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Old 26th September 2010, 03:02 AM   #1
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Default use of SMD components?

Hello, I have some questions here. I wonder if an amp kit using SMD components would be better or more reliable than common parts like resistors, caps, transistors etc, let say it is the same amp kit but one using SMD components and the other regular board. I ask this ,because I have seen some amps kits using regulars boards with big components and I think these kits with these big parts would be more reliable than those tiny SMD components and if the big parts get damaged they will be easier to change than SMD components.What I see in the SMD design is they are compact and all the voltages and signal are really short. I hope you understand what I am trying to say.Thanks

Last edited by lanchile; 26th September 2010 at 03:04 AM.
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Old 26th September 2010, 03:16 AM   #2
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My view would be that the reliability of the resulting amplifier is more a function of the care its put together with than the choice of through-hole/SMT. Myself I'd go for the SMT kit if choosing between them because smaller size means less area to collect RFI - this seems to be something you've noticed too. For audio I consider this quite a significant advantage. That is assuming the kit doesn't have parts smaller than 0603 which I might find difficult to see let alone solder

Sometimes desoldering SMT is easier - to change a part becomes simpler because everything's on one side. No need for a solder sucker with the smaller parts, they're so tiny that both ends get hot enough with a touch on one pad. But of course its very easy to lose components - fortunately they're so cheap it barely matters.

I'd check in the case of an amp kit that the parts that will encounter significant voltage across them are adequately specified. Thinking here of feedback resistors around amps that might introduce excess distortion if made from too-low rated parts. An 0603 (0.1W) as a feedback resistor I think wouldn't be such a good idea in a 100W/8R amp.
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Old 26th September 2010, 03:28 AM   #3
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Well, I agree with the smaller size means less area to collect RFI thing.
But are these SMD components strong enough to handle long term work?
I ask this because I see these tiny components and they look so "weak" to me. I always worked with real parts lol not these tiny staff. Maybe I am getting to old for this "new" era lol.
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Old 26th September 2010, 03:34 AM   #4
rlaury is offline rlaury  United States
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There are 2 benefits of using SMD. #1. Reduced manufacturing cost of medium to high volume products. It can improve quality in the short term, but the process requires high supervision. Rework or repair can be "not practical". #2. Space saving allowing more complex circuitry in a smaller area. In general SMD components in high gain critical analog stages produce unwanted noise and distortion unless you use certain types. Not a problem for digital. In general SMD is not an advantage in low volume DIY projects.
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Old 26th September 2010, 03:37 AM   #5
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Well I think that given most consumer electronics these days is SMT, if it was a major reliability problem there would be more widespread failures in the field. I just recently fixed my LCD TV - the PSU board looked conventional on the top side so I thought it had 'grunt', when i turned it over there were hundreds of SMT components on the reverse and I became a bit concerned. I agree they do look 'weak' but this is one of those times its best to put aside our perceptions and go along with the evidence Now its fixed, the TV is working fine (a non-SMT MOSFET had failed) - its power supply is a similar power level (around 250W) to an audio amp, but all switching of course.
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Old 26th September 2010, 06:07 AM   #6
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One big advantage of SMD is the reduced parasitics. The residual capacitance and inductance are much smaller than in components with leads.

The down side is that you have to be almost a watchmaker to work on this modern stuff. The ICs especially are very small pitch and the circuit traces very fragile, so rework is difficult. And the old tools just won't do.

Many of the components rely on conduction for heat dissipation, while old style stuff relied on convection.
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Old 26th September 2010, 07:44 AM   #7
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I'm for SMD, at least for those components that qualify. Those little SMD film resistors are actually pretty good in terms of noise, unless you over drive them. I can solder 0201 (20mW) resistors, and SOT-923 transistors, on a good day, it takes a steady hand. I used a 40W soldering iron and replaced the tip with a small finishing nail. With that, some flux, and a small flathead screwdriver, , 0402's are a breeze.

Pd can be increased by using heavier weight copper and larger pads, if possible. The only thing is if you intend to hand solder as opposed to using solder paste and an oven, then for the 0402 and smaller you should not bury pads into large copper areas because the smaller iron will not be able to heat the pad and component properly. The bake oven is the best thing for SMD's but for prototypes, leave a leader in the PCB layout to act as a terminal for the tiny components. Besides, you should not be using 0201 size anyway unless Pd is a few mW or in the uW range, <20V.
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Old 26th September 2010, 12:33 PM   #8
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For the designs I do at work I use SMT components (except for connectors, relais, trimpots and electrolytic capacitors) because of their advantages already mentioned. In my DIY project I use SMT resistors and some SMT diodes but all other components are THT.

Things to consider using SMT for DIY:

Ceramic capacitors might crack because of handling and soldering. They are very fragile and even crack for no apparent reason soldered during professional industrial assembly process. Land patterns are very important, thermal soldering profiles are as well. I discourage using ceramic SMT capacitors for DIY.

Using SMT resistors seems to be less critical since they are mechanically more robust. However I wouldn't go below 1206 for DIY. Land patterns are less critical. Since they are rated at less power than an average THT part they might heat up more and thus cause problems in some applications, for example thermal distortion or thermal noise when run too hot.

I wouldn't recommend SMT transitors for DIY because if you do you're stuck with the pinout and can't use another transitor with different pinout without serious solder acrobatics. Thermal stress during soldering is an issue with all SMT components and might degrade the transistor inside the package because of the short leads good heat transfer from solderjoint to die.

Why I don't use certain SMT components for my designs at work:

SMT connectors are mechanically less tough connected with the PCB and might be ripped off more easily including the solder pads connected to them. They are good for extreme high density boards or crappy throw-away consumer electronics that are desired not to last too long.

Some SMT electrolytic capacitors might be damaged during reflow or vapour phase soldering since lead free soldering requires higher temperature profiles and not every electrolyte fluid might survive this. For applications where the PCB experiences a lot of mechanical stress (acceleration, vibration) a heavy SMT ElCap might have a too weak mechanical connection to the board (see connectors). Bigger land patterns can help. I can remember having ripped off a cap from a PCI card when I pulled it out of a slot. In this case the solder joint failed.
For hand soldering I see no reason why not to use SMT electrolytic capacitors.

I don't trust SMT relais since I heard a story from another pcb designer who used SMT relais that failed later in the field because moisture trapped inside the relais expanded during vapour phase soldering and cracked the housing of the relais. During wahsing the pcb solvents came into the relais and degraded the relais contacs over time.

I don't like SMT trim pots because the unsealed ones are crappy. For the sealed ones one might have similar problems like with the SMT relais.

General thing to consider using SMT parts:

THT parts can compensate board flex easily since they have long leads that can deform. Many SMT parts can't because the leads are very short or missing at all (metallised edges). SMT ceramic capacitors are especially vulnerable. Thermal cycling causes different thermal expansion in different materials and thus mechanical stress on the solder joints and components leads of the SMT parts. In some cases this might cause the connection or the part to fail. It might be rare but it happens sometimes.

SMT parts are not designed to be handled manually. Many are fragile and could be damaged if not handled carefully.
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