Surface Mount Technology - Design Guidelines
These notes were written a few years ago but the points considered remain valid today. They don't contain hard and fast "rules" but attempt to make the designer think logically about his design. Of course, there are some contentious points raised which are bound to raise comments such as "Why should I change I've always done it in such-and-such a way?" My answer is "Why not? You won't know unless you try it!" I worked for fifteen years as a designer and I didn't simply pull these ideas out of a hat. Hopefully you will find them interesting and thought-provoking.
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 Return to Main Index Page 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 (C)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
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