Need some advice to connect boards to boards

[bask185]

I have a small choise dilemma and I could use an extra thought.

For others I am designing PCB control panels which also acts as front end panel.

Such a panel will be unique per customer. I use through hole leds and tactiles because I want to avoid soldering on the front side. Soldering makes things dirty so I only want to solder on the back side. I also prefer diffused tht leds because they look better imo. And I can buy many sets of tactiles with nice caps in tht.

Every PCB needs some kind of a controller. That red stamp sized thing on the bottemright is my first variant. It is my own ‘arduino’ variant with 5v, 3v3 regulator, i2c eeprom, max485 and atmega. It has castellated holes which I learned to do on this great forum.

The controller must be made using JLCPCB smt assembly.
The panel must be handsoldered because smt assembly would be too expensive and the resistors would be on other side than the switches and leds. Doing smt on 2 sides would even cost me more.

Idea #1 is to solder such a castellated hole controller underneath the board. This particular one needs redesigning because it is too big (impractical shape) as it is now. If I lack IO which is the case for this panel, I would need a suplementing similar device. This one could do without all chips and would… well just be an atmega with crystal and some expensive castellated holes. It would be smaller than a pro-mini and communicate via I2C.

Soldering the castellated is considered acceptable amount of labour. It does up the the price over €1,25 per board and forces me to order atleast 50 pieces because of it. This is tollerable. (and I learned that I can sand PCBs to make own castellated holes )

Idea #2 is putting the controller on distant and connect via 20p jtag flat cable. That is where the bottom SMD connector was ment for.

The most obvious drawback is that the controller also need a connector and this means more labour work unless this connector can be done via smt assembly.

This is the prototype controller with supplementing IO extender.

Idea #3 looks a lot like idea #1 but the castellated holes are replaced by smd board 2 board connectors. Instead of soldering the controller to the panel’s back, I simply press it on the panels back. Because there will be some distance between the 2, the controller could hover over THT parts which the castellated variant cannot.

This idea has my preference. It solves the size issue of idea #1 and is less labour than #2. However I find it hard to implement because I cannot find good board 2 board connectors. There are just soo many of them and the search functionality by jlcpcb’s component library is a bit crappy tbh.

The panel could have the male or female conenctor and the controller would be the other gender.
The connector I am searching for needs to be able to be soldered by hand as well as by smt assembly service.

One of the connectors I thought of was this one used on model train decoders

The male connector is not plugged in on top but via the bottom side. This one is called a Plux22 but I think it is a proprietary one? Anyways jlcpcb does not have this one. I do not know the general name for these kind of connectors. I do can buy them and solder by hand. I would have the female connector be on the controller side and the male on the panel.

Any input like ideas, connector suggestions, recommendations or brainfart… would be greatly appreciated.

Kind regards,

Bas

[teletypeguy]

Hi Bas:

I started using an M.2 connector after playing with sparkfun micromods.
sparkfun micromod stuff

I made a micromod (with kicad) and had it fabbed and assembled at jlcpcb. Sparfun is the red one and mine is the green one (0.8mm thick pcb with enig connector pads and a chamfer to plug in):

I have used it in a couple of kicad projects now:

The m.2 connectors are available in several versions and you can define whatever pinout works for you (I stayed with the mmod pinout). I have soldered several of the connectors with flux and a solder-loaded angled tip, but it is a bit of a pain. If you are having the smt done at an assy house, I give a big thumbs up to m.2 connectors.
gil

[Piotr]

@bask185

The controller must be made using JLCPCB smt assembly.

Consider next idea - to assemble everything manually. If your processor is 0.65mm raster than you should not have big problem with assembling it manually.
You need the good tin with rosin (I use 0.5mm diameter) and soldering iron with sharp tip. Good tin means not PbFree.
You fix atmega with two pins (you can correct positioning many times till you are sure it is good) and then you solder pin, by pin.
The other method (I never tried) is to solder all pins together and then take away the excess tin using the copper braid of some shielded wire soaked in rosin. To do that (when I have two pins shorted) I use
100W transformer (gun) iron as heating that braid needs more power.
The word ‘braid’ I took form google translator and hope it is what I wanted to say.

If you have your pcbs with atmega footprint you can do some experiments before designing the whole big PCB with atmega at its back.

The other subject is RS485.
Typically communication wires need some protection against pulses that can come and damage IC.
In past I met some people who installed RS485 drivers in socket because “it is an element that wears out and sometimes needs to be replaced (like a light bulb)”. They simply didn’t know how to protect it.
When near the building that your devices operate lightning will strike two devices, both connected to that building grounding system can have a difference between their grounds up to about 50V with infinity current efficiency (looking from electronic perspective).
If that happens when both devices are in two buildings (even touching themselves with their walls) the difference can reach several kV.

[der.ule]

At least SAMTEC name those kind of connectors “bottom entry” or “pass through”

.100" Pitch Socket Strips, 2.54 mm Square Post Socket Strips | Samtec

[bask185]

Thank you for your answers. That M2 connector looks nice, I didnt know about this type to connect boards 2 boards.

Soldering those things by hand seems labouring to me, and as you point out that it is somewhat a pain…

Impatient as I am I continued to work out some ideas. And I think the best trade off between costs, labour and space is to use plain 2.0mm male and female connectors.

The baseboard get’s a SMD male header. This safes space and is not that labouring to solder as the 2.0mm pitch is relative large.

Same thing on a different panel

That connector also has SDA, SCL, 485 A, 485 B and an analog pin routed. One of the boards will be I2C slave and the other is the master. Their software will be identical and they can tell which is who by means of soldering a single resistor on the carrier board to make a restistor divider.

The mating controller gets a THT female. Like der ule points out, those connectors I wanted are called bottom-entry or top or bottom entry. Unfortunately JLCPCB don’t have any in stock (for as far as I can find). They do exist in their inventory list. So I don’t know if they will ever come in their inventory. As such I chose for THT. And I could redesign the contoller without that much work should I swap to a bottom-entry connector.

Those 3 solder pads on the bottom are the address pins for the EEPROM. The master controller should be able to use the eeprom of an adjacent slave though this will not be needed… I think…

In past I met some people who installed RS485 drivers in socket because “it is an element that wears out and sometimes needs to be replaced

They are not to expensive to manufactor, I could simply send new controllers if this ever happens

I do know how to smd solder, that is not an issue but I simply do not want to solder at al. It is simply far more cheaper to let a jlcpcb do this for me. I already get me enough work to solder the control panel itself.

Anyways, thank you all for your contributions. I will keep those m2 connectors in mind.

Kind regards,

Bas