e032032.pdf

GENERAL INTEREST

SMDs? Don’t panic!

Part 2: practical tips on SMD mounting

By C. Tomanik

In this second and concluding instalment we concentrate on the

techniques necessary for soldering and desoldering SMD components.

In the last article we looked at some of the different compo-

nent outlines available for SMDs and some of the basic tools

necessary to begin work. This article looks more closely at

the techniques necessary for soldering and desoldering SMD

components. A standard soldering iron with an interchange-

able bit together with some additional solder paste are the

only soldering tools necessary for most SMD work. A normal

soldering iron would be something similar to the Ersa type

Analogue 60 A. This iron is rated at 60 W and has inter-

changeable bits. Weller and Antex also produce soldering

irons with a similar speciﬁcation. For the ERSA iron it is nec-

essary to fit a fine soldering bit type 832 UD. This bit has a

tip diameter of 0.4 mm. The disadvantage of the simple con-

ical soldering bit is that it does not provide an even heat dis-

tribution when applied to the joint area and pad but nonethe-

less, with a little care, it is still possible to produce good qual-

ity soldered joints.

The iron temperature should be set between 350 and

400 °C. This may seem a little hot but a high bit temperature

speeds up the soldering process and actually reduces the risk

of overheating both component and pad.

iron bit and tack down one end of the component with this

solder. Don’t worry about making a perfect joint just yet

because we are just ﬁxing the component in the correct posi-

tion.

Now go to the other end of the component and place the sol-

dering iron tip in contact with both the component lead and

the PCB pad. The iron will be at 90º to component lead axis.

Feed solder into the joint between the component and pad.

The ﬂux will ﬂow over the joint, drawing solder neatly around

the pad and component by capillary action. Don’t feed in too

much otherwise a ball will start to form. The optimum amount

will give a concave shape to the finished joint surface a bit

like a miniature version of a trumpet bell-end.

Resistors, ceramic and tantalum capacitors

These component packages are broadly similar and can be

treated identically for the purposes of soldering.

Place the component down on the PCB pads and line it up

accurately using tweezers. Put a small bead of solder on the

Back to the ﬁrst joint, we can now re-heat it with the iron and

add a touch more solder to make it ﬂow around the pad and

lead. Take the iron away as soon as possible to avoid compo-

nent overheating and don’t add too much additional solder.

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MELF Diodes

The MELF outline is cylindrical which makes it a little tricky

to handle and the lead area in contact with the PCB pad is

less than a resistor outline, producing a much smaller capil-

lary effect when soldering. A little additional flux added to

the PCB pads will help here. Otherwise follow the procedure

for soldering resistors.

SOT23 Transistors and chip electrolytics

These components have relatively widely-spaced short leads

for soldering to the PCB pad. We will show the procedure for

an SOT 23 type transistor package.

dering iron, tack down two of the component legs (choose

diagonally opposing corner legs). This time it’s easier if the

soldering iron bit is held along the component lead axis rather

than at 90º to it.

As before, position the component and tack down one of its

legs with a small blob of solder.

Now we get to the tricky bit, ﬁrstly this is your last chance to

check that the component really is accurately centred. Each

component leg is now soldered identically: With the bit and

component lead in line, quickly heat up the lead and solder

pad then feed solder onto the bit.

Now to the remaining leg(s), hold the iron bit at 90º to the leg

axis using light pressure and feed solder over the leg, from

above this time, so that solder coats the leg and ﬂows down

over the pad. This will produce a more evenly ﬁnished joint. Go

back to the ﬁrst tacked leg and reheat it with a little extra sol-

der as above.

Tackling components with ﬁnely pitched leads.

As a example for soldering these types of IC’s we will use a

QFP type of package outline. The leg spacing is 0.635 mm

(not much bigger than the soldering iron bit diameter).

Dab some ﬂux paste over each of the PCB pads.

Position the component accurately over the pads (make sure

pin 1 on the IC corresponds to pin 1 on the PCB). Using a sol-

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Too much solder will cause a blob to form which bridges over

to an adjoining pad. Don’t worry because this can be simply

removed with the application of solder wick. Position wick

over the blob and apply light pressure with the iron bit until

solder is drawn into the wick. Finally re-solder with a little

additional ﬂux and solder.

Soldering components with ﬁnely pitched leads

Once again we will use a QFP packaged device. As before,

the pads should ﬁrst be treated with ﬂux.

The end of this microwell soldering bit is offset at 30° and has

a small cup-shaped tip for holding a pool of solder.

Position the component on the PCB and ﬁx it by tacking two

diagonal opposed leads to their pads with solder.

Desoldering SMDs

Desoldering can be a bit of a headache with a standard sol-

dering iron because the component is rigid and mounted

directly on the PCB surface. First begin by removing as much

of the solder in the joints as possible with solder wick, the

application of a little ﬂux will maximise the amount of solder

removed. Chip components can be heated to 450°C. Place the

soldering iron on one side of the component and tug gently

with tweezers until the component can be moved. Once

removed the component should be discarded.

Components with lots of leads need a certain amount of

patience. One technique is to heat up a group of adjacent

leads and bend each leg upwards with the help of tweezers to

prevent them re-attaching to the pad. With QFP outlines it is

best to start at one corner of the chip and desolder individual

legs alternately left then right, proceeding away from the cor-

ner and around the outline. Alternatively you can snip

through the leads with suitable side cutters and desolder

each severed leg, removing them with tweezers. As before

clean up the pads with solder wick.

Working with the cup bit it is necessary to ﬂux the joint and

then place a small amount of solder into the cup. The cup

shape ensures that the solder will remain in place until the

tip is placed over the component lead, when the flux will

draw the solder out of the cup and into the joint. Judging the

right amount of solder in the cup can be tricky, too much will

result in a solder bridge (use solder wick to clean up) but with

experience this technique is quite effective.

Using a solder station

Desoldering QFP outline ICs

Removing these components with the special desoldering

tweezers is much quicker and easier than is possible with a

standard soldering iron. For each different type of component

housing there is a corresponding set of tweezers to desolder

During the preparation of this article a solder station type

SMT Unit 60 A (Figure 58) produced by ERSA was used to

explore the advantages of working with a dedicated SMD sol-

dering station. This station comprises a soldering iron with

interchangeable bits and a pair of heated desoldering tweez-

ers. Antex and Weller are also well known soldering iron man-

ufacturers and produce similar soldering stations. The tech-

niques of soldering and desoldering using these stations are

slightly different compared to the standard soldering iron that

we have been using up until now.

and remove the IC. As an example we will look at the proce-

dure for removing an IC with a QFP type of outline.

Position the pre-heated tweezer jaws over the component

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leads and wait for the solder to ﬂow.

A gentle lift of the tweezers will pull the IC clear of the PCB.

Finally use some solder wick to clean up the pads.

period is a sure way to destroy the component and loosen the

PCB pad bonding. If you anticipate a lot of SMD work in the

future then it will be worthwhile investing in a specialist sol-

der station. These are not cheap (the ERSA SMT unit 60A

used in this article retails at around £366 and is stocked by

RS Components www.rswww.com ). In the long run these

speed up SMD work especially if you will be handling large

ICs with ﬁnely pitched leads.

Providing you have the right tools, a steady hand and are

prepared to spend a little time practicing, there really is no

need to panic, even if you only have a standard soldering iron

it is quite possible to be successful with SMDs.

Conclusions

So working with SMD components need not be so stressful

after all, as long as you keep in mind the basic rules: choose

a ﬁne soldering bit and work quickly, remember that the joint

area is quite small so that it reaches soldering temperature

very quickly. If you haven’t made the joint after a few sec-

onds, take the iron off and try again later when everything

has cooled down. Leaving the bit on the joint for a prolonged

(020305-2)

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