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Old 6th February 2013, 05:15 AM   #2701
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Quote:
Originally Posted by dchisholm View Post
Regarding your capacitor C2, is there a consensus about passing a trace between the legs of a part in 0805-package?
Actually, capacitors C1, C2 are 1210 in the layout shown, but if the trace is thin enough and the PCB fab can handle it, it should be fine with 0805 and 0603 also. I have used 12-mil traces between the pads of 0603 resistors on an ENIG board earlier, and the PCB house had no problems, and I was able to hand-solder the resistors without problems. If the solder-mask is good enough (i.e. doesn't lift or have bubbles/inclusions), it works fine.

Crosstalk isn't an issue since it's at right angles, but there may be a small amount of parasitic capacitance (~ 1 pF or less) between the component, dielectric solder-mask and the trace.

If the trace between the pads is a problem, then a zero-ohm 1206 jumper or routing it on the solder side are options.

Edit: Regarding coplanarity, it's not a problem for the Panasonic ECHU 1210 parts for C1, C2. They have a raised flange at the pads which essentially lifts the central section of the capacitor clear off the board. Panasonic probably did this to allow solvent penetration below the device, but whatever the reason, it seems to be good idea for capacitors.

Last edited by linuxguru; 6th February 2013 at 05:22 AM. Reason: addendum
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Old 6th February 2013, 06:39 AM   #2702
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Originally Posted by linuxguru View Post
Actually, capacitors C1, C2 are 1210 . . .
OK, must be time to send the old tri-focaled eyeballs out for calibration.

But the production problems described by VivaVee and rsavas seem to still apply to 1206 and 1210 packages. To be honest, I haven't had enough experience with those SMD sizes to consider whether the same practices would apply to their layout.

I don't recall if a trace under the component was a factor but I once spent time staring at customer returns through a toolmaker's microscope to discover absence-of-solder under component contacts. The insidious part was that the assemblies passed production test but failed after a short service - presumably, because the clean, new, component made enough physical contact with the (solderless) pad to establish electrical continuity at the time of manufacture, but eventually separated from the PWB during service. And to REALLY complicate the case, sometimes the symptoms would "clear while testing" - sometimes (not always) to reappear after the assembly was returned to service. Based on this behavior, somebody convinced the Senior Manglers that the inept and over-paid design engineers had created a design that was randomly breaking into oscillation. After finding several instances of dry solder pads, I showed them to the production engineer who said he'd fix the problem - and I guess he did, though I never heard how.

And, as you noted, it's not a problem in cases where the component body is deliberately inset from the end contacts. That's why it's not an issue with SOT23 packages: going from memory, I believe the official JEDEC standards call for at least a thousandth or two of daylight under the body when it sits on its leads.

It's just as true that it won't be a problem in a hand-soldering operation, where the operator forms a solder fillet between the PWB and component contact. (Assuming you can make the lead-free stuff behave that way.) This would be a "normal" manual assembly process - not the unnatural acts that design engineers have coerced components to perform during development and prototyping.

So I guess the two sides of this question continue to glare at each other over the wall, as they have for a couple of decades now.

Dale
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Old 6th February 2013, 04:15 PM   #2703
bcarso is offline bcarso  United States
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I wonder if anyone makes a zero-ohm jumper that has enough clearance to go over the top of another package?
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Old 6th February 2013, 04:40 PM   #2704
rsavas is offline rsavas  Canada
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Quote:
I wonder if anyone makes a zero-ohm jumper that has enough clearance to go over the top of another package?
I call it a wire!! :-)
You probably can use those SMT test points that are "C" shaped". Designed for scope probes etc.
Some SMT current shunts are of the same construction as well!!
As a suggestion, use SMT comps with folded end terminations to get around the coplanarity issue.
Would expect DFM issues to be the last of the concerns.
I can vouch that Contract MFG's like to add a premium for any DFM issue that they uncover, as part of a design review. So start adding those imaging fudicials & other DFM features as well!!
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Old 6th February 2013, 05:02 PM   #2705
bcarso is offline bcarso  United States
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Originally Posted by rsavas View Post
I call it a wire!! :-)
You probably can use those SMT test points that are "C" shaped". Designed for scope probes etc.
Some SMT current shunts are of the same construction as well!!
As a suggestion, use SMT comps with folded end terminations to get around the coplanarity issue.
Would expect DFM issues to be the last of the concerns.
I can vouch that Contract MFG's like to add a premium for any DFM issue that they uncover, as part of a design review. So start adding those imaging fudicials & other DFM features as well!!
Yes, I was thinking of those current shunts in fact.
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Old 18th February 2013, 07:22 AM   #2706
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The attached files are a documentation package for a proposed implementation of the SW-OPA Discrete Opamp. This documentation is a revised version of Post # 2620 in this thread, at < Discrete Opamp Open Design >.

The attached files incorporate revisions based on comments received in response to the previous post. Significant changes:
1. The input stage balance potentiometer has been relocated to the drain circuit of the input stage. Associated component values changed accordingly.

2. Protection diodes added to the output stage.

3. Minor component value changes based on reported parts availability.

4. Reference designators reassigned.

5. Thick-film resistors replaced by thin-film resistors in Parts List.

6. Ground plane removed from vicinity of input pins.

Changes #1 and #2 resulted in significant changes to printed wiring board layout and routing.

Dale
Attached Files
File Type: pdf SW-OPA_Post2620_SchemPL.pdf (391.2 KB, 157 views)
File Type: pdf SWOPA2620PWB1_RevB_ALL.pdf (437.9 KB, 108 views)
File Type: zip Post2620_Docs_RevB_Part2.zip (937.5 KB, 52 views)
File Type: zip Post2620_Docs_RevB_Part1.zip (667.1 KB, 51 views)

Last edited by dchisholm; 18th February 2013 at 07:32 AM. Reason: Corrected atch file
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Old 18th February 2013, 08:40 AM   #2707
RNMarsh is online now RNMarsh  United States
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Will this one day be available with all parts installed... thats about the only way I would get to hear it. Just too small parts for me to see/handle/solder. Thx-RNMarsh
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Old 18th February 2013, 09:18 AM   #2708
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I agree with Mr. Marsh. My 62 year old eyes and fumble fingers can't handle such small parts. Ray
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Old 18th February 2013, 02:44 PM   #2709
rsavas is offline rsavas  Canada
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"I agree with Mr. Marsh. My 62 year old eyes and fumble fingers can't handle such small parts. Ray "
If you guys want me solder up a few boards for you, then send me the parts & I'll do it for you. I can test them for operation as well, so you do not get DOA's.
This is great thread, so i can offer help to some good contributors.
At 53 and being very near-sighted, this is a cinch for me. Lots of experience doing 0.5 mm pitch, no problem. That and my 10x magnifiers, thin solder, fine tips etc.
Send me a PM and I'll supply my address, I live around Toronto,Canada

Cheers
Rick
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Old 18th February 2013, 04:09 PM   #2710
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Quote:
Originally Posted by dchisholm View Post
The attached files are a documentation package for a proposed implementation of the SW-OPA Discrete Opamp. This documentation is a revised version of Post # 2620 in this thread, at < Discrete Opamp Open Design >.

Thank you.
As far as I remember, this isn't Scott's latest design, is it?
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