Hobbyist 1 or 2 boards - PCB or PCBA ?

[jmf11]

Hello,

The question:
I'm working on a stm32H7 + USB3300 project for audio applications. I deals with LQFP and QFN pachages, 4 layers PCB. I may need only 1 or 2 boards. My previous project PCB was OK from the first time, but I don't know if it was due to luck or good reviews.

I wonder what are the best options for Hobbyist and this type of one-off projects, and how you proceed in similar cases.

My understanding:

PCBA pros (ex JLCPCB):
- difficult soldering done with industrial quality,
- address all (most of) the small 0402, 0603 packages
- prices of parts are often much cheaper than direct order to Digikey/Mouser (often offsets the assembly prices),
- no need for stencil (which adds cost to bare PCBs)
 
PCBA cons:
- minimum 5 boards,
- if my design is wrong, I have 5 useless populated boards,
- higher shipping costs,
- few parts may not be available (and no equivalent), which means "rework" without stencil (in my case some audio quality oscillators mear the MCU).

Bare PCB pros:
- Bare PCBs are cheap,
- If I have errors in the PCB, I cane have those redone for cheap
- I can order just the components I need (no board minimum number)
- higher shipping costs,
- I can solder/reflow the board in one stage, having sourced all parts.
 
PCBA cons:
- stencil seems to significantly increase shipping costs... but are so useful (mandatory?) for QFN and LFQP,
- prices of parts (much) more expensive from Digikey/Mouser, and have to reach 50€ in parts for free shipping. Not that easy for a 20€ board (PCB+parts),
- some packages difficult to solder correctly with just hot air,
- risk to damage components in the soldering process
- population of 0402, 0603...

How do you proceed in this type of situation? Any advices on mitigating the "cons"?


The context:

From time to time, I'm in design and making boards for audio niche applications. My previous project was https://github.com/jmf13/NeatAmpTAS3251/tree/master. I have basic soldering skills, with iron and hot air.

This is a hobby, open hardware oriented. I base my design on other ones, application notes, reference designs. I try do it well and review a lot, but it is not my dily job and may make errors.

Best regards,
JMF

[bostonman]

Seems as if you're asking what most hobbyists are confronted with often when they come up with a design.

Only you can determine what is worth it based on your budget. I would buy bare PCBs and populate them myself, but I don't know much about your design, soldering skills, and equipment.

[liudr]

To mitigate design errors, you could add test points, lengthen and isolate some traces for cutting, and bring out pins that you thing aren't used to pads or holes so you can more easily fix your mistakes without having to order new boards.

Regarding PCB + own work vs. PCBA, are you a hands-on person or a hands-off person? If you don't wish to stock parts and supplies for hands-on approach, you are better off with PCBA, which is basically at the mercy of the assembler. If you prefer hands-on, you need to stock up on some parts and supplies. I prefer hands-on because despite best efforts of PCBA, there will be repairs and rework anyway unless you are willing to throw away any board that has issues.

The project you provided as an example looks pretty good but also is not a tiny board so your stencil cost will be around maybe $10 without frames at jlcpcb and should be able to be shipped with your boards, unless you were spec'ing for framed stencils that always ship separately, thus increasing shipping cost by about $30. In my opinion, stenciling is a necessary skill at your level. You have to face it eventually if you're trying to avoid it now. Maybe one of your designs becomes popular and you want to make a few dozen boards to sell to your community. Then you will want to think about populating the boards on your own, with components you buy from trust-worthy vendors such as digikey. I can trust jlcpcb for boards, most of the times, if they don't mess up panelizing them for me from time to time. But I will put my trust of parts with digikey. Reworking a board that size just for a few components that the fab doesn't have is a hassle on its own, which I only bother in order to repair boards that failed QA.

[jmf11]

Thank you for your replies and advices. In both messages, wind seems to head toward doing the first boards "hands-on", populating and soldering by myself. This helps to confirm that it makes sense for prototyping, even if PCBA prices are low.

I believe that I'm in between hands on and hands off people. I already played with stencils. So I believe I can do it (at least for LQFP, to be confirmed for QFN). Doing by myself relieves some stress as it lowers impacts of potential errors. This is a good thing.

When working on prototypes, are there ways to buy part "kits" with small quantities of very frequent parts (instead of buying capacitors/resistors one by one...)?

For my project, I will do my homework and check my BOM on Octopart, to compare with JLCPCB part prices. I recognize that I appreciate the JLCPCB "basic parts" feature, which helps narrow down the choices. Digikey / Mouser are sometime overwelming when looking for small basic things

Best regards,

JMF

 

[rich t]

I build boards in my toaster oven all the time, down to 0603 components and 0.5mm pitch QFN.

You can see some of my PCB experiences here in step 3: https://hackaday.io/project/192598/instructions

I elongate the pads of QFN somewhat so I can rework with a fine tip soldering iron if needed -- I limit paste on the QFN flag to 25%.

Using hot air rework station, I can reuse QFNs close to a dozen times -- I have much worse luck with QFPs, which is why I actually prefer QFN.

I usually start without a stencil (just tinning pads by hand with a soldering iron and a Kester 2331-ZX flux pen), but as the quantity grows, I use a stencil and paste (my favorite is ChipQuik SMDLTLFP).

You can see my PCBA experiences here: https://rtestardi.github.io/usbte/pcbafd.pdf

The things I'd call out if you are just beginning are:

1. the flux pen is your friend -- use it generously and components will just snap into place on reflow -- do clean flux if you are using water soluble like I do.
2. heat the underside of the board with hot air first if you need to remove a QFN from the top.
3. if panelizing boards, keep mlcc chip capacitors a bit away from the edge.
4. if using smt usb connectors, a through-hole in the main ground pads makes the connection much stronger (even though some sucking of paste can occur there)
5. when laying out 0.1" header holes as is common, I stagger then by +/- 0.0045" so that the header does not need to be held for soldering!

PS if you want to see how to build a toaster over controller for SMT: https://rtestardi.github.io/usbte/toaster.pdf

[Kim Christensen]

When working on prototypes, are there ways to buy part "kits" with small quantities of very frequent parts (instead of buying capacitors/resistors one by one...)?

You can buy SMD resistor and capacitor books. (example only) Other configs and prices can be found elsewhere.

Another option for buying Digikey parts is to team up with other hobbyists locally and combine your shipments. People used to do that at my old workplace. They'd send out an email with a deadline and people could place their orders. Works only with trusted people of course.

[jmf11]

I usually start without a stencil (just tinning pads by hand with a soldering iron and a Kester 2331-ZX flux pen), but as the quantity grows, I use a stencil and paste (my favorite is ChipQuik SMDLTLFP).

How do you proceed to tin the pads by hands? I imagine not just with flux.

I have seen some videos about soldering LQFP by sliding the iron over the pads. But can you solder QFN without stencil, paste and hot air?

By the way, I was checking prices in Digikey, and the stm32H7 are more than 2x the price from JLCPCB when PCBA (11€ vs 4.8€). But I have to be consistent with my query: needs and constraints are not the same when prototyping and for batch production...

And also Thanks Kim for the link to capacitors books.

JMF 

[rich t]

> How do you proceed to tin the pads by hands? I imagine not just with flux.

I just use a solder blob on the tip of the iron -- and lots of flux on the board -- as you drag the iron across a pad, the pad ends up with a bump of solder on it when you are done.  Then I flux again and place components on the fluxed bumps -- and then off to the toaster oven.  The upside of this is you don't need a stencil at all.  The downside is you have to touch each pad and the resulting "bumps" are hard (unlike paste), so if you have something like an 0603 capacitor and the bumps are different heights, the component can be quite tilted until reflow.  Occasionally, if one bump melts before the other (like especially when they are very different heights and the component is tilted), then the component can "tombstone" up on one side, rather than lay flat.

But whenever I am building just one or two boards, this works fine with no stencil, and occasional rework after reflow.

> But can you solder QFN without stencil, paste and hot air?

Absolutely, yes, 0.5mm QFN without stencil or paste or hot air.  (Though hot air is absolutely needed for QFN reuse -- for the first two months, I had to reuse a handful of QFNs until Microchip production ramped up!)  See: https://forum.microchip.com/s/topic/a5C3l000000MfQtEAK/t388630

With the method above (no paste, no stencil), the typical failure is "open" pins.

With paste and stencil, otoh, the typical failure is "shorted" pins.

In both cases the rework is identical -- lots of flux on the board and drag the very fine tip of the iron (either dry for reworking shorts or with a bit of solder for reworking opens) along the pads and suck up the extra solder.

Really, QFNs are easy this way -- easier than QFP -- because you can't damage the pins with the soldering iron.  I did a couple of dozen boards of my most recent design without stencil or paste (https://hackaday.io/project/192598-flea-scope-usb-oscilloscope-18-18-msps-webusb) before I started using stencil and paste (which is easier mostly because component placement is easier -- you can push the 0603 into the paste and it sticks).

On my larger builds, I have a small turntable so I build 10 boards at a time next to each other, and they all go thru the toaster oven together.

PS if you want to see how to build a toaster over controller for SMT: https://rtestardi.github.io/usbte/toaster.pdf

[rich t]

PS you can also flux the QFN component pads and tin them just like the board -- then when you lay the QFN down on the board, you have bumps on top of bumps -- you end up with less "opens" that way...

Snipping this out from the document I linked to above:

Toaster Oven Tips
Fine pitched SMT reflow soldering can be an intimidating task to the newcomer, but I have found that I can
reliably reflow “Flea-Scopes”, including 0.5mm pitch QFP’s and even 0.5mm QFN’s with just a few tricks:
 
• To mount a QFN (leadless) package, I first flux the component pins and then tin them with my soldering
iron; each pin ends up with a little bump of solder on it.  I then flux the board pads and tin them, with a
similar and opposing bump (it definitely helps to have a solder mask on the board, but I have done it
without).  Then I flux everything again and align the bumps on the QFN pads with the opposing bumps on
the board, and prepare for reflow using your new Toaster Oven Temperature Profile Controller.
• To mount the QFP (leaded) packages, I use a similar procedure but do not tin the delicate component pins.
• After reflow, if touchup is needed (the typical problem will be “opens”, not “shorts”), I use a generous
amount of flux at the pin/pad interface and then drag the ever slightest amount of solder across it with my
soldering iron.  Note that the flux is critical in this step to avoid any bridging/shorts.
• I used a Weller WES51 soldering iron with an ETP (1/32" screwdriver) tip, Chip Quik SMDLTLFP
solder paste, Kester Pocket-Pak lead-free rosin-core solder, a Kester #2331-ZX water soluble flux pen,
and a PanaVise PV Jr. Model 201 (to hold things).

[liudr]

You can buy SMD resistor and capacitor books. (example only) Other configs and prices can be found elsewhere.

Another option for buying Digikey parts is to team up with other hobbyists locally and combine your shipments. People used to do that at my old workplace. They'd send out an email with a deadline and people could place their orders. Works only with trusted people of course.

LOL I have those books. Been using components from those books often and refilling/adding more of them whenever I order from digikey etc. This way I always have a couple dozen parts that I can hand place vs reels of components that are fixed to my pick-n-place machine.
It's a great idea to find others to share components especially chip resistors and capacitors. Some are so cheap by the reel like $20 per reel for some 10K resistors you are better off getting a full reel to share with others.
It's also a good idea to consolidate components. Sometimes you must use certain value resistors for some ICs but other times you just need a voltage divider and involving a common resistor like a 10K will save part counts. Connecting multiple 10K to make 5K or 20K is also a good idea to reduce parts count. It might be hard for you since you deal with audio so some analog components require specific resistor values.

[m98]

For single-sided reflow, get one of those Chinese temperature-controlled hot plates. Components with a high thermal mass need some hot air assistance, but other than that, gets me consistently perfect results every time, even for really delicate stuff like WLCSP.

[mskeete]

With JLCOCB, You can actually order 5 PCBs and have only 2 of them assembled (ubless thats changed recently)

[jmf11]

Thanks a lot for all the feedbacks.

They helped me understanding some finally obvious things, like that doint the prototype is not doing a small serie of boards. Maybe the first prototype could have a slightly different board to optimize debugging.

Trying to cut costs too much for the proto could lead to a bad situation, with several boards, assembled but not usable.

Assembling the prototype seems the common way compared to PCBA (which is good for small series).

Using a stencil increases a bit the price of the proto, but I believe that it eases a lot the assembly (even if possible to do without as explained by rich t). I will by a small reflow plate to complement the hot air rework station.

Not many hobbyists around to share parts orders...

I don't find the adafruit R and C samples book in France unfortunately. I don't know if I can trust the Aliexpress ones (no indication on parts brands most of the time). 

Now horizon is clear and I just need to do it ;-)

JMF

[Kim Christensen]

I don't find the adafruit R and C samples book in France unfortunately. I don't know if I can trust the Aliexpress ones (no indication on parts brands most of the time).

You can get resistor kits at DigiKey France, but it's going to be more expensive than Amazon or AliExpress.