DIY PCB's: Headers on topside.

[Blueteeth]

Hey,

Well I've got all the stuff for DIY PCB's, and I'm confident after a bit of practice, I can make some reasonable single sided PCB's...and maybe even some crafty double sided ones.

That said, a lot of my projects use CPLD's and microcontrollers, and so, I'm going to need plenty of headers on boards. Because many of these devices are SMT (oo less drilling for me!) I'm hoping to mount them on the topside of the board, however...connecting top side copper tracks to pin headers is tricky without plated through holes.

Any tips? I've thought of use plating techniques, and rivets and even lifting up the plastic housing from the header, soldering the pins on the top side, then pushing the plastic holder back down..which is a bit dirty. I'm aware of SMT 0.1" headers, but these are rare and expensive, not to mention they don't give as good mechanical conneciton to the board as through-hole.

Just wondering how other people do this, or do you all use loads of via's to make sure all header connections are on the bottom side only? One CPLD has 100 pins, so the idea of soldering 80 or so vias isn't thrilling...especially considering it can all be done on the top side, with a minimal of wire links on the bottom.

Blueteeth

 

[Torben]

Why not call the side with the SMTs the bottom? Then you can do the headers through-hole, as well as any larger components you might have. I see this done in a lot of the equipment I rip apart.

Some other stuff uses flat ribbon "wire"--actually more like copper traces sandwiched in plastic which slide into a ZIF SMT socket on the PCB, like the ribbon that connects a laptop's keyboard to the motherboard. You could run that to a daughterboard with the header on that. More expensive, though. And I don't know what that connector is called.

Just a couple of random thoughts. I haven't actually had to deal with the problem myself yet but I intend to, so I'm interested in hearing more ideas on this topic, too.


Torben

 

[Boncuk]

If you use normal pin headers and mount them on the "solder side" you use the top layer to solder them in place properly. Taken from the library each solder pad will be on the top and on the buttom layer. Just don't print the buttom layer if everything is soldered on the component side.

Since they are mounted 180 degrees "false" (upside down) the pin count will be reversed. Using R/A (right angle) pin headers the overall thickness of the finished board won't be that much more.

I usually select pin headers with a pin distance of 2.54mm (1/10"). They are also available with a pin distance of 1.27mm (1/20").

Regards

Hans

 

[ericgibbs]

Why not call the side with the SMTs the bottom? Then you can do the headers through-hole, as well as any larger components you might have. I see this done in a lot of the equipment I rip apart.

Some other stuff uses flat ribbon "wire"--actually more like copper traces sandwiched in plastic which slide into a ZIF SMT socket on the PCB, like the ribbon that connects a laptop's keyboard to the motherboard. You could run that to a daughterboard with the header on that. More expensive, though. And I don't know what that connector is called.

Just a couple of random thoughts. I haven't actually had to deal with the problem myself yet but I intend to, so I'm interested in hearing more ideas on this topic, too.


Torben

Morning Torben,

Is this the cable you had in mind. Flexi-cable.

http://www.dalbani.co.uk/electronic_components/cd_flat_flexi_cable.php?p=

http://shop.ebay.co.uk/items/_W0QQ_nkwZflexiQ20connectorsQQ_armrsZ1QQ_fromZQQ_mdoZQQ_sopZ12

 

[Torben]

Morning Torben,

Is this the cable you had in mind. Flexi-cable.

http://www.dalbani.co.uk/electronic_components/cd_flat_flexi_cable.php?p=

http://shop.ebay.co.uk/items/_W0QQ_nkwZflexiQ20connectorsQQ_armrsZ1QQ_fromZQQ_mdoZQQ_sopZ12

Morning Eric! Yes, that's just what I was talking about.

Thanks for the links,


Torben

 

[jpanhalt]

It is possible to solder right-angle headers on the same side that they sit, but difficult. That is, the headers with the pins that come out of the plastic and bend to a right angle. It is hard to do, and you lose the mechanical advantage of the through hole.

Therefore, I do as suggested above. If I want through hole, I mount the header on the normal component side and keep SMT's on the copper side. Other large components, e.g., relays, power inductors, large electrolytic capacitors, are also mounted on the component side.

If you do that "hybrid" SMT/through-hole construction, I strongly recommend using a schematic capture program like EAGLE for making the board. It will help keep the inversions straight.

Remember, inversions are not an electronic problem with 2-pin devices and other devices with the pins in a plane (unless orientation of the case, such as the tab on a TO-220, matters). With staggered 3-pin devices and other devices with more pins not in a single plane, inversions will make a difference.

John

 

[Mikebits]

SMT headers are fairly strong if soldered well.

**broken link removed**

 

[3v0]

I have to agree with John about using a schematic capture PCB layout. When you start flipping parts from side to side it is too easy to mess up.

So far I have done as suggested by others and put the headers on the bare side of SS boards and the SMT parts on the copper.

I have one DS board design where I used a DIP chip instead of the SMT part because I needed the signals on both sides of the board. The DIP when soldered top and bottom acts as vias. With home brew non plated boards through hole parts can be useful even if the board is mainly SMT.

For SS boards you may be able to push the pins flush into the plastic strip. Insert the header with the plastic on the bare side and solder the pins on the other. The plastic would help reduce torque on the solder joints.

 

[Blueteeth]

Hey guys, cheers for the responses ^^

I did of course consider the idea of having the usual bottom copper, for SMT, with through-hole on the top. I guess the fact I'm hesistant to do that is more a question of aestetics, gotta love a PCB covered in little SMT chips.

Thankfully eagle can sort out that easily without problems, just mirror the SMT parts and they go on the bottom side. Thats a lot safer than designing it with topcopper and trying to work out for yourself any inversion problems.

Good call on the flex cables! A second board for actual 'headers' could always be atteched to the other board.

Mikebits: Agreed, they are fairly mechanically stable, and I would happily use them but maybe I'm missing something..tried RS AND farnell/newark (I'm in the UK so digikey is out for small orders) and I can only find a handful of them, plus they are ridiculously expensive, where-as 2x40 pin headers can be had from ebay for just a few cents, I literally have hundreds of them.

3V0, again, agreed, when parts have easy access to top side and bottom side, like DIP chips, they make vias. I've no problems with wires made with a bit of wire, but using a part as the vias themselves would make it neater.

As an FYI, the three idea's I'm going with are the following :

1) Pushing the plastic carrier so it is halfway up the pins, placing the hdeare in the holes with matrix board underneath to hold it, then hand soldering the pins on the topside. Then pushing the plastic carrier back down, and soldering the bottom.....like the DIP chips, these will also act as vias.

2) Partitioning a design into two parts. First contains the 'brains' like micro's and CPLDs/FPGAs as a 'header board'. With pinheaders pointing down, and the SMT parts on top. This can then be plugged into female headers mounted on the base board. Advantage of that is you can reuse the header boards for different carrier boards! In effect, as we are doing is use two boards for double sided....one as top copper, and one as bottom copper, with the headers acting a 'via's. This would especially be handy for PQFP208. (yes, some projects I have use chips with lots of pins, FPGA's, DRAM, etc..)

3) Not worth the hassle but purely as an academic approach, making my own copper bails for easily plating holes. I've spoken to a chemistry friend about plating copper onto tin solder (RoHS) using 'available' chemicals, and I reckon it could be done.
The 'copperset' system is no longer being made, and the bails are 24UKP for 500. Given that even with small amounts of chemicals I could plate maybe a hundred metres of solder, if I need loads of bails, making them en mass isn't tough.
I'm always up for 'true DIY', not like those makezine amateurs But chances are I'll fail miserably.

Thanks for the help guys, I'll let you know which method proves to be the best. Again, this is really only for complicated DS boards, that in fact would probably be cheaper the get professionally made up, but hey, its a challenge, plus I'm a cheapskate, if it costs more than 30 quid, I'm not doing it.

Blueteeth