip needs to be pointed thus giving access
to individual SMT joints.
Pointed Tips when used on SMT Pads have an added
advantage of less heat capacity. So once placed on
an SMT pad the temperature drops rapidly. This has
the advantage that quick joints can be made on the
tiny pads without them getting damaged.
=> Tweezers
DON'T use SHARP ones! These may DAMAGE delicate SMT
resistors. Flat Ended types are preferable as rounded
ones can't be used to pick up the smaller SMT devices.
Solder Braid Use the narrowest grade you can. Using
fat ones take too long to heat up which may damage the
pads.
=> Inspection Eyeglass
These are available in different magnifications.
A X8 magnification is adequate. Using any higher doesn't
display enough board area. There are some types available
with a built-in measuring facility. These are important
for checking actual PCB pad and drill hole sizes.
=> Cut and Crop Tool
Also known as Cut and Clench or Cut and Crimp The tool
automatically squashes the wire flat before cutting
above it. This tool is used in the VIA Technique described
later.
=> Heat Gun
This type of tool is required for removing (reworking)
SMT components, This is the only tool that needs a certain
amount of training to use successfully. This will be
described later.
Heat Guns come in 2 forms:-
=> Paint Strippers
=> Hot Air Tools
=> Paint Strippers
These are relatively cheap and generate a wide column of
very hot air with limited control over the air flow. These
are more suited for removing the larger SMT devices or where
heating a larger surface area of the PCB is required.
=> Hot Air Tools
These are designed to heat small areas and usually have a
proper air velocity control to regulate air flow and thus
the temperature. Thus these are more suited for removing
the smaller SMT devices.
Generally though, as we're more interested here in a Low
Cost solution, the techniques described later will be
referring to the Paint Stripper type of tool.
+----------------------------------------------------------+
+ => Good and Bad Joints:-
+----------------------------------------------------------+
The knowledge of how to create Good and Bad Joints is
Essential. For those of you already familiar with the
subject, please bear with me as beginners may be
participating.
The reasons for a bad joint:
=> The copper cladding is oxidised
- Clean with non-abrasive methods:
--- Scrub with small brush using IPA solvent
--- Re-solder within a few minutes before oxidisation
occurs again
=> Soldering iron bit needs Wetting
- Press the tip into the special wetting paste provided.
=> Not applying the solder tip and solder to the joint
simultaneously
=> Not enough heat being applied to both the component leg
and pad.
<* Facts about PCBs and Soldering *>
=> The copper cladding is glued to the fibre glass laminate
=> The melting point of the solder is 180°C
=> The melting point of the glue is 150°C
Therefore when the solder is molten, the copper cladding
is "floating" on molten glue!
The reason why the glue's temperature is LOWER than the
solder's is BY DESIGN.This is to insure that the copper
cladding's EXPANSION is UNIMPEDED by the glue.
This means that PCB pads are VERY VULNERABLE during soldering.
+----------------------------------------------------------+
Graphical Examples of:-
=> A Badly Soldered Joint: bad-joint.jpg
=> A Good Soldered Joint: good-joint.jpg
=> PCB Construction: PCB-makeup.jpg
+----------------------------------------------------------+
+----------------------------------------------------------+
+ => Soldering Rules:-
+----------------------------------------------------------+
The Technique of How to Solder is documented in the Topic
Link below. Again, for those of you already familiar with
them, please bear with me.
=> Take care not to press TOO HEAVILY on the pads, otherwise
the pads will lift up and will be very difficult to repair.
=> Moving components BEFORE the solder has solidified may
again either LIFT the copper tracks or cause a DRY JOINT
explained previously.
=> Apply the soldering iron tip and solder to the joint
SIMULTANEOUSLY and keep there until the solder flows
correctly over the joint.
=> Remove the soldering iron tip IMMEDIATELY after the smoke
from the flux within the solder disappears approximately
2 seconds after the solder melts.
=> Only use SILVER-LOADED SOLDER for SMT joints (2% Silver)
<* Tin Plating *>
=> Don’t Tin Plate your PCB UNLESS the plating solution is
NEW!
=> The solution can become heavily contaminated from being
used just a few times. These contaminants make it MUCH
MORE DIFFICULT to solder.
=> This rule is ESSENTIAL particularly for SMT PCBs.
=> Prototype PCBs DON'T NEED Tin Plating anyway. They still
remain clean looking YEARS LATER.
=> They WON'T go GREEN unless exposed to severe climatic
conditions.
+----------------------------------------------------------+
Graphical Example of:-
=> Not Using Tin Plating: tin-plating-example.jpg
+----------------------------------------------------------+
+----------------------------------------------------------+
+ => SMT Pads:-
+----------------------------------------------------------+
Surface Mount Pad sizes seem to vary with every designer.
They all have their own critia. Here we are Designing for
Prototype so need to have pads that are Longer than normal.
<* PAD Specification for SMT Components *>
The SMT Pad MUST be designed in the CAD STAGE SPECIFICALLY
for HAND SOLDERING and:
=> CAN be the same WIDTH or wider as the SMT leg.
=> MUST be LONGER than the IC LEG positioned on it, to
allow ease of VISUAL INSPECTION and hand soldering of
the joint.
+----------------------------------------------------------+
Graphical Example of:
=> SMT "Gull Wing" IC Leg: gull-wing.jpg
=> SMT "J-Lead" IC Leg: j-lead.jpg
+----------------------------------------------------------+
+----------------------------------------------------------+
+ => VIA Holes:-
+----------------------------------------------------------+
Double Sided PCBs made as a Prototype probably won't have
Plated-Through-Holes. So how can you cope with VIA Holes?
Some of the tracks on a double sided PCB connect from one
side of the board to the other. Most of them connect via a
component’s leg, but some don’t. These connections that
don’t are called VIAs.
On commercially manufactured PCBs these VIAs are produced
by a system called Plated-Thru-Hole. This process electro
plates all the inside of the holes with copper.
However a simple technique is described below that doesn't
require the EXPENSIVE plated-thru-hole technology.
<* VIA Connection Technique *>
A special Cut and Crop Tool is used to produce a reliable
connection through the VIA hole pads.
There are various names used for this tool:
=> Cut and Clench
=> Cut and Crimp
Using 24 SWG TINNED COPPER WIRE:
=> Place the wire in the VIA hole. (Hole should be 0.7mm
Diameter)
=> Use the cut and crop tool to produce the result.
=> Solder both sides of the wire link.
+----------------------------------------------------------+
Graphical Example of:-
=> VIA Connection Technique: via-technique.jpg
+----------------------------------------------------------+
+----------------------------------------------------------+
+ => IC Sockets:-
+----------------------------------------------------------+
Without Plated-Through-Hole Technology, IC Sockets have to
be soldered both sides to act as VIA connections.
There are 2 COMMON types of IC SOCKETS in use today:-
=> DIL - Dual-in-Line
=> PLCC - Plastic-Leaded-Chip-Carrier
<* DIL SOCKETS *>
Available in various sizes from 8 to 64 pins. However their
QUALITY can vary. By FAR the BEST is the TURNED PIN TYPE.
Unfortunately they're the MOST EXPENSIVE. However they do
give BETTER ELECTRICAL CONTACT with the IC leg, which is
VERY IMPORTANT for RELIABILITY.
When creating a DOUBLE-SIDED PCB tracks need to be connected
to the DIP socket from BOTH sides of the board. The SOLDER
SIDE connections are EASY, but the COMPONENT SIDE connections
get OBSTRUCTED by the socket's PLASTIC BODY, making soldering
difficult if not IMPOSSIBLE.
A SPECIAL JIG is used to EXTRACT the pins from
their FORMER. This makes the soldering of the COMPONENT SIDE
pin MUCH easier.
The JIG described is for EXTRACTING the DIL IC SOCKET PINS.
The IC sockets used are a 16 WAY TURNED PIN type. These are
used as they represent the CHEAPEST WAY of purchasing the
socket pins.
The JIG shown below can be constructed from a 4" VICE MOUNTED
VERTICALLY. (contact: harvey_twyman@yahoo.com for details.
<* PLCC SOCKETS *>
Available in various sizes from 20 to 84 pins. The SURFACE
MOUNT type described here are purchased WITH a CENTRE SECTION
as shown in the LEFT PHOTO below.
The CENTRE SECTION needs to be REMOVED by gently pressing it
out with a SCREWDRIVER. The result is shown in the RIGHT PHOTO
below.
The reason for REMOVING the PLCC socket's centre is so that
VIA HOLE LINKS, discussed PREVIOUSLY, can be placed in the
CENTRE of the PLCC socket, i.e. UNDERNEATH the PLCC device.
The socket RAISES the PLCC device OFF the PCB ENOUGH to
accommodate for the HEIGHT of the VIA LINKS but ONLY when the
CENTRE SECTION of the PLCC socket is REMOVED.
If PLATED-THRU-HOLE TECHNOLOGY isn't available to you, but you
need to produce DOUBLE SIDED PCBs, then this technique described
is a solution.
+----------------------------------------------------------+
Graphical Example of:-
=> Pin Extraction Jig (Upper): pin-extractor-1.jpg
=> Pin Extraction Jig (Lower): pin-extractor-2.jpg
=> PLCC Socket With Center: plcc-skt-w.jpg
=> PLCC Socket Center Removed: plcc-skt-wo.jpg
+----------------------------------------------------------+
+----------------------------------------------------------+
+ => Soldering SMTs:-
+----------------------------------------------------------+
Surface mount pads need to be tinned with solder prior to
mounting the component.
Commercial PCB manufacturers use a process called HOT
SOLDER LEVELLING which produces a "flatter" solder joint.
If this process is not available there is an ALTERNATIVE
TECHNIQUE using SOLDER BRAID to achieve the same result.
=> Scrub the pads with a small brush while applying IPA
SOLVENT. Apply the solder (MUST use the 2% SILVER Type)
to the SMT pad.
=> Place the solder braid on the pad NEAR to the track
connecting the pad.
=> Apply the soldering iron to the top of the braid resting
on the pad. While the solder is molten, SLOWLY DRAG
the braid AWAY from the pad in the OPPOSITE direction
to the track connecting it.
=> TAKE CARE when DRAGGING the solder braid. As stated
previously, the pad is FLOATING on molten glue which
is holding the pad on the board. This is the reason for
dragging the braid AWAY from the track that connects
to the pad. The track prevents the pad from moving out
of position.
+----------------------------------------------------------+
Graphical Example of:-
=> Solder Levelled Pad: solder-levelled-pad.jpg
=> Levelling Technique: solder-levelling.jpg
+----------------------------------------------------------+
<* Soldering a 2 Padded SMT Component *>
=> Tin BOTH Pads
=> Solder Level only ONE of the 2 pads as described previously,
leaving the other pad just TINNED
=> Hold the sides of the component with tweezers so its
terminals aren't OBSTRUCTED
=> Apply solder and heat the TINNED pad
=> While the solder is molten, move the component to align
on BOTH pads
=> If alignment is proving to be DIFFICULT then remove the
iron OFTEN to prevent the pads from being DAMAGED by the
excessive heat
=> When the component is correctly ALIGNED on the pads then
solder the LEVELLED PAD properly
=> Finally RESOLDER the first pad AGAIN as almost certainly
that joint will be DRY from all the extra heating during
the alignment procedure
+----------------------------------------------------------+
<* Soldering a Multi Padded SMT Component *>
=> Tin ALL the component's pads
=> Solder Level all pads EXCEPT one CORNER pad
=> Hold the component with tweezers so that its terminals
aren't OBSTRUCTED
=> Apply solder and heat to the UNLEVELLED corner pad
=> While the solder is molten, move the component to align
on ALL its pads
=> If alignment is proving to be DIFFICULT then remove the
iron OFTEN to prevent the pads from being DAMAGED by the
excessive heat
=> When the component is correctly ALIGNED on ALL the pads
then solder the DIAGONALLY OPPOSITE corner pad and
RE-CHECK alignment
=> REPEAT the task of soldering the corner pads until
alignment is correct
=> Now solder all the OTHER pads
=> Finally RESOLDER the ORIGINAL 2 corner pads AGAIN as
almost certainly those joints will be DRY from all the
extra heating during the alignment process
+----------------------------------------------------------+
+----------------------------------------------------------+
+ => Fine Pitch SMTs:-
+----------------------------------------------------------+
Components are shrinking in size all the time so the ability
to be able to solder these Tiny devices "By Hand" is
essential.
The fact that SMT components are getting smaller is common
knowledge. The problem is that the LARGER SMT components are
becoming UNAVAILABLE TOO!
Therefore the techniques described previously HAVE to cope
with the SMALLER more common components as well.
<* A Special Technique *>
=> Normally the device pads are soldered INDIVIDUALLY.
However this is NOT POSSIBLE with these FINE PITCH devices
as the pitch between the device's leads are TOO SMALL.
=> Therefore place the soldering iron BETWEEN 2 ADJACENT leads.
=> Apply solder and heat as normal. REMEMBER not to keep the
heat applied for VERY LONG, particularly as the pads are
SO SMALL, the GLUE will MELT holding the pad.
=> This will almost certainly SHORT CIRCUIT the 2 leads
TOGETHER.
=> The excess solder can be REMOVED using SOLDER BRAID
described in the SOLDER LEVELLING Technique mentioned
previously.
+----------------------------------------------------------+
+ => Handling SMTs:-
+----------------------------------------------------------+
Handling SMT devices correctly is essential to prevent both
Damage and Contamination.
The PHYSICAL SIZE of SMT components demands the use of
TWEEZERS. However SMT components are VERY FRAGILE and thus
a BLUNT NOSE Type is essential. Sharp tweezers can easily
damage SMT resistors.
The resistive film on SMT resistors for example is VERY THIN
and damage could either CHANGE the value or even make it
completely OPEN CIRCUIT!
Resistors come in various physical sizes:-
=> 1206 - 0.12" by 0.06" This size is the easiest to handle
=> 0603 - 0.06" by 0.03"
=> 0402 - 0.04" by 0.02" These will blow away if breathed on!
The component value is printed on them in the form:
=> 1K5 1500 Ohm
=> 390R 390 Ohm
=> 1M8 1,800,000 Ohm
Capacitors are available in the same sizes as above but
have NO IDENTIFICATION marks on them AT ALL!
<* So DON'T MIX THEM UP! *>
It's important that you use the largest of the SMT
components available, for ease of handling.
Some devices are so small and light that just BREATHING ON
THEM will send them flying away! The smaller devices may
damage more easily as described.
The pads on SMT components are pre-tinned and SEALED in
airtight packaging. This is to prevent the pads OXIDISING.
They also have use-by dates similar to food stuffs.
Therefore:
<* ONLY OPEN THE PACKAGING JUST BEFORE USE *>
The use of CAROUSELS to hold lots of UNPACKED SMTs is
NOT ADVISED for the above reasons.
ALL SMT components MUST ONLY be touched with TWEEZERS!
Fingers will CONTAMINATE them with grease, which will be
very difficult to remove and subsequently DIFFICULT to
solder.
ELECTROSTATIC DAMAGE can occur when CMOS inputs are
exposed to high voltages as low as 60 Volts. However it’s
more likely to be KVolts.
The use of WRIST STRAPS and EARTH MATS are essential while
handling any CMOS device.
When handling devices away from the electrostatic-free area
ALWAYS:-
=> Hold the device tightly in the palm of your hand,
keeping its pins discharged through your damp skin.
+----------------------------------------------------------+
Graphical Example of:-
=> SMT Resistor: smt-resistor.jpg
+----------------------------------------------------------+
+----------------------------------------------------------+
+ => Removing SMTs:-
+----------------------------------------------------------+
This is called REWORK in the manufacturing trade, as the job
has to be repeated. Reworking a job costs a company MONEY.
Therefore it’s important to get things right every time by
monitoring QUALITY at all stages of assembly to REDUCE the
necessity for rework.
EXCESSIVE HEAT during rework may damage the:-
=> Components
=> The PCB's PADS
=> De-Laminate the Fibre Glass PCB itself i.e. a bulge appears
in the PCB due to trapped gases expanding.
Therefore priorities have to be established when reworking
as to which is more important, saving the COMPONENT or the
PCB’s PADS.
The PADS are usually treated MORE IMPORTANTLY than the
COMPONENTS. PCB repair is always MORE DIFFICULT and TIME
CONSUMING when compared to the cost of purchasing new
components.
<* Removing a 2 Pinned SMT Device *>
=> Place the soldering iron tip "flat" over the 2 component pads.
=> Place extra solder to the tip to improve "thermal conduction".
The "surface tension" of the extra molten solder will "suck"
the component off the PCB and onto the tip,
<* Removing Multi Pinned SMT Devices *>
There are several options, but the CHEAPEST are discussed below:
<* Using a HEAT GUN *>
This may cause DAMAGE to the surrounding components and PCB if
TOO MUCH heat is applied. This can be a VERY EFFECTIVE method
if the surrounding area is PROTECTED from the heat. This can be
achieved by either using special HIGH TEMPERATURE SELOTAPE
availalble or by keeping the Heat Gun IN MOTION over the PCB
area to control the temperature.
As long as care is taken to LOCALISE heating, the only PROBLEM
will be that the solder on the ADJACENT SMALL COMPONENTS may
ALSO become MOLTEN. They may even get BLOWN AWAY! However as
long as the heat gun has a WIDE funnel of air, the air pressure
shouldn't be enough for this to occur.
Once the solder is MOLTEN the component can be either taken off
with TWEEZERS or more easily by TAPPING the PCB on the bench.
However BEWARE, this may also DISLODGE other adjacent components
that have molten solder.
<* CUTTING the chip's LEGS OFF *>
This method involves sacrificing the component, to save the PCB:-
Cut all the legs off the component using either a fine pair of
SIDE CUTTERS or if the legs are very small then a SHARP KNIFE
can be used.
Great CARE is required in not DAMAGING the PCB's PADS when
pressing down with the knife.
Using SOLDER WICK, prepare the pads for a new component by
applying the SOLDER LEVELLING TECHNIQUE described previously.
Of the 2 methods described above the HEAT GUN method has an
advantage that:-
It's LESS DAMAGING to the PCB's pads from the SHARP KNIFE as
long as the Heat Gun is kept IN MOTION as described above.
And Finally:-
Another popular method of rework is to use a proprietry product
called LOW MELT(TM). This is a special LOW MELTING POINT solder
but DOES REQUIRE extra equipment support so isn't discussed here
in detail. Further information is available from the
manufacturers at:
www.zephyrtronics.com
+----------------------------------------------------------+
Graphical Example of:-
=> Removing SMT Resistor: rework-smt-resistor.jpg
+----------------------------------------------------------+
+----------------------------------------------------------+
+ => Frequently Asked Questions (FAQs):-
+----------------------------------------------------------+
There are always Many ways of solving the Same problem.
Here are some Frequently Asked Questions (FAQs) from
Feedback E-mails received.
=> Question:
You mention using a POINTED TIPPED soldering iron - surely
wouldn't a WIDE TIP be better?
=> Answer:
If your PCB has a SOLDER MASK LAYER then a wide tip can be
used. A Solder Mask Layer will encourage the solder to FALL
either side of the mask so reducing BRIDGEING.
This Web Page is intended for PROTOTYPE PCBs that don't have
a Solder Mask Layer. Therefore a pointed tip is easier to
solder individual SMT pads.
=> Observation:
The fine tip has a very low THERMAL CAPACITY because of it
size. Therefore the temperature drops extremely quickly when
it comes into contact with the pad.
Once off the pad it regains its higher tempereature also very
quickly. The higher thermal capacity of the rest of the bit
CHARGES up the tip quickly as well.
This DROP in tempereture has the advantage that the solder
melts quickly on the pad but the temperature dropping protects
the pad and its glue from overheating and causing damage.
=> Question:
You use SOLDER WIRE intead of SOLDER PASTE - Why?
=> Answer:
SOLDER PASTE requires a lot of handling considerations:-
=> Storage in a Refridgerator
=> Thawing out for at least 30 minutes to room temperture
=> Manual application to SMD pads using applicators is MESSY
due to its VERY THICK consistency
Manual Soldering using solder paste can ALSO cause SOLDER
BALLING where the temperature is accelerated TOO HIGH that
causes the solder to BOIL, BUBBLE and SPIT its solder balls
EVERYWHERE over the board with a possibility of causing SHORT
CIRCUITS. However by using SOLDER WIRE - NONE of these occur.
=> Question:
Wouldn't it be better to use a HOT AIR PENCIL instead of a
Soldering Iron?
=> Answer:
Hot Air Pencils are DIFFICULT to set up in terms of their
AIR TEMPERATURE. The Air velocity controls are usually very
fiddly to adjust the flow to achieve the correct temperature.
If the air pressure is set TOO HIGH, it can be strong enough
to BLOW the SMT devices OFF their pads.
You are usually heating an AREA of the PCB so it takes longer
to melt the solder. However a SOLDERING IRON solders quickly
WITHOUT all the other problems mentioned.
+----------------------------------------------------------+
Graphical Example of:-
=> An Example PCB: Wind_Chill_PCB.jpg
+----------------------------------------------------------+
=> End of Article
+----------------------------------------------------------+