Techniques for reliable high-speed digital circuits

[BigDumbDinosaur]

GARTHWILSON wrote:

Thanks for the reassurance. I just made my 44-pin PLCC socket PCB components. I'll see how the room compares and decide if I want to solder the PLCCs to the board directly, or socket them. I expect sockets will win, in spite of the extra inductance and the fact that they take room on every layer.

As you do your layout and consider the socket-or-no-socket dichotomy, keep in mind my POC V1.2 unit runs at 20 MHz without being on the ragged edge. I have my doubts the little bit of extra inductance added by a socket will matter in your application—it obviously didn’t in mine.

One other thing in favor of using a socket is the ease at which a part can be removed without damage, should the need arise. That has allowed me to salvage “expensive” parts from older POC units for use in new ones.

[BigEd]

(We've mentioned before that PCB technology keeps moving on: for example, four layer boards are now much more affordable. In the same vein, I gather assembly is now affordable from some suppliers: they will deliver assembled, or partially-assembled, boards, if the parts in question are in their catalogue. I think it's worth considering this option, especially when there's some anticipated difficulty with some surface mounted components. A board which already has PLCCs or RAMs soldered in, and perhaps also tiny capacitors, maybe even sockets, can then be finished off when it turns up.)

[plasmo]

GARTHWILSON wrote:

I'm laying out an '816 board. I thought of just soldering PLCCs directly to the board, but I would need space around them to solder them, meaning sockets wouldn't take any more room. Thru-hole sockets kind of precludes putting parts on both sides of the board right there, but SMT sockets cannot be hand-soldered. I'm also staying away from parts so small I'm not sure I could finish a board and not have soldering defects, so I don't think I want to try the PQFPs for this.

It is possible to hand solder SMT PLCC socket. With sufficient practice, the level of difficulty is comparable to hand soldering 0.5mm pitch QFP. I use a fine tip with 30 degree bend, Metcal STTC-026. The bend helps get around the plastic webbing of the SMT PLCC socket. SMT PLCC socket is bigger than PLCC IC, so you do need to allocate more space around a PLCC if you plan to use a socket.

BigEd wrote:

(We've mentioned before that PCB technology keeps moving on: for example, four layer boards are now much more affordable. In the same vein, I gather assembly is now affordable from some suppliers: they will deliver assembled, or partially-assembled, boards, if the parts in question are in their catalogue. I think it's worth considering this option, especially when there's some anticipated difficulty with some surface mounted components. A board which already has PLCCs or RAMs soldered in, and perhaps also tiny capacitors, maybe even sockets, can then be finished off when it turns up.)

SMT parts are generally cheaper and easier to obtain; pc boards with SMT parts are smaller thus faster & cheaper; and it is actually easier to solder SMT components than comparable through-hole parts once the soldering skill has developed. From cost point of view, if a designer is proficient with SMT soldering, I think an economical approach is to offer kits with SMT components already installed by the designer so the end users don't have to deal with SMT soldering.
Bill

[GARTHWILSON]

plasmo wrote:

GARTHWILSON wrote:

I'm laying out an '816 board. I thought of just soldering PLCCs directly to the board, but I would need space around them to solder them, meaning sockets wouldn't take any more room. Thru-hole sockets kind of precludes putting parts on both sides of the board right there, but SMT sockets cannot be hand-soldered. I'm also staying away from parts so small I'm not sure I could finish a board and not have soldering defects, so I don't think I want to try the PQFPs for this.

It is possible to hand solder SMT PLCC socket. With sufficient practice, the level of difficulty is comparable to hand soldering 0.5mm pitch QFP. I use a fine tip with 30 degree bend, Metcal STTC-026. The bend helps get around the plastic webbing of the SMT PLCC socket. SMT PLCC socket is bigger than PLCC IC, so you do need to allocate more space around a PLCC if you plan to use a socket.

What brand of PLCC sockets are you using? I was thinking of this kind, where the contacts are not accessible at all for a soldering iron, being completely underneath and covered:

The problem is the access, not the pitch.

[BigDumbDinosaur]

plasmo wrote:

SMT parts are generally cheaper and easier to obtain; pc boards with SMT parts are smaller thus faster & cheaper; and it is actually easier to solder SMT components than comparable through-hole parts once the soldering skill has developed.

Of course, it helps to be able to see those dinky parts and their equally-dinky leads whilst building one’s contraption.

[plasmo]

Garth,
These are unusual SMT PLCC sockets. SMT socket's contacts should be visually inspect-able to check for proper solder joints. Photo shows my SMT PLCC44 socket on the right; the PLCC44 socket on the left is through-hole PLCC socket.

[pjdennis]

BigDumbDinosaur wrote:

Of course, it helps to be able to see those dinky parts and their equally-dinky leads whilst building one’s contraption.

I'm getting there as far as struggling to see the tiny things. I'm jealous of the microscope camera I recently saw being used at this point in this video: https://youtu.be/nhAisp4dWq8?t=1054

[BigDumbDinosaur]

pjdennis wrote:

I'm jealous of the microscope camera I recently saw being used at this point in this video: https://youtu.be/nhAisp4dWq8?t=1054

You need steady hands, as well. I didn't watch the video for more than a few seconds before it was interrupted by advertising, at which point I quit. I don't patronize Youtube "content creators" who allow advertising in the middle of their videos so they can make a few extra bucks, euros, pounds, rubles, etc.

[floobydust]

plasmo wrote:

Garth,
These are unusual SMT PLCC sockets. SMT socket's contacts should be visually inspect-able to check for proper solder joints. Photo shows my SMT PLCC44 socket on the right; the PLCC44 socket on the left is through-hole PLCC socket.

The attachment DSC_69930703_F.jpg is no longer available

I also found a PLCC-32 SMT socket that has mounting studs. This should make alignment much easier before hand-soldering one of these.

[plasmo]

The mounting studs are nice alignment feature. The hard part about soldering the SMT PLCC socket by hand is not burning the central plastic webbing which is located very close the the soldering tabs.
Bill

[Dr Jefyll]

plasmo wrote:

The hard part about soldering the SMT PLCC socket by hand is not burning the central plastic webbing

I'd be tempted to take an X-acto knife and cut the little strands, then remove that little rectangular "floor" before the soldering even begins. Is the darn thing even necessary?

Alright, I guess maybe it's supposed to limit the depth of insertion of the PLCC into the socket. But you could wait until the soldering is done then put the little rectangle back where it belongs... secured to the PCB with a dab of adhesive. Hmmm...

-- Jeff

[BigDumbDinosaur]

Dr Jefyll wrote:

plasmo wrote:

The hard part about soldering the SMT PLCC socket by hand is not burning the central plastic webbing

I'd be tempted to take an X-acto knife and cut the little strands, then remove that little rectangular "floor" before the soldering even begins. Is the darn thing even necessary?

Alright, I guess maybe it's supposed to limit the depth of insertion of the PLCC into the socket. But you could wait until the soldering is done then put the little rectangle back where it belongs... secured to the PCB with a dab of adhesive. Hmmm...

That stuff might be more temperature-resistant than one might think. After all, the socket has to be able to withstand reflow temperatures of ~450°F for at least 30 seconds.

[BigDumbDinosaur]

floobydust wrote:

I also found a PLCC-32 SMT socket that has mounting studs.

Where did you find it?

[BillO]

plasmo wrote:

The mounting studs are nice alignment feature. The hard part about soldering the SMT PLCC socket by hand is not burning the central plastic webbing which is located very close the the soldering tabs.
Bill

If your using a low temp solder paste the temperature should not affect the plastic.

[BillO]

floobydust wrote:

Do you have side/angle shots that show how far the contacts extend below the bottom of the socket base?

I guess it's about the same as the stand-off height.