SMD design and assembly techniques

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I’ve been getting started on both designing for SMD, and assembling it, and i’d like to compare notes.

Everything i say in this thread is obviously 99% opinion, so pay attention if people disagree rather that taking it as solid info.

Component sizes

The two common sizes in DIY synths are 0805 and 0603, and the chips are SOIC, easier to handle than 0805 stuff. People say 0805 and 0603 are equally difficult to assemble, but i personally feel there is enough of a difference to prefer 0805 when feasible. However, 0805 components will be more expensive, and when availing yourself of JLCPCB’s assembly service, only 0603 components will be available as “basic” components that entail no surcharge. In general, when assembling by hand, i think it’s always safe to put a 0603 component on a 0805 footprint or the other way around. I’m not sure it’s fully advisable with automated assembly, but i know people get away with it.

In general i intend to design for 0805 but suggest people don’t hesitate to place equivalent 0603 on it.

Footprints

The footprints provided in KiCad often have two variants: one best for automated assembly, and one best for hand assembly. They generally have “HandSolder” in the name. It’s not clear if the ones optimized for hand assembly perform worse on a pick and place machine.

JLCPCB PCBA (PCB Assembly) service

They have a small amount of super common parts in their “basic” catalogue. The general idea is that any time you force someone to load a reel in the pick and place machine, they charge you a few currencies for the privilege. So if you design for PCBA, the economics of your board can change, for example, using two TL072 will be much less expensive than a single TL074, as the latter isn’t a basic part.

This plugin makes things much easier:

It takes care of searching JLC’s component database for you, takes care of component rotations, and outputs files in the format JLC expect. It’s not super user-friendly (don’t hesitate to use this thread if you have questions about it) but i strongly recommend it.

Soldering techniques

At home, there are basically seven options:

  • Soldering iron. Unlike the other options that use solder paste, this one uses small gauge solder. I don’t like this option, I don’t have the dexterity for it.
  • Hot air gun. Definitely a good way to replace components, but doing a full board with it is time consuming.
  • Using a toaster oven dedicated to this purpose. I have not tried this.
  • Using a cooking hot plate dedicated to this purpose. Tried it, wasn’t happy with the poor temperature control.
  • Using a reflow oven. Even the hobbyists models are expensive.
  • Using a PCB designed to heat up safely. Sounds like a cool but scary hack, not tried. Only suitable for small boards.
  • Using a PCB hot plate. I got one from Aliexpress for 60 Euros with a 150×150 surface, very happy with it.

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Solder paste

Solder paste has a short shelf life (6 months or so) and i can attest its quality makes a world of difference. Low quality old stock made me think the reflow videos i saw online were simply not something mere mortals could achieve. But with a tube of paste manufactured two months ago that was constantly refrigerated, I built a board with about ~50 components with only one defect: two pads on a SOIC chip were bridged - a common issue that’s easy to fix (NE5532 at the right of this photo).

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Paste is one of those things you shouldn’t buy from AliExpress unless you find a store there you really trust. I’m sure the stuff they sell you was high quality when it left the factory, but it’s worthless if it sits in a hot warehouse for two months before reaching you.

Some solder is advertised as being more convenient to work with due to having bismuth, requiring a lower temperature. It’s a trap - the joints it produces are more brittle.

Stencils

The fab can make a laser-cut stencil for your PCB. I have not tried this. What i have tried, however, is a 3D printed stencil - and I can attest it gave me excellent results.

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The tool i’m using takes a Gerber and gives you a 3D printable stencil in return. The only setting i changed was increasing the size of holes. Because of that, all the pads of a chip are linked into a single opening, it’s proving not to be a big issue in practice.

The stencils use very little filament, but are probably good for no more than 3 uses before getting too gunked up, and cleaning might damage them.

https://solder-stencil.me

Magnification

A way to magnify your work is crucial to inspect the board. I have a cheap microscope i am happy with, but am not sure everyone ought to get one, when jeweler’s loupes / binoculars are very cheap and get the job done.

I found it particularly useful to make sure I placed diodes in the correct direction.

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Tweezers

Decent tweezers help! If you use the cheap stuff, sometimes it will simply NOT release the component at all. I bought the cheapest “legit” brand AliExpress has to offer, Sanhooii, and it was a very worthy upgrade.

That’s all i got for now. I’m generally happy with the process, and will probably go with SMD rather than THT in my future projects.

Your thoughts?

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As someone who does SMD very rarely - and then pretty only much when no other option is available - I can say that the smaller components are an order of magnitude more difficult to work with. Working with components smaller than a sesame seed is maddening.

I am currently stalled on a project simply because I can not tell the orientation of the diodes. I know you can check with a voltmeter - but the combination of the fact that the diodes themselves have no discernable orientation coupled with the fact that the orientation is not printed on the board… its been a serious issue for my brain.

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That i had to double-check diode orientation to assemble this board today made me realize i will have to alter my footprint in the future, i’ll add an additional line and a K letter towards the cathode.

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You have a very good grasp of the challenges in SMD use. The method is only important to you and your own workflow.

I mix it up between hot air and hot plate depending on size, the only exception is with large or crowded boards where I use cut up sections of a silicone baking sheet to mask off the board so I can work on one section at a time with hot air.

Component placement. Only have one type/value of component out at a time. Incorrect parts stand out more. If you lay out all the smd components before you start you are more likely to make mistakes or sneeze your BOM off the desk.

Keep your solder paste in the freezer until you need it and warm it in a cup of warm water to body temp or a smidge higher for better flow. Old solder paste can sometimes be revived with a stint in the freezer but ultimately just buy enough for your project. I also fix the solder tube to a multitool and give it a huge shake to assist in flow control.

Flux paste, not liquid, and plenty of it is more important than the state of your solder paste. Warm it too.

If you have no silk screen a sheet of clear PET plastic and a marker will allow you to mark pads on the sheet and then stamp or cut out the holes (seen it, but never done it)

I use suction tools to place parts, tweezers and a wee tungsten probe to refine or hold while heating. For me the #1 tool at this point is a £7 pair of tweezers that are connected to a multimeter/tester, making sure of values and polarity before heat.

Clean your tweezers with alcohol or brake cleaner and they will release easier, giving the components a quick mist will also help (or make a wet mess if overdone). Also make sure not to flatten out the angle of the tweezers tip by squeezing. Only the very tip of each leg needs touch the part.

A tiny dot of CA glue applied with a pin can help with wandering parts.

Avoid the distraction of smd parts storage and other supporting nonsense. Most accessories are made for the industry and add no benefits.

A pal who makes pedals had JLB PCB make a tiny set of solder screens in three component sizes which he sliced up even smaller and soldered to copper pins (mostly for repairs but like me he’s got shaky hands and any support helps).

Magnification, I have it all; jewelers lupe, wireless pen camera magnifier (sold as a zit popper on Ali express), large and larger lenses in a variety of frames and retorts. I even have my grandfather’s brass microscope which came to me when he retired. However… (Don’t hate the messenger) If you have a phone with a 4mp camera or higher then you already have the ability to ‘zoom’ in well enough to work with any size of component and better than most hobby electronics magnifier. Your only issue is keeping the phone still and ‘awake’.

My only design comment is be really sure your project NEEDS to use smd parts. Must you have your oscillator fit on a postage stamp? That said, using smd components without a PCB to add diodes or resistors to a jack socket or led etc. is very satisfying. Perf board (plated holes like stripboard) is a fantastic when prototyping with smd parts. I’ve made a few mini motor drivers for my RC plane pals using perf and smd parts and any passive circuit you fancy can be reduced but be sure or you will hate your creation and regret the time spent and that is the opposite of the ethos here. (IMHO)

Ok, that’s my tuppence worth.
My best to all

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My smd probe tweezers.

One other tool I use is a jewelers flat screwdriver as the straight tip makes shoving awkward parts about easier than waltzing a resistor around the pads with a pointed probe.

Also, apologies for talking about screens/silk screens when these days they call them stencils.

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For me the big issue with SMD is solder paste. I’m a hobbyist and not a particularly prolific one. The contents of a tube of paste is a lot larger than the amount I would realistically use in the time it takes to go bad. I am still using the spool of wire solder I bought over four years ago at a cost only 33% more than a single tube of paste — and in that time I’d presumably need to go through about eight tubes.

For the most part I don’t need the compactness of SMD — not for Kosmo modules, usually — and it’s rare I need to use a component that isn’t available as through hole. SMD parts can be cheaper but not by enough to offset the difference in solder cost.

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Is the cost that bad? The recent syringe that gave me great results cost me 5 euros and is from a no-name Aliexpress shop (but a highly rated one that specializes in this product) and the cheapest ones i see on Mouser go for 15 euros. The way i see it, even if i don’t use it all, it’s at worst a €30/year expense.

And by going SMD, i can probably often avoid making a controls PCB and a separate logic PCB, and figuring out their interconnection. Fewer boards means less expense, too.

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Most Kosmo modules with board mounted panel components need two PCBs anyway, one for jacks and one for pots/switches. Granted, smaller components may still mean smaller total PCB area, which may mean cheaper fabrication.

Aside from the cost of paste, there’s putting 80% of the lead I buy into the waste stream in six months, which doesn’t sit well with me.

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I do almost exclusivly SMD builds now and use standard solid solder. I have exactly the same problem with paste and it expiring before i get anywhere near using a full tube.

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Agreed, virtually all of my PCBs’ sizes are limited by the convex hull of the front panel parts. On top of that, I’m pretty sure the area term in the PCB cost calculation is based on (max width) x (max height) , so there’s little point in going for any shape except rectangular.

Take the polymorph per example: Polymorph analog VCO . That’s a 250+ part project, yet there’s plenty of space for THT parts and routing.

The only upside of SMD I see is the possibility for automated fabrication. But then again, I don’t really mind the soldering. Is it really DIY if you’re not sniffing them vapours?

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After getting used to soldering SMD resistors and caps, trimming the excess leads with THT and having them all over the bench, floor and in my socks is a pain in the ass, and often a pain in the toe. RIP robo vac too.

Desoldering components is much easier with SMD not to mention storage and costs are much lighter.

I have to admit SMD is kinda less fun and theres a certain theraputic hands-on feeling to THT, but when you just wanna get things done or its a big layout, it can feel like a chore with all the extra steps that come with THT.

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