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Practically Speaking Hands-on techniques for turning ideas into projects – by Mike Hibbett Introduction to surface-mount technology – Part 2 T his month, we carry on from the previous Practical Speaking (April 2020) to take a closer look at SMDs (surface-mount devices), specifically how to select, purchase and use them. This month, we start with passive components; and in the next article we will cover the more complicated transistor and IC parts. Resistance is futile! There are many benefits to constructing circuits with SMD devices. Used predominantly in high-volume consumer electronic products, they are often cheaper than wire-ended equivalents, due simply to the scale of SMD use. By the same token, many interesting components are only available in surface-mount packages because they have been created for a specific high-volume application. Another benefit to hobbyists is that you do not need to drill holes in a circuit board to fit them, and as you do not need a hole passing through the PCB to mount them, you can get more components on a board – should you be adventurous enough to do double sided component placement. There is one major drawback however: size. SMD devices are small; sometimes absolutely tiny, so they pose challenges to hand soldering by the inexperienced. Let’s take a look at some of the details, starting with the simplest of devices – resistors. SMD resistors come in a range of packages. They are named after their dimensions, two digits for length and two for width, with those numbers being a multiple of 10 thousandths of an inch. For example, an 0805 package measures 0.08-inch × 0.05-inch. Fig.1 shows resistors in sizes 1206, 0805, 0603, 0402 and 0201. If you look carefully you may spot the 01005 part we pushed to one side (a rogue for breaking the package naming convention). For someone new to SMD soldering and with good eyesight 0805 packaged parts are a reasonable starting point; the author prefers to use 0603, as with time these become no harder to solder but do offer great space saving, which is important when you are trying to design small boards. The larger package 44 sizes such as 1206 are only used when a larger power rating (1/4W) is useful. As the size decreases, so does the maximum working voltage, simply because the gap between the device’s terminals reduces. An 0805 resistor is good to 150V, but an 0603 package is limited to 75V. This is rarely an issue for hobbyist projects but does need to be understood and remembered. The resistors shown are all the same value and can perform identically in many applications. The key point to look out for when choosing a small component package is power rating; an 0805 resistor is typically rated for 1/8W, 0603 half that again. So be careful in power supply circuits or driving larger output loads with your choice of component sizes. If a resistor does not have sufficient power rating, put two resistors (of double the value) in parallel, or use a larger package (although there are benefits to standardising on a single size, as we will mention.) For typical hobbyist digital and audio projects these limits rarely come into play and will be highlighted by the project author or application note if they do. Resistors come with their value printed on the part, at least in package sizes down to 0603. Below that size they all look the same – so storage and handling requires care. If you drop an SMD on the floor, our advice is to leave it and take another from the store. At less than half a penny each, the risk of picking up the wrong component is not worth it. Assuming you find it again, of course! 1206 0805 0603 0402 0201 01005 Fig.1. Various SMD resistor sizes, compared to a 1/4W wire-ended part – the smallest ‘dot’ is a 01005 device. traditional wire-ended components; visiting the Farnell website and entering ‘4K7’ in the search bar yielded 161 options for through-hole resistors, and over 800 options for SMD, as shown in Fig.2. Where do you start?! When you click on ‘Chip SMD Resistors’, a set of filters appear (thankfully!) as shown in Fig.3. From here, you can further refine your search. Before we do, let’s take a moment to analyse the list. We are showing eight of the ten filters shown on the actual website. The voltage rating is something we rarely think about with-wire ended resistors; some of these parts are limited to just 15V. This Component selection Surface-mount components can be ordered from the usual online suppliers such as Farnell, RS Components and Digikey. Selecting a part can be challenging at first, so let’s follow an example selection through, assuming you are looking for a 4.7kΩ resistor. When choosing a traditional wire-ended resistor you would typically be facing just a few questions that can be easily answered – metal film, carbon or wire-wound? What power rating? What accuracy? The selection criteria for surfacemount components is broader than with Fig.2. 4K7 (4.7kΩ) resistors available on the Farnell website: 1217 different types! Practical Electronics | June | 2020 Fig.3. Component selection filter options on the Farnell website. is a consequence of the size reduction. Thankfully, most of the parts are rated at 50V and above, and so are suitable for most hobbyist designs. The ‘Resistor Element Type’ refers to how the resistance of the part is created, and it’s no surprise that ‘Thick Film’ is the most popular, as it is the main technology outside of specialised resistors with very high accuracy Fig.4. Typical list of resistors on the Farnell website. Practical Electronics | June | 2020 45 or temperature stability. The resistor tolerance is something we can largely ignore; unless specifically required, 5% tolerance (E24 series) will be fine. Finally, we have the key parameter ‘Resistor Case Style’. There is a blistering array of choice here; the scroll box shown in Fig.3 has 19 options. The selection here is easy – choose your favourite size. A good starter size is 0805, but when you get more comfortable with SMDs you will probably move down to 0603. Note that when we say ‘0603’ we are referring to the imperial measurement, not the metric size. PCB design is one of those rare professions where it is common to mix measurement units. PCB dimensions and hole sizes are often quoted in metric, while track widths, pin spacing and component sizes are quoted in Imperial. Be careful when reading datasheet dimensions that you know whether a value is in thou (aka ‘mil’) or mm! When we click on ‘Apply Filters’ we get to see the list of potential purchases – a choice of 123 different parts matching our specification. This may be daunting at first, but there is a simple trick – click on the icon shown in Fig.4. to sort by increasing price, and pick the cheapest – there is really no reason to be more selective than that with general purpose resistors. This filter will show the minimum priced device first, but does not filter on minimum quantity – so to pay 0.0032 Euro for a resistor, you will have to buy 5000 of them. You may laugh at that, but remember this is only 15 Euro for a lifetime supply of 4.7kΩ resistors! You have to scroll down a little to find the price for parts offered in more sensible quantities – at a minimum order quantity of 10 parts, you will pay half a Euro Cent each. Do pay attention to the ‘Price For’ column, as it indicates how the part will be supplied – on a reel or cut tape. You can see the two bulk packaging options in Fig.5. A reel is simply a reel of tape, so the components are basically shipped in the same format. This only becomes important if you are buying components to send to a PCB assembly house – they will need the components on a reel. SMD capacitors Now let’s take a look at capacitors, where the story gets slightly more complicated. At values above 1µF, you need to pay close attention to the working voltage of the part. For any value of capacitance, you need to pay attention to the ‘series’ of the capacitor (commonly stated in selection parameters as CoG or X5R) which really refers to its stability across temperature range and working voltage. (This is not a parameter specific to SMD devices, however – the series rules apply equally to wire-ended devices). Where the choice of an SMD capacitor does get interesting is in the use of higher value capacitors. Many of us are used to working with leaded electrolytic or tantalum capacitors, but when choosing SMD components, ceramic capacitors are often the first point of call. Ceramic capacitors are easy to use because they are not polarised, but do be aware that they get more expensive as the value increases above 10µF. They are, however, our capacitor of choice because the parts remain small and have long storage and operational life. Ceramic capacitors have no value marking, even in the larger SMD sizes. However, high-value tantalum capacitors are marked, The lack of marking on ceramic capacitors is due to the material they are constructed from not taking well to the printing process (ie, it would be too expensive to do it effectively with a bespoke printing process.) Buying components Fig.5. Examples of components on reel and cut tape. 46 With SMD components being so cheap, it’s rare to buy a single component when you need one. Typically, the minimum quantity is 10, so it’s very easy to build up an unorganised collection of SMD parts in your component ‘stock’. There are two approaches to this. First, standardise on a particular size of component as your ‘default’ size. Having done that, buy a reel of some of the common values. For resistors, a 5000-part reel of a given value can be bought on eBay for less than £5; it’s worth the wait for a lifetime supply. Second, it’s possible to buy a complete set of ‘all values’ of a component in a book format. This consists of perhaps 20 – 50 parts of each and every value in the E24 range – over 4000 individual components – all in a specific package size. The author has three books of resistors in 0805, 0604 and 0402 package sizes. Each book measures 8 × 6 × 1 inches in size, so it’s very compact. For example, at the time of writing, eBay item 291874912665 with 4250 pieces at £15.99 including delivery. Books of components save lots of space, are easy to use and offer excellent value for money. For around £15 you get over 4600 resistors – at a cost of under 1/3 of a penny each. The components are supplied in tape form and slot into the book, so when you run out of a strip they can be replaced easily by purchasing through a normal distributor like Farnell. Even the plastic pages can be replaced. Books of capacitors follow the same approach but are approximately twice the price, which is to be expected as large value capacitors are expensive. We find our component books invaluable, and a terrific space saver in the lab. It won’t stop you needing to purchase the occasional exotic part, but you will be buying things less frequently – and avoiding extra shipping costs. PCB design Using surface-mount components implies you are using a PCB, either home-made or purchased from one of the low-cost Far-east suppliers. This means that you will be using a PCB design tool, which brings up its own set of challenges. With surface-mount components it is essential that you select the correct footprint – the area of exposed copper on the PCB – when laying out the PCB. Leaded components can be ‘adjusted’ to fit incorrect pads; SMDs have no such capability. Resistors and capacitors follow relatively simple footprint standards but ICs, connectors and other solderable parts come with varying or non-standard footprints. If you have the components to hand in advance of designing the PCB, print the board out at a 1:1 scale on paper and verify the components fit. We’ve been caught out on more than one project with incorrect-width IC packages. With all SMD component footprints, ensure that the pads are a little longer than the component placed on it; compact solder pads are great for professional soldering machines and help free up board space, but remember that you will be soldering these components by hand with an iron. When laying out the PCB design, it’s tempting to bring components close together – that may be why you chose SMD components in the first place. When placing components close to other parts at the design stage, be mindful of how you will actually solder them down. Take Fig.6 Practical Electronics | June | 2020 Fig.6. A potentially troublesome SMD component layout – soldering order matters! for example; It would be very challenging to solder the IC or resistor if the capacitors had been placed down first – you would have difficulty getting the soldering iron into the small pads. A PCB laid out like this requires that the IC be soldered first, followed by the resistor and only then the two capacitors. Next up In our concluding article in the next Practically Speaking column we will move onto the more complex task of IC and transistor selection. Blast from the past Our previous Practically Speaking column caught the eye of one reader – the article included a photograph of the cover of the March 1965 issue of Practical Electronics. That cover showed a ‘build it yourself’ oscilloscope project. Reader David Hannaford reached out to say he built that scope in 1965, and still has it. He kindly sent us a photograph. David Hannaford wrote: ‘Mike – Love your articles in PE and especially your reference in the latest magazine to the 1965 PE Oscilloscope that I built at that time as my first big PE project. I still have it [see photo above] but not sure about running it now as 55-year-old capacitors and 600V floating around sounds a bit dangerous.’ Many thanks for the feedback and photo David, I have a soft spot for that issue of the magazine as it was the month I was born. I’m glad to report I am still running! STEWART OF READING 17A King Street, Mortimer, near Reading, RG7 3RS Telephone: 0118 933 1111 Fax: 0118 933 2375 USED ELECTRONIC TEST EQUIPMENT Check website www.stewart-of-reading.co.uk Fluke/Philips PM3092 Oscilloscope 2+2 Channel 200MHz Delay TB, Autoset etc – £250 LAMBDA GENESYS LAMBDA GENESYS IFR 2025 IFR 2948B IFR 6843 R&S APN62 Agilent 8712ET HP8903A/B HP8757D HP3325A HP3561A HP6032A HP6622A HP6624A HP6632B HP6644A HP6654A HP8341A HP83630A HP83624A HP8484A HP8560E HP8563A HP8566B HP8662A Marconi 2022E Marconi 2024 Marconi 2030 Marconi 2023A (ALL PRICES PLUS CARRIAGE & VAT) Please check availability before ordering or calling in PSU GEN100-15 100V 15A Boxed As New £400 PSU GEN50-30 50V 30A £400 Signal Generator 9kHz – 2.51GHz Opt 04/11 £900 Communication Service Monitor Opts 03/25 Avionics POA Microwave Systems Analyser 10MHz – 20GHz POA Syn Function Generator 1Hz – 260kHz £295 RF Network Analyser 300kHz – 1300MHz POA Audio Analyser £750 – £950 Scaler Network Analyser POA Synthesised Function Generator £195 Dynamic Signal Analyser £650 PSU 0-60V 0-50A 1000W £750 PSU 0-20V 4A Twice or 0-50V 2A Twice £350 PSU 4 Outputs £400 PSU 0-20V 0-5A £195 PSU 0-60V 3.5A £400 PSU 0-60V 0-9A £500 Synthesised Sweep Generator 10MHz – 20GHz £2,000 Synthesised Sweeper 10MHz – 26.5 GHz POA Synthesised Sweeper 2 – 20GHz POA Power Sensor 0.01-18GHz 3nW-10µW £75 Spectrum Analyser Synthesised 30Hz – 2.9GHz £1,750 Spectrum Analyser Synthesised 9kHz – 22GHz £2,250 Spectrum Analsyer 100Hz – 22GHz £1,200 RF Generator 10kHz – 1280MHz £750 Synthesised AM/FM Signal Generator 10kHz – 1.01GHz £325 Synthesised Signal Generator 9kHz – 2.4GHz £800 Synthesised Signal Generator 10kHz – 1.35GHz £750 Signal Generator 9kHz – 1.2GHz £700 HP/Agilent HP 34401A Digital Multimeter 6½ Digit £325 – £375 HP33120A HP53131A HP53131A Audio Precision Datron 4708 Druck DPI 515 Datron 1081 ENI 325LA Keithley 228 Time 9818 Practical Electronics | June | 2020 HP 54600B Oscilloscope Analogue/Digital Dual Trace 100MHz Only £75, with accessories £125 Marconi 2305 Modulation Meter £250 Marconi 2440 Counter 20GHz £295 Marconi 2945/A/B Communications Test Set Various Options POA Marconi 2955 Radio Communications Test Set £595 Marconi 2955A Radio Communications Test Set £725 Marconi 2955B Radio Communications Test Set £800 Marconi 6200 Microwave Test Set £1,500 Marconi 6200A Microwave Test Set 10MHz – 20GHz £1,950 Marconi 6200B Microwave Test Set £2,300 Marconi 6960B Power Meter with 6910 sensor £295 Tektronix TDS3052B Oscilloscope 500MHz 2.5GS/s £1,250 Tektronix TDS3032 Oscilloscope 300MHz 2.5GS/s £995 Tektronix TDS3012 Oscilloscope 2 Channel 100MHz 1.25GS/s £450 Tektronix 2430A Oscilloscope Dual Trace 150MHz 100MS/s £350 Tektronix 2465B Oscilloscope 4 Channel 400MHz £600 Farnell AP60/50 PSU 0-60V 0-50A 1kW Switch Mode £300 Farnell XA35/2T PSU 0-35V 0-2A Twice Digital £75 Farnell AP100-90 Power Supply 100V 90A £900 Farnell LF1 Sine/Sq Oscillator 10Hz – 1MHz £45 Racal 1991 Counter/Timer 160MHz 9 Digit £150 Racal 2101 Counter 20GHz LED £295 Racal 9300 True RMS Millivoltmeter 5Hz – 20MHz etc £45 Racal 9300B As 9300 £75 Solartron 7150/PLUS 6½ Digit DMM True RMS IEEE £65/£75 Solatron 1253 Gain Phase Analyser 1mHz – 20kHz £600 Solartron SI 1255 HF Frequency Response Analyser POA Tasakago TM035-2 PSU 0-35V 0-2A 2 Meters £30 Thurlby PL320QMD PSU 0-30V 0-2A Twice £160 – £200 Thurlby TG210 Function Generator 0.002-2MHz TTL etc Kenwood Badged £65 Function Generator 100 microHz – 15MHz Universal Counter 3GHz Boxed unused Universal Counter 225MHz SYS2712 Audio Analyser – in original box Autocal Multifunction Standard Pressure Calibrator/Controller Autocal Standards Multimeter RF Power Amplifier 250kHz – 150MHz 25W 50dB Voltage/Current Source DC Current & Voltage Calibrator £350 £600 £350 POA POA £400 POA POA POA POA Marconi 2955B Radio Communications Test Set – £800 47