Need someone to fabricate this for me

Hi,

I cannot volunteer to do this, but I can suggest an alternative; if you don't have the tools or experience for this, you're better off ditching the castellated board which makes no sense if it is carrying just the microcontroller, crystal and a couple of passive components, and just redesigning the board on the left to carry the TQFP version of the chip, which has fairly large 0.8mm pin spacing, and is hand-solderable with a normal iron, no need for hot air tool skills. The TQFP part will take up less space than the castellated board, so it will fit. It would be a cleaner, lower-profile and more reliable solution.

Castellated boards kind-of make sense for (say) ESP boards because they contain more circuitry as a module, and also radio circuitry to simplify the end product. Here it's not simplifying, it's making it more complicated.

Also, the left side board is large, you could also go for a larger crystal and passives, if you feel you have the space for that. I can't tell if you're using 0603 or 0402.. 0402 is more unreliable for manufacture, I don't think it's worth the effort (unless there's a technical reason for using 0402).

Hi,

I cannot volunteer to do this, but I can suggest an alternative; if you don't have the tools or experience for this, you're better off ditching the castellated board which makes no sense if it is carrying just the microcontroller, crystal and a couple of passive components, and just redesigning the board on the left to carry the TQFP version of the chip, which has fairly large 0.8mm pin spacing, and is hand-solderable with a normal iron, no need for hot air tool skills. The TQFP part will take up less space than the castellated board, so it will fit. It would be a cleaner, lower-profile and more reliable solution.

Castellated boards kind-of make sense for (say) ESP boards because they contain more circuitry as a module, and also radio circuitry to simplify the end product. Here it's not simplifying, it's making it more complicated.

Also, the left side board is large, you could also go for a larger crystal and passives, if you feel you have the space for that. I can't tell if you're using 0603 or 0402.. 0402 is more unreliable for manufacture, I don't think it's worth the effort (unless there's a technical reason for using 0402).

Hi, The reason for the Castellated holes version is so that i can use the same module for other projects in the future, Instead of having a full arduino with pin headers taking up valuable room, i could instead go for a module that keeps a low profile while providing everythhing i need, Also i regrettably went with 0402 and it was a terrible mistake, I only have space where it is on the PCB due to how the xbox from panel connects to the front. I guess i could go with the TQFP version as well but preferrably would want to keep with this module idea for future projects.

Thank you for your feedback though and i guess i could make a prototype using the TQFP version. Also do you suggest any other crystals to use because they are really small and again, hot air rework/soldering those sized modules is really difficult

How many of these do you need to solder ?

You will get the best results if you use leaded solder paste, a stencil to apply it and reflow the boards in a decent oven.

If you don't have all these things, you can hand solder the parts on using a soldering iron and plenty of flux, but only if you have a decent sized hole under the chip centre pad.

If you have only the tiny holes shown in the data sheet or on hole you will have to use solder paste for the centre pad.

In my experience hand held hot air guns are worse than useless for soldering parts on.

You can use a hot plate.

The really cheap way:

Get some leaded solder past and some really runny flux ( https://www.rapidonline.com/warton-metals-future-315-low-residue-no-clean-flux-500ml-bottle-87-0728  )

This is the flux for hand work - I've tried lots. Rapid's cheapo solder wick is OK too.

Apply paste to the board using a pin, cocktail stick or whatever. You will end up with it smeared across the processor pads but this doesn't matter. Only put a tiny bit on the centre pad, especially if you have no holes through the pad.

Reflow the boards on a hotplate, (if you don't have one use a clean non stick frying pan (at a pinch a dirty sticky one )). Calibrate the hotplate if you have the means. You want it to be at 215C for lead free solder paste.

Place the solder pasted and componented board in the pan and wait untill the solder melts where you can see it on the processor and other component pins. Wait a further 30 seconds because the solder under the processor needs more time. remove the board.

Some of the pins on the processor will be shorted, clean it up with a fine tip soldering iron, solderwick and lots of flux.

Clean the board. Washing up liquid in water will do. Rinse lots in tap water. Rinse twice in de-ionised water. Dry thoroughly before you apply power (this is important !!!!).

This process works, using a Chinese £150 hotplate just permanently set to 215C I've soldered hundreds of boards like this. It's not ideal and I have better gear now. Leaded solder makes it all much, much easier. If you are going to use lead free solder just pay someone to do the job for you. (Perfectly legal to use leaded solder for prototoypes.)

With packages like the one you are using for the ATMEGA it is a really good idea to have a decent sized hole (1 - 1.5 mm diameter) in the middle of the centre pad. It will let excess solder paste escape, allow you to check if the pad is soldered or solder the pad if required. Ignore the manufacturers notes about lots of tiny holes - that's fine with a proper production process but not for home brew.

By the way - you don't need the castellation for prototypes unless you are really pushed for space - you can solder through holes to pads quite easily.

The picture shows a tiny board which replaces a chip on the main board that didn't quite work as planned. I used holes rather than castellations because it is massively more expensive to get single tiny boards with features on the edges. I bought all my tiny boards in a 16 board lump and cut them out with a very non-precision guillotine - that way they cost about £4 each rather than £40 each on a 2 day turnaround.

Hope this helps - happy soldering.

MK

Ideally i only need maybe 5 of these, The castellations don't actually cost any more when ordering via JLCPCB so its just $2 for the 5 plus the shipping cost from china.

The problem i have is i dont know whether the PCB's designed are actually good in that i havent made a mistake or that i am not soldering the QFN chips properly. As when i try to use arduino via ISP it always reports device signature 00000.

Ideally i am looking for some assistance into whether my PCB is correct or if not is anyone able to assist in designing a PCB of the same size (give or take) that fits this project perfectly and others in the future that i decide to make.

The hot air gun that is apart of my soldering station seems to do the job. After preheating the board at 150C on medium air for 5-10 minutes then when ready to solder using paste, pushing the temperature to 400c for the 10-15 seconds it takes to reflow

(i use 400 because i doubt its actually pushing out 400C air).

I will look into that flux you reccommended and while i cannot get the hotplate currently, it will be something i will look into for future uses.

Hi Alex,

I don't know if you're using an 8 MHz crystal or a different value, but this 5 x 3.2 mm 8 MHz crystal5 x 3.2 mm 8 MHz crystal might be an option. That is a popular size, and easy to solder.

Next popular size up is 7 x 5 mm. There are also larger curved metal can ones (available in through-hole or surface-mount) if you have the space for them.

If you post the schematic of your pcb here I'll look at it (as will many others who may be more Arduino expert than me).

The track layout would help as well.

Please post it as a .pdf or .jpb so that everyone can read it.

MK

Thanks for the tip of using a large hole under the center pad Michael. I hadn’t tried that and getting too much paste without a stencil is easy to do.  Also agree that hot air rework is difficult with these parts.  I use a cheap $30 toaster oven and watch a thermocouple for temperature control which works OK.

Here are closeup images of the PCB layout, i believe i have all the bare minimum components needed for this to function, the 100nF cap on the AREF so i can use analog inputs and the 10k resistor on the reset and VCC to stop random resets and allow the resetting of the ATMega.

The 16Mhz crystal not sure if actually needed to get it working as i can use 8Mhz i believe.

Thanks

From an EMC point of view it's not ideal - the ground plane is nearly cut in two.

I don't see any load capacitors for the crystal (if it's actually a ceramic resonator it may not need them).

The system should start up on it's internal clock so the crystal shouldn't matter if you don't use it.

How are you trying to programme it - does this programming process work with other parts ?

MK

They are ceramic Resonators yeah, And as for programming i'm using Arduino as ISP as you should be able to do that with these. i have done it successfully before using a DIP package ATTiny and a QFN version of the ATTiny but this doesnt want to play nicely.

Can you post a picture of your programming set up and explain exactly what happens when you try.

MK