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Mounting PCBs.

Contents


Introduction.

This is a description of how I usually mount PCBs.
A lot of the information is based on my personal preferences and should not be considered "hard facts".
The exception is where electrical safety is mentioned.


Tools.

Everybody wants professional tools, but most hobbyists can not justify the expense.
It can be tempting to buy second-hand professional tools as they "last forever" or can be repaired if necessary.
Check spare parts prices first.
A tip for a professional soldering iron may cost more than a decent hobby soldering iron.
If you are in doubt if you really need a particular tool, start by buying a cheap one and upgrade later if you use it a lot.

Soldering iron

Make sure that the soldering iron you use is safe and legal to use where you live.
Do not buy it from a dubious web-shop - it may have the required safety markings without being safe.
A safe soldering iron does not "tickle" your fingers when you use it - not even the hand holding the solder-wire.

This list some suitable types:

The most common tip shapes are "pencil point", "chisel shaped" and "angled face" ( descriptions from a manufacturer catalog ).
I find the "pencil point" almost useless for PCB work as there is very poor heat-transfer to the tip.
The tip I most often use for trough-hole PCBs is a 1.5 X 0.8 mm "chisel shaped". Approximately half the size for SMD PCBs.

In addition you will need a sponge for cleaning the tip and possibly a stand for the soldering iron.

A gas soldering iron may be useful in the few cases you need a LOT of heat. It is very difficult to use without breaking the PCB.

Soldering wire

I normally use 0.7 mm rosin based solder wire for through-hole PCBs and 0.5 mm for SMD.
60/40 ( sn/pb ) is the easiest to solder with, but may not be legal where you live.
Lead-free solder requires a higher temperature and the finished solderings looks matte compared to sn/pb solder.
Silver solder wire is one of the above with a silver content of 1%..2%. This lowers the melting point making it easier to solder with.
Any claim that silver solder wire "sounds" better is completely BS. It is - by the way - quite expensive.

De-soldering braid

De-soldering braid is by far the easiest way to remove excess solder from a PCB.
Sizes from 0.5 mm to 2 mm will cover most applications.
I have occasionally used the shield from a thin coax cable as de-soldering braid ( the "real thing" is better as it contains flux ).

I had some de-soldering braid from a web-shop that cost around 10% of the "real thing". One minor issue is that it does not work at all.

De-soldering pump

This is useful for removing large amounts of solder from a PCB.
I find that I spend a lot of time just keeping it clean, so I normally use desoldering braid.

Wire cutter

A type with a cutting capacity of 0.4  to 1.2 mm copper wire and a length of 110 mm to 140 mm will cover most applications.
Some very low-cost types have a very sharp tip so they occasionally cut the PCB tracks. This can be fixed with a piece of sand-paper.

Tweezers

I generally prefer straight stainless steel types with a handle length of around 120 mm, a tip size of 1.5x1 mm for general purpose work and 1.5x0.1 mm for SMD work.

Lead forming tools

Lead forming can be done with ordinary snipe nose pliers, but it takes a long time.
There are a lot of special tools available for this, but they are generally very expensive.

The best value for money I have found is from Production Devices [1].
Be aware that there are useless copies ( probably 3-D printed ) of these tools available from some web-shops.

Ohmmeter

For mounting PCBs? Yes.
Some resistor values are difficult to read. The first 4 bands are the value and the last band the tolerance. There is a larger distance between the 4. and 5. band, but it can often be difficult to see.
A resistor marked brown-red-black-brown-brown reads brown-brown-black-red-brown from the other end. The first is 1.2 kΩ 1%, the second 11 kΩ 1%. Both are E24 standard values.
Any cheap ( preferable auto-ranging ) ohmmeter will do.

PCB holder

A good PCB holder is "half-the-work", but they are very expensive.
I find the types with crocodile clips ( "helping hands" ) useless for PCB work.
One way to hold the PCB during soldering is to put screws in the mounting holes and set it on the screws.
It may be useful to stick the screws to a piece of cardboard with double-sided tape.

PCB soldering stand.
Fig.1: PCB soldering stand.

I had to use this soldering stand for this PCB as the components was too close to the edge to use a PCB holder.
4 screws and 4 nuts are normally enough, but I did not have sufficiently long screws.


Parts list.

Mount the PCB with the correct components the first time. This is much faster than having to look for errors later. Just take your time.
Do not attempt to mount PCBs with more than a few components after the schematic.
Use a parts list ( BOM ).
If you do not have one, make one.
Use a spread-sheet and print it.

Table 1: Parts list example.
DescriptionQuantityDesignatorFittedPackage markingNote
Resistor, 100 Ω4R1, R4, R8, R9FittedBN-BK-BK-BK-BN
Resistor, 1 kΩ1R3FittedBN-BK-BK-BN-BN
Capacitor, X7R, 10n, 50V,
2.5mm pin-spacing
4C1, C5, C8, C9Fitted10n
IC, TL072, DIP1U1FittedTL072
Resistor, 100 Ω1R2FittedBN-BK-BK-BK-BNMount on end
Resistor, 100 Ω1R7Not FittedBN-BK-BK-BK-BN

Description Description of the component. Put any description that help you identify the component.
Quantity I should probably write something here?
Designator I should probably write something here?
Fitted If a component should be installed or not. I normally put the "Not Fitted" components in the bottom of the list.
This allows you to check that the component positions are empty when all components are installed.
Package marking This should help you identify the physical component.
The color codes shown here are according to IEC60757. You can find them here [2].
Instead of BN-BK-BK-BK-BN you could write Brown-Black-Black-Black-Brown.
Many SMD component are marked with a 2-character code as there is no room for the part-number.
Note I should probably write something here?

The list above is sorted after how tall the components are, as this is the order you will mount them in ( there are some exceptions to this ).

Use the printed component list to make sure you have the components you need at hand and check them in the Quantity column.
If you want to check Not Fitted components or not is up to you.
Note that many components have no marking on the component. Keep these in the container / bag until you need them.

Table 2: Parts list example.
DescriptionQuantityDesignatorFittedPackage markingNote
Resistor, 100 Ω4 ✔R1, R4, R8, R9FittedBN-BK-BK-BK-BN
Resistor, 1 kΩ1 ✔R3FittedBN-BK-BK-BN-BN
Capacitor, X7R, 10n, 50V,
2.5mm pin-spacing
4 ✔C1, C5, C8, C9Fitted10n
IC, TL072, DIP1 ✔U1FittedTL072
Resistor, 100 Ω1 ✔R2FittedBN-BK-BK-BK-BNMount on end
Resistor, 100 Ω1 ✔R7Not FittedBN-BK-BK-BK-BN

Next step is to put the components in / on the PCB.
As you go along, simply underline the designators in the parts list ( underline is shown as bold here ).

Table 3: Parts list example.
DescriptionQuantityDesignatorFittedPackage markingNote
Resistor, 100 Ω4 ✔R1, R4, R8, R9FittedBN-BK-BK-BK-BN
Resistor, 1 kΩ1 ✔R3FittedBN-BK-BK-BN-BN
Capacitor, X7R, 10n, 50V,
2.5mm pin-spacing
4 ✔C1, C5, C8, C9Fitted10n
IC, TL072, DIP1 ✔U1FittedTL072
Resistor, 100 Ω1 ✔R2FittedBN-BK-BK-BK-BNMount on end
Resistor, 100 Ω1 ✔R7Not FittedBN-BK-BK-BK-BN

General information.

If possible, print a drawing of the PCB lay-out before you start mounting the PCB.
Some PCBs do not have room for all or some of the designators on the PCB itself or they may be printed under the component

Do not assume that f.ex. the 2 channels of a stereo unit is symmetrical or identical for the 2 channels.
Some PCBs are made to look symmetrical for aesthetic reasons, but what is R1 in one channel may be in an other position in the other.


Through-hole PCBs.

Removing components from the tape.

When you remove components from the tape, cut the leads near the tape.
If possible, do not pull the components from the tape. This may leave adhesive on the leads that may end up in the PCB holes, making soldering difficult.
Cutting components of a tape one by one can take quite some time. Small stainless-steel poultry scissors works very well for up to 100 resistors per cut.
Some components ( like TO-92 cases ) have too short leads if you cut them from the tape. Check for adhesive residue after you pull them from the tape ( in the leads - not in the case ).

Lead forming

When you bend the component leads, you should always hold the lead ( using pliers ) between the component and the place you bend the lead.
This is specially important for semiconductors with heavy leads like TO-126, TO-220, TO-247 etc.
Many small metal cased semiconductors ( like TO-71 ) use a glass seal for the leads. This may crack if you just bend the leads without holding them.

Lead formed resistors.
Fig.2: Lead formed resistors.

Fig.2a:Leads formed by tool like [1].
Fig.2b:Lead formed over 1.5 mm drill shaft for mounting on end.
Fig.2c:Kinked leads to keep resistor body clear of PCB. This is typically used for fusible resistors.
Fig.2d:Kinked leads to keep resistor body clear of PCB. Compact power resistors like this one can have a body temperature of up to 250 °C. Bend the leads so you can read the value after the resistor is mounted.
Fig.2e:Inrush current limiters can get very hot and should be mounted clear of the PCB.

Mounting the PCB.

Start with the lowest components, put them in the PCB and bend their leads.
A typical order can be: Diodes, resistors, ceramic capacitors, IC sockets, film capacitors and electrolytic capacitors.
Polarized components should of course be installed in the right orientation.
Resistors should be installed so they "read" in the same direction. That makes debugging the board much easier.
Use a screwdriver to bend the leads on components with short leads. Be careful not to scratch the PCB.
The board may then look something like this:

PCB with components.
Fig.3: PCB before cutting the leads.

Cut off excess leads at the edges of the pads.
You may want to do this a couple of times while you insert the components to prevent the build-up of leads on the PCB solder-side to get too high.
Solder the components.

There are some components where you should not bend the leads:
Relays, switches, transformers, connectors and some film capacitors.
You have to put these in one-by-one, cut their leads and solder them.

Mounted PCB.
Fig.4: Mounted PCB. Note that the resistors all "read" in the same direction.


Surface mount PCBs.

Most boards just can just lie on the table while you mount them.
Some boards are too small, so they will move around. Simply stick them to a piece of cardboard with double-sided tape.
If there are components on both sides, make sure to remove all adhesive residue before mounting the second side.
Note that the PCB shown here is specifically designed for hand-soldering, so the pads are larger than on most SMD PCBs.
Start by applying solder to one pad for each component. It should look something like this:

PCB with solder on one pad.
Fig.5: PCB with solder on one pad for each component.

For some reason this photo look like I put a lot of solder on each pad.
I have actually only put the minimum amount that will just cover the pad.
Install all components. It is important that the components are put flat against the PCB so that all pins touches the PCB.
It should look something like this:

PCB with components mounted.
Fig.6: PCB with components mounted.
Solder all components. It should look something like this:

Soldered PCB.
Fig.7: Finished PCB.

Inspect the PCB carefully using a magnifying glass.
If there are any shorts, remove them with de-soldering braid.


Repairing PCBs.

Some components like transformers or relays are normally easy to remove from a PCB without damage to the component or the PCB.
Other components, like ICs, can be very difficult to remove without damaging the component or the PCB ( unless you have a special - very expensive - tool ).
In general, you will want to be able to reuse the PCB.
Simple cut the component leads as close to the component body as possible, remove the component and remove the leads one-by-one.
Straight leads are simply heated and pulled up from the boards component side.
Bend leads are straightened on the PCB component side, heated and put through the PCB.
Components like film capacitors or electrolytic capacitors are a little more difficult. You have to remove the solder from the leads/pads ( use de-solder braid ), pry the leads free of the PCB ( without ruining the via ) and pull the component out of the PCB.
Next you need to clean the pads. You can do this with de-solder braid, but I normally use a stainless steel probe ( the same type as the dentists use ).

SMD chip components are most easily removed from the PCB using 2 soldering irons.
On less dense PCBs - like the one above - it is often possible to use a large tip, heat the entire component and push the it of its pads.
Components with more than 2 leads are most easily removed by cutting their leads, removing the component and cleaning the PCB.
Several SMD components can only be removed by special tools.

I some cases you are interested in rescuing one or 2 components from a PCB ( but you do not need the PCB ).
This is easily done with a paint-stripper ( add molten, airborne solder to the list of warnings that come with the paint stripper ) and a screwdriver.
You will need a way to hold the PCB upside down. A heat resistant PCB holder is the best, but 4 screws as shown above can be used.
Heat the paint stripper. When it is at full temperature, hold it 1..2 cm above the PCB and poke the component(s) you need out with the screwdriver.
I have used this successfully with 100+ pin connectors, 40-pin ZIF sockets, VFD glasses and relays.


References.

[1] Production Devices Lead Forming Tools
[2] Resistor color codes and e-series.

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E-mail: pp@poulpetersen.dk

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