younger designers. I mentioned, in a previous article about design

with certain types of solder resist. However the possibility of using

with board area. Provided that the interconnection method is suitable,

width. Local "necking" of tracks is often unavoidable, for instance

width of the solder machine tracks (has it been standardised for

width and soldering will still be satisfactory. It is more important

wider tracks. Wrinkling, however, may be the lesser evil because bare

which might occur. Where a decrease in spacing is unavoidable it

which is required to be free from components; the material of the

which can produce poor solder joints or even prevent the formation of

where a track must pass between the legs of an I.C. However, while it

volume of solder is minimised (reduced cost) and provides a degree of

visual inspection method uses a comparison method: a known good board

visible area while, at the same time, increasing eye fatigue. The best

varies between day time and night time, it can be accelerated

unreliable. Magnification is necessary and reduces the instantaneously

unless you try it!" I worked for fifteen years as a designer and I

unless the soldering process is tailored specifically to meet their

Unfortunately even the "synthetic" fluxes leave considerable residues

tracks is important, therefore, where hot components are connected to

to maintain adequate gaps between pads. Note 2: Stress fracturing will

to carry a significant amount of current but widths greater than 2.5mm

to avoid sag during soldering. Surface Mount Devices are potentially

through holes (Vias). When Chlorinated Fluorocarbon solvents are used

thou) wide, then the solder pad should be separated from that track

thou) so the artwork should allow for this tolerance by making

This file may be downloaded for private and personal use but NO part

they become disasters in production. Inspection Infra Red reflow Infra

them interesting and thought-provoking. The orientation of the

the mother panel should be narrow, or it should be supported, or a

the moisture has penetrated it can be removed only slowly by long term

the heat and make that component more reliable. However, if that heat

the direction of travel over the wave, the direction must be decided

The designer must appreciate that close track spacing increases the

The designer must also appreciate that large areas of copper act as

The chemical reactions are accelerated by the flow of current and by

the case of high voltage- or high impedance- circuitry, the spacing

the boundaries and at places where the resist layer is thinnest. Once

the board before the application of resist, all serve to increase the

that a comprehensive functional test should also be carried out.

than white or reflective parts. Unfortunately we can have very little

than their conventional counterparts. A small number of samples

Tests will indicate that component leads may cover only 50% of the pad

testing to ensure that the equipment will meet its specification

test is limited in its ability to test all components it is important

supporting structure (metal can affect the wave temperature); the

such fluxes in order to avoid the problems and expense of washing

such a short article. Consequently, I have concentrated on those

substrates which do not flex. Non photo-definable solder resist can

specified on the relevant drawing. If the copper tracks are tinned,

spaced evenly and not bunched together in localised areas unless

Solder Water soluble flux provides a reasonable compromise, being

solder used increases considerably; the joint becomes rigid and stress

solder resist. Moisture will penetrate solder resist. The ingress of

software will minimise the inspection requirement. Since automatic

small daughter boards is to be soldered then, again to minimise sag,

should, therefore, be subjected to environmental conditioning and

should be restricted to as few places as possible. Tracks should be

should be minimised or else a supporting structure should be utilised

should be considered most carefully. If the decision is in favour of

should be considered during the early stages of design, since the

should be 0.5mm (20 thou) wide wherever space permits. In general,

risk of short circuits, leakage current, and inter-track capacitance,

risk of reduced reliability or damage to the PCB and adjacent

rigid pallet should be used to carry the daughter boards. Whichever

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resist-free areas larger than the corresponding solder pads or test

resist is applied there will always be unprotected copper at the

requirements. Preheat control is especially important with synthetic

requirement can not be met then provision must be made for testing the

relied upon to melt the solder. The colour of the components and leads

reliably. Such conditioning is more meaningful if an electric current

reliably only by a Vapour Phase process. Conventional leaded

reliability, that a lot of design knowledge and experience fails to be

relevant production drawing. While a vibrating wave technique can

reduces the risk of short circuits and poor joints, since solder will

Red Reflow is NOT a hot oven technique. It is a direct radiation

recommended is 0.2mm (8 thou) but tracks of this width should be used

realistically possible, in accordance with the repeatability of the

punched slots between which run the interconnecting tracks. The lines

prone to whiskering (dendritic growth) which forms conductive paths.

produce a good joint. Small, circular or oval pads create less stress

process easier. Cleaning Because of the small size and large number of

previous boards?); the direction of travel; the amount of sag which

predefining the lines along which the bending will occur: the

placement machine and the ability of the soldering equipment to

place. Synthetic fluxes can also cause "webbing" of solder when used

photo-definable solder resist, albeit at a cost. The accuracy will

peeling during soldering. Wider tracks, therefore, should be divided

passed on to younger designers, so mistakes continue to be made.

pads. Accuracy of resist placement can be improved by the use of

overlooked and results in poor equipment reliability. If a number of

out of sight beneath the plastic moulded body and can be soldered

other components. This type of design consideration is invariably

only where absolutely necessary since they can cause a significant

of that capacitor will be greatly reduced! Necking (narrowing) of

of it may be published in any form without the prior permission of the

of greatest weakness thus contain only tracks. The width of the board

of a solder wave sufficiently to increase its viscosity; an effect

number of "fall outs" during bare board manufacture, thereby

not suffer from problems of stress in the way that chip SMDs do.

not be placed with a guaranteed accuracy of better than 0.25mm (10

naturally assume a curved meniscus. With larger pads, the amount of

must be borne in mind. The effect of the moisture, when combined with

more prone to moisture ingress and the effects of changing temperature

more "necks" of no more than 1mm (40 thou) width in order to avoid

Moisture It would be impossible to cover all SMT design aspects in

moisture is usually slow but can be increased in humid air by raising

moisture ingress occurs by electrolytic action. Solder Resist Rework

minimise the risk of short circuits during bare board manufacture. In

mind that it will probably be equivalent to a board using conventional

method where any part of a component which is exposed to the radiating

method of supporting structure is chosen for the daughter boards, it

may need to be still larger. In addition, it may be necessary to

maximised in order to accommodate component misplacement is not valid.

leaves behind. Vias should not be positioned beneath components where

large board. The maximum practical size of board is about 100mm (4

joints. This requirement also applies to test pads and to plated

its susceptibility to corrosion. Unless the copper is tinned before

it. Rework involves the application of heat to components with the

it in such-and-such a way?" My answer is "Why not? You won't know

is used as a control standard and compared with new production boards

is of importance because black parts can become appreciably hotter

is illuminated alternately. The operator sees only one image which

is epoxy based but the exact type should be chosen with care and

is conducted to, say, an electrolytic capacitor, then the reliability

is applied to the components since most in-service failure due to

is accepted that such necking is often necessary, the fact that it can

inter-track spacing should be at least 0.5mm (20 thou) in order to

integrity of the through connection electrically and the height of the

inspection is impossible and where flux can be trapped. If this

Infra-Red radiation or damage may occur. The area of the board is

inevitably decrease as the board size is increased. Most solder resist

increasing the unit price. Long term reliability will also be affected

increased and there are severe restrictions imposed on their use. Wave

include a ground potential "guard rail" to trap any leakage currents

inches) square, if reliability is not to be prejudiced, but bear in

in the form of a ladder to permit the solder resist to adhere to the

in the component than do larger pads. The small size ensures that the

important because the effect of bow and twist, as the board is heated

however, undesirable in military applications or where vibration will

However, pad areas should be minimised to avoid the risk of short

Hopefully, through articles such as this, I can help to redress the

heatsink effects, during soldering, which can cause stress and dry

heat sinks during the soldering process and can lower the temperature

guided manual assembly techniques together with carefully written test

ground planes are required they should be divided by cross-hatching.

greatly minimise both shadowing effects and dry joints caused by the

generally more critical. High or wide components should not have other

general, a wide track running from a hot component helps to dissipate

for most of its circumference. The connection should be via one or

for cleaning they can represent an additional hazard to health as well

fluxes since the solvent must be evaporated before soldering takes

flexibility in the joint. The lack of sharp corners on the pads

exposure to hot, dry air and might never completely be removed.

essential then the effect of bow and twist can be minimised by

entrapment of flux vapours, the technique should not be relied upon to

eliminate bad design practice. The minimum track width currently

elements can become hot enough to melt solder and any part not exposed

electrolyte which conducts current readily between tracks. Tin is

didn't simply pull these ideas out of a hat. Hopefully you will find

dictated by the contact area of the lead. These leaded components do

devices may be circular or oblong, preferably with rounded corners, as

Devices (SMDs) Pads for Gullwing, J Lead and Spear- or Butt-Lead

detrimental to the reliability of the components and must NOT be

design. Of course, there are some contentious points raised which are

costly process which should be avoided if at all possible.

corrosion of the metal. Salts of copper are soluble and form an

correct electronic functioning specifically requires close spacing.

copper, unprotected by tin-lead, can cause reliability problems due to

copper being left exposed but wrinkling of the resist can occur on

control over these colours, at present, but it should be borne in mind

considered remain valid today. They don't contain hard and fast

considerations which are most often overlooked or even unknown to

components. Rework Qualification Tests Pads for all chip SMDs should

components, such as electrolytic capacitors, should not be exposed to

components within the "shadow boundary". Since this is determined by

components of three to four times its area. If a large board is

components is important but the spacing between components is

components are put into individual clusters, separated by milled or

component with care. Certain I.C. carriers, for instance, have J Leads

component width. Indeed, the pads should be made as small as is

component above the board must be a minimum of 0.25mm (10 thou) for

cleaning to be feasible (if required). Version 1.1 updated on 3/5/99

cleaning then a flux with very low solids content will make the

Cleaning of SMD assemblies is highly undesirable. It is an extra,

circuits between leads. The argument that pad areas should be

choice is affected by: the solder machine requirements; the border

cheaper and less harmful to the environment than CFC solvents. In

caused by thermal contraction or mechanical bending of the P.C.B. can

cause fractures in the component which may not be discovered until

can not. Build up of hot air in the equipment should be regarded as

can have greater reliability than the same components combined on one

can be tolerated - and other considerations. Gaps Between Tracks Email

can be difficult and every effort should be made to avoid the need for

by the increased probability of fractures in tracks of such small

by means of a lens and mirror system where each of the pair of boards

by Martin T. Pickering B.Eng. Wider tracks may be required if they are

boundary of the resist; especially when a screen is used to apply the

bound to raise comments such as "Why should I change I've always done

boards are immediately apparent to the observer. Automatic assembly or

board material between the areas of copper. For the same reason, if

board manufacture and there is a risk of solder resist wrinkling and

before the solder resist is applied, there will be less chance of bare

be experienced. In such cases, the SMDs will be mounted on ceramic

be discussed in a follow-up article. Note 1: Joint flexibility is,

be circular or oval and the width should not exceed 75% of the

balance by making designers aware of some potential problems before

author. These notes were written a few years ago but the points

atmospheric oxygen, is to react with metal to form salts. Active flux

at the very beginning of the design and must be stated clearly on the

as a serious drawback of the Infra-Red method. In general, tracks

as a potential risk to the environment. The cost of CFC solvents has

artificially in a cyclic humidity chamber. Penetration is fastest at

area of copper (e.g. a power supply track), which is more than 1mm (40

appears to flicker as the lights alternate. Differences between the

and makes the unit very prone to failure in humid conditions. In

and lowering the temperature in cycles. The effect is known as

and cooled, puts enormous stress upon the SMDs and tends to increase

an increase in temperature. The designer should select the types of

almost identical components, visual inspection is difficult and often

agents (e.g. halides) and any impurities or reagents, not cleaned from

after the unit has been put into service. Pads for Surface Mount

affect reliability and can result in solder shorts during manufacture

addition, water tends to remove corrosive flux activators which CFC

a number of small boards connected to a mother board after soldering

a joint altogether. Where pads adjoin - or form part of - a larger

*Martin Copyright ©1997 Martin Pickering Copper Track Width Conclusion

(100 thou) should be avoided because they can cause problems with bare

"rules" but attempt to make the designer think logically about his

"breathing" and, although it occurs naturally as the temperature

Copyright ©1997 Martin Pickering
Version 1.1 updated on 3/5/99
This file may be downloaded for private and personal use but NO part of it may be published in any form without the prior permission of the author.

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