So we have this application that requires a circuit to maintain intimate
contact with a curved biological surface, namely the abdomen of a
pregnant woman.

Flex is the obvious thing, but since the curvature is relatively gentle,
we're thinking of using thin FR4. We can get a 4-layer board with
overall thickness of 0.4 mm. The board needs to be about 20 x 100 mm,
and we can make it pretty spidery--each of the individual detector/TIA
pairs can be on its own paddle with a long skinny "cable" attaching it
to one end of the board.

Have any of you lot tried something like that?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Fri, 16 Jul 2021 13:25:45 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>So we have this application that requires a circuit to maintain intimate
>contact with a curved biological surface, namely the abdomen of a
>pregnant woman.
>
>Flex is the obvious thing, but since the curvature is relatively gentle,
>we're thinking of using thin FR4. We can get a 4-layer board with
>overall thickness of 0.4 mm. The board needs to be about 20 x 100 mm,
>and we can make it pretty spidery--each of the individual detector/TIA
>pairs can be on its own paddle with a long skinny "cable" attaching it
>to one end of the board.
>
>Have any of you lot tried something like that?
>
>Cheers
>
>Phil Hobbs

No, but I have done 3-layer boards.

On 16/07/2021 18:25, Phil Hobbs wrote:
> So we have this application that requires a circuit to maintain intimate
> contact with a curved biological surface, namely the abdomen of a
> pregnant woman.
>
> Flex is the obvious thing, but since the curvature is relatively gentle,
> we're thinking of using thin FR4.  We can get a 4-layer board with
> overall thickness of 0.4 mm.  The board needs to be about 20 x 100 mm,
> and we can make it pretty spidery--each of the individual detector/TIA
> pairs can be on its own paddle with a long skinny "cable" attaching it
> to one end of the board.
>
> Have any of you lot tried something like that?
>
> Cheers
>
> Phil Hobbs
>

I did have a 2 sided board 0.4mm that we used as poor-man's flex but it
was surprisingly rigid and had to be heated with a hot air gun to get it
bendy enough to adopt desired shape, once cool it retained shape without
tendency to straighten. I suspect it will not be conformable enough for
your application but is cheaper to try than flex.

piglet

Piglet wrote:
> On 16/07/2021 18:25, Phil Hobbs wrote:
>> So we have this application that requires a circuit to maintain
>> intimate contact with a curved biological surface, namely the abdomen
>> of a pregnant woman.
>>
>> Flex is the obvious thing, but since the curvature is relatively
>> gentle, we're thinking of using thin FR4.  We can get a 4-layer board
>> with overall thickness of 0.4 mm.  The board needs to be about 20 x
>> 100 mm, and we can make it pretty spidery--each of the individual
>> detector/TIA pairs can be on its own paddle with a long skinny "cable"
>> attaching it to one end of the board.
>>
>> Have any of you lot tried something like that?
>>
>> Cheers
>>
>> Phil Hobbs
>>
>
> I did have a 2 sided board 0.4mm that we used as poor-man's flex but it
> was surprisingly rigid and had to be heated with a hot air gun to get it
> bendy enough to adopt desired shape, once cool it retained shape without
> tendency to straighten.  I suspect it will not be conformable enough for
> your application but is cheaper to try than flex.
>
> piglet

Good wisdom, thanks.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On 2021-07-16 19:25, Phil Hobbs wrote:
> So we have this application that requires a circuit to maintain intimate
> contact with a curved biological surface, namely the abdomen of a
> pregnant woman.
>
> Flex is the obvious thing, but since the curvature is relatively gentle,
> we're thinking of using thin FR4.  We can get a 4-layer board with
> overall thickness of 0.4 mm.  The board needs to be about 20 x 100 mm,
> and we can make it pretty spidery--each of the individual detector/TIA
> pairs can be on its own paddle with a long skinny "cable" attaching it
> to one end of the board.
>
> Have any of you lot tried something like that?

not tried to use in real application, but seen and bent in my hands:
https://ats.net/products-technology/product-portfolio/flexible-rigid-flexible-pcbs/semiflexible-pcbs/

cheers
Michal

On Friday, July 16, 2021 at 10:25:54 AM UTC-7, Phil Hobbs wrote:
> So we have this application that requires a circuit to maintain intimate
> contact with a curved biological surface, namely the abdomen of a
> pregnant woman.

A 3-d curved surface isn't ideal for any transverse-stiff material (it has to stretch or
shrink) in a thin plate. You can relieve the stretch/shrink stresses by
a circumferential kerf cut (think make-a-spiral) or some variant.

Conductive elastomers are an unlovely, but possible, variant circuit technology.

On 17/07/21 00:15, whit3rd wrote:
> On Friday, July 16, 2021 at 10:25:54 AM UTC-7, Phil Hobbs wrote:
>> So we have this application that requires a circuit to maintain intimate
>> contact with a curved biological surface, namely the abdomen of a
>> pregnant woman.
>
> A 3-d curved surface isn't ideal for any transverse-stiff material (it has to stretch or
> shrink) in a thin plate. You can relieve the stretch/shrink stresses by
> a circumferential kerf cut (think make-a-spiral) or some variant.
>
> Conductive elastomers are an unlovely, but possible, variant circuit technology.

I have this vision of a matrix of spring-loaded pogo pins.

I'm not sure whether to be worried by that :)

Michal wrote:
> On 2021-07-16 19:25, Phil Hobbs wrote:
>> So we have this application that requires a circuit to maintain intimate
>> contact with a curved biological surface, namely the abdomen of a
>> pregnant woman.
>>
>> Flex is the obvious thing, but since the curvature is relatively gentle,
>> we're thinking of using thin FR4.  We can get a 4-layer board with
>> overall thickness of 0.4 mm.  The board needs to be about 20 x 100 mm,
>> and we can make it pretty spidery--each of the individual detector/TIA
>> pairs can be on its own paddle with a long skinny "cable" attaching it
>> to one end of the board.
>>
>> Have any of you lot tried something like that?
>
> not tried to use in real application, but seen and bent in my hands:
> https://ats.net/products-technology/product-portfolio/flexible-rigid-flexible-pcbs/semiflexible-pcbs/

Thanks. Dunno if we can make do with 2 layers, but maybe.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

whit3rd wrote:
> On Friday, July 16, 2021 at 10:25:54 AM UTC-7, Phil Hobbs wrote:
>> So we have this application that requires a circuit to maintain
>> intimate contact with a curved biological surface, namely the
>> abdomen of a pregnant woman.

>> Flex is the obvious thing, but since the curvature is relatively
>> gentle, we're thinking of using thin FR4. We can get a 4-layer
>> board with overall thickness of 0.4 mm. The board needs to be
>> about 20 x 100 mm, and we can make it pretty spidery--each of the
>> individual detector/TIA pairs can be on its own paddle with a long
>> skinny "cable" attaching it to one end of the board.
>
> A 3-d curved surface isn't ideal for any transverse-stiff material
> (it has to stretch or shrink) in a thin plate.

Thanks.

The board's aspect ratio is 5:1, and we expect to cut it up into paddles
joined together with long skinny 'cables'. So it's basically a 1-D
problem. It ought to bend fine--my worry is stuff like traces cracking
or progressive damage to the glass fibres.

The current plan is to use L2 and L3 for the 'cable' wiring to reduce
metal fatigue compared with L1 and L4.

Using Piglet's suggestion for hot forming the average curve might be a
win--we obviously need to accommodate some range of curvatures, but they
should all be convex at least.

> You can relieve the stretch/shrink stresses by
> a circumferential kerf cut (think make-a-spiral) or some variant.
>
> Conductive elastomers are an unlovely, but possible, variant circuit
> technology.

We need to solder actual circuitry on them though--several
photodetector/TIA pairs, as I mentioned, including largish through-hole
photodiodes. So the choices seem to be

1. Serious 4-layer flex circuit$$
2. Rigid-flex circuit$$$$$
3. Semi-flexible FR4.

So #3 looks like a good option if it can be made to work reliably.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Fri, 16 Jul 2021 13:25:45 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>So we have this application that requires a circuit to maintain intimate
>contact with a curved biological surface, namely the abdomen of a
>pregnant woman.
>
>Flex is the obvious thing, but since the curvature is relatively gentle,
>we're thinking of using thin FR4. We can get a 4-layer board with
>overall thickness of 0.4 mm. The board needs to be about 20 x 100 mm,
>and we can make it pretty spidery--each of the individual detector/TIA
>pairs can be on its own paddle with a long skinny "cable" attaching it
>to one end of the board.
>
>Have any of you lot tried something like that?
>
>Cheers
>
>Phil Hobbs

I might do my first rigid-flex. I'll have a metal block with an e/o
modulator bolted to the top and a heater on the bottom. I'm thinking
about machining a pit in the block and putting a little pcb in there,
pushing against the bottom of the eom, with a thermistor wheatstone
bridge. A shallow trench could let the flex get out to the outside
world and not conduct much heat. The other end of the flex could run
to a connector on my control board.

I wonder if an FFC jumper could be soldered directly to a board
somehow.

https://www.digikey.com/en/products/detail/parlex-usa-llc/100R7-102B/660120

Probably a hassle.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

On Fri, 16 Jul 2021 21:21:19 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>whit3rd wrote:
>> On Friday, July 16, 2021 at 10:25:54 AM UTC-7, Phil Hobbs wrote:
>>> So we have this application that requires a circuit to maintain
>>> intimate contact with a curved biological surface, namely the
>>> abdomen of a pregnant woman.
>
>>> Flex is the obvious thing, but since the curvature is relatively
>>> gentle, we're thinking of using thin FR4. We can get a 4-layer
>>> board with overall thickness of 0.4 mm. The board needs to be
>>> about 20 x 100 mm, and we can make it pretty spidery--each of the
>>> individual detector/TIA pairs can be on its own paddle with a long
>>> skinny "cable" attaching it to one end of the board.
>>
>> A 3-d curved surface isn't ideal for any transverse-stiff material
>> (it has to stretch or shrink) in a thin plate.
>
>Thanks.
>
>The board's aspect ratio is 5:1, and we expect to cut it up into paddles
>joined together with long skinny 'cables'. So it's basically a 1-D
>problem. It ought to bend fine--my worry is stuff like traces cracking
>or progressive damage to the glass fibres.
>
>The current plan is to use L2 and L3 for the 'cable' wiring to reduce
>metal fatigue compared with L1 and L4.
>
>Using Piglet's suggestion for hot forming the average curve might be a
>win--we obviously need to accommodate some range of curvatures, but they
>should all be convex at least.
>
> > You can relieve the stretch/shrink stresses by
>> a circumferential kerf cut (think make-a-spiral) or some variant.
>>
>> Conductive elastomers are an unlovely, but possible, variant circuit
>> technology.
>
>We need to solder actual circuitry on them though--several
>photodetector/TIA pairs, as I mentioned, including largish through-hole
>photodiodes. So the choices seem to be
>
>1. Serious 4-layer flex circuit$$
>2. Rigid-flex circuit$$$$$
>3. Semi-flexible FR4.
>
>So #3 looks like a good option if it can be made to work reliably.
>
>Cheers
>
>Phil Hobbs

We use some LEDs that surface mount on the top of a board but shoot
light down, through a hole to the bottom. That backlights a front
panel.

Will you need to sterilize the rig between uses? Throw it away?

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

jlarkin@highlandsniptechnology.com wrote:
> On Fri, 16 Jul 2021 21:21:19 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> whit3rd wrote:
>>> On Friday, July 16, 2021 at 10:25:54 AM UTC-7, Phil Hobbs wrote:
>>>> So we have this application that requires a circuit to maintain
>>>> intimate contact with a curved biological surface, namely the
>>>> abdomen of a pregnant woman.
>>
>>>> Flex is the obvious thing, but since the curvature is relatively
>>>> gentle, we're thinking of using thin FR4. We can get a 4-layer
>>>> board with overall thickness of 0.4 mm. The board needs to be
>>>> about 20 x 100 mm, and we can make it pretty spidery--each of the
>>>> individual detector/TIA pairs can be on its own paddle with a long
>>>> skinny "cable" attaching it to one end of the board.
>>>
>>> A 3-d curved surface isn't ideal for any transverse-stiff material
>>> (it has to stretch or shrink) in a thin plate.
>>
>> Thanks.
>>
>> The board's aspect ratio is 5:1, and we expect to cut it up into paddles
>> joined together with long skinny 'cables'. So it's basically a 1-D
>> problem. It ought to bend fine--my worry is stuff like traces cracking
>> or progressive damage to the glass fibres.
>>
>> The current plan is to use L2 and L3 for the 'cable' wiring to reduce
>> metal fatigue compared with L1 and L4.
>>
>> Using Piglet's suggestion for hot forming the average curve might be a
>> win--we obviously need to accommodate some range of curvatures, but they
>> should all be convex at least.
>>
>>> You can relieve the stretch/shrink stresses by
>>> a circumferential kerf cut (think make-a-spiral) or some variant.
>>>
>>> Conductive elastomers are an unlovely, but possible, variant circuit
>>> technology.
>>
>> We need to solder actual circuitry on them though--several
>> photodetector/TIA pairs, as I mentioned, including largish through-hole
>> photodiodes. So the choices seem to be
>>
>> 1. Serious 4-layer flex circuit$$
>> 2. Rigid-flex circuit$$$$$
>> 3. Semi-flexible FR4.
>>
>> So #3 looks like a good option if it can be made to work reliably.
>>
>> Cheers
>>
>> Phil Hobbs
>
> We use some LEDs that surface mount on the top of a board but shoot
> light down, through a hole to the bottom. That backlights a front
> panel.
>
> Will you need to sterilize the rig between uses? Throw it away?

The real one will be potted in flexible black silicone with clear
silicone for windows. We imagine using a peelable layer for asepsis,
but it might be possible to get it cheap enough to be disposable.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Fri, 16 Jul 2021 21:21:19 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>
>
>1. Serious 4-layer flex circuit$$
>2. Rigid-flex circuit$$$$$
>3. Semi-flexible FR4.

There is more than one kind of flex (either as part of rigid-flex or
alone). If you need to survive dynamic flexing there are rules to
follow and things to specify or it will die early.
--
Best regards,
Spehro Pefhany

Spehro Pefhany wrote:
> On Fri, 16 Jul 2021 21:21:19 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>>
>>
>> 1. Serious 4-layer flex circuit$$
>> 2. Rigid-flex circuit$$$$$
>> 3. Semi-flexible FR4.
>
> There is more than one kind of flex (either as part of rigid-flex or
> alone). If you need to survive dynamic flexing there are rules to
> follow and things to specify or it will die early.
>

It isn't a printhead application--the bending will be quite gentle and
relatively infrequent.

Any references for the flex rules?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Saturday, July 17, 2021 at 10:31:15 AM UTC+10, Tom Gardner wrote:
> On 17/07/21 00:15, whit3rd wrote:
> > On Friday, July 16, 2021 at 10:25:54 AM UTC-7, Phil Hobbs wrote:
> >> So we have this application that requires a circuit to maintain intimate
> >> contact with a curved biological surface, namely the abdomen of a
> >> pregnant woman.
> >
> > A 3-d curved surface isn't ideal for any transverse-stiff material (it has to stretch or
> > shrink) in a thin plate. You can relieve the stretch/shrink stresses by
> > a circumferential kerf cut (think make-a-spiral) or some variant.
> >
> > Conductive elastomers are an unlovely, but possible, variant circuit technology.
> I have this vision of a matrix of spring-loaded pogo pins.
>
> I'm not sure whether to be worried by that :)

It's a bed-mails-test fixture. Most of the boards made at Cambridge Instruments got one, and the production engineers, the printed circuit layout drafts-people and the design engineers would haggle about the little circles of copper that the pogo-pins stuck into. Production wanted each node to have it's test point, and there were nodes where the extra capacitance was lethal.

Some of the pogo pins had quite a long throw.

--
Bill Sloman, Sydney

On Saturday, July 17, 2021 at 11:21:26 AM UTC+10, Phil Hobbs wrote:
> whit3rd wrote:
> > On Friday, July 16, 2021 at 10:25:54 AM UTC-7, Phil Hobbs wrote:
It ought to bend fine--my worry is stuff like traces cracking or progressive damage to the glass fibres.

The Hewlett-Packard X-Y recorder used flexible cables to accommodate the excursions of printing head along the X and Y axes.

The write-up in the Hewlett-Packard Journal laid some emphasis on keeping the flexible cables symmetrical, and putting the metal in the middle to get million excursion life-times.

> The current plan is to use L2 and L3 for the 'cable' wiring to reduce
> metal fatigue compared with L1 and L4.
>
> Using Piglet's suggestion for hot forming the average curve might be a
> win--we obviously need to accommodate some range of curvatures, but they
> should all be convex at least.
>
> > You can relieve the stretch/shrink stresses by a circumferential kerf cut (think make-a-spiral) or some variant.
> >
> > Conductive elastomers are an unlovely, but possible, variant circuit
> > technology.
>
> We need to solder actual circuitry on them though--several photodetector/TIA pairs, as I mentioned, including largish through-hole
> photodiodes. So the choices seem to be
>
> 1. Serious 4-layer flex circuit$$

Not if HP was right. But you aren't going to flex the circuit anything like as often as the X-Y recorder did.

> 2. Rigid-flex circuit$$$$$
> 3. Semi-flexible FR4.
>
> So #3 looks like a good option if it can be made to work reliably.

If. There are other printed circuit materials - we used a layer of isocynate resin-bonded Teflon cloth at one point - purely for it's lower dispersion - but it would have been softer than epoxy glass.

--
Bill Sloman, Sydney

On Fri, 16 Jul 2021 22:45:39 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>Spehro Pefhany wrote:
>> On Fri, 16 Jul 2021 21:21:19 -0400, Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>
>>>
>>>
>>> 1. Serious 4-layer flex circuit$$
>>> 2. Rigid-flex circuit$$$$$
>>> 3. Semi-flexible FR4.
>>
>> There is more than one kind of flex (either as part of rigid-flex or
>> alone). If you need to survive dynamic flexing there are rules to
>> follow and things to specify or it will die early.
>>
>
>It isn't a printhead application--the bending will be quite gentle and
>relatively infrequent.
>
>Any references for the flex rules?
>
>Cheers
>
>Phil Hobbs

Search for design guides and maybe include "dynamic flex" about how
copper should be arranged. Multiple layers are much worse than two
double layer flex circuits on top of each other. Also, probably
obviously, take care where flex emerges from rigid to prevent bending
at the joint. The suppliers can use a better form of copper too (may
not be necessary in your case). Did a rigid-flex design a couple years
ago for a space instrument that had to work in a vacuum- some of the
sensors were gimballed so we had to take all that stuff into account.

I've got samples of minimal thickness FR4, it's okay for small bends,
maybe. I imagine mounting parts on flex might involve some additional
tooling.
--
Best regards,
Spehro Pefhany

On Friday, July 16, 2021 at 1:25:54 PM UTC-4, Phil Hobbs wrote:
> So we have this application that requires a circuit to maintain intimate
> contact with a curved biological surface, namely the abdomen of a
> pregnant woman.
>
> Flex is the obvious thing, but since the curvature is relatively gentle,
> we're thinking of using thin FR4. We can get a 4-layer board with
> overall thickness of 0.4 mm. The board needs to be about 20 x 100 mm,
> and we can make it pretty spidery--each of the individual detector/TIA
> pairs can be on its own paddle with a long skinny "cable" attaching it
> to one end of the board.
>
> Have any of you lot tried something like that?
>
Nope, but a crazy idea, can you cut some slots and such in FR4 such as to make it more bendable at two or three places? Do you need bendiness on more than one axis?

George H.
> Cheers
>
> Phil Hobbs
>
> --
> Dr Philip C D Hobbs
> Principal Consultant
> ElectroOptical Innovations LLC / Hobbs ElectroOptics
> Optics, Electro-optics, Photonics, Analog Electronics
> Briarcliff Manor NY 10510
>
> http://electrooptical.net
> http://hobbs-eo.com

Am 17.07.21 um 04:45 schrieb Phil Hobbs:
> Spehro Pefhany wrote:
>> On Fri, 16 Jul 2021 21:21:19 -0400, Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>
>>>
>>>
>>> 1. Serious 4-layer flex circuit$$
>>> 2. Rigid-flex circuit$$$$$
>>> 3. Semi-flexible FR4.
>>
>> There is more than one kind of flex  (either as part of rigid-flex or
>> alone). If you need to survive dynamic flexing there are rules to
>> follow and things to specify or it will die early.
>>
>
> It isn't a printhead application--the bending will be quite gentle and
> relatively infrequent.
>
> Any references for the flex rules?

Around 2003, we made these XFP transceivers:

<
https://www.flickr.com/photos/137684711@N07/51316513581/in/dateposted-public/lightbox/
>

ROSA and TOSA are attached with a flexible Kapton tape.
The metal bar behind the solder location was absolutely
necessary to make it work.

I don't think that FR4 might work. Epoxy does not like to bend
nor does glass fiber around a tight corner. I don't think that
Kapton costs too much, they use it in mass market things like
printers and there even seem to be pre-fabricated "cables".

BTW, these ROSA/TOSA tapes were a major 3D electromagnetics
simulation nightmare. Endless HFSS and ADS runs.
The hot signals and GND had to flip sides somewhere,
at 10 GBPS without damaging the eye diagrams.

I had some bad mechanical experience recently with Rogers TMM-6.
Bends easily and then breaks suddenly. It feels somehow like a
rubber eraser. Then we made a multilayer with TMM6 + FR-4.
The board house swapped the sides. The uwave synthesizer and the
microstrips ended up on FR-4, the DC stuff on TMM-6. :-((

Gerhard

On Sat, 17 Jul 2021 09:34:26 +0200, Gerhard Hoffmann <dk4xp@arcor.de>
wrote:

>Am 17.07.21 um 04:45 schrieb Phil Hobbs:
>> Spehro Pefhany wrote:
>>> On Fri, 16 Jul 2021 21:21:19 -0400, Phil Hobbs
>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>
>>>>
>>>>
>>>> 1. Serious 4-layer flex circuit$$
>>>> 2. Rigid-flex circuit$$$$$
>>>> 3. Semi-flexible FR4.
>>>
>>> There is more than one kind of flex  (either as part of rigid-flex or
>>> alone). If you need to survive dynamic flexing there are rules to
>>> follow and things to specify or it will die early.
>>>
>>
>> It isn't a printhead application--the bending will be quite gentle and
>> relatively infrequent.
>>
>> Any references for the flex rules?
>
>Around 2003, we made these XFP transceivers:
>
><
>https://www.flickr.com/photos/137684711@N07/51316513581/in/dateposted-public/lightbox/
> >
>
>ROSA and TOSA are attached with a flexible Kapton tape.
>The metal bar behind the solder location was absolutely
>necessary to make it work.
>
>I don't think that FR4 might work. Epoxy does not like to bend
>nor does glass fiber around a tight corner. I don't think that
>Kapton costs too much, they use it in mass market things like
>printers and there even seem to be pre-fabricated "cables".
>
>BTW, these ROSA/TOSA tapes were a major 3D electromagnetics
>simulation nightmare. Endless HFSS and ADS runs.
>The hot signals and GND had to flip sides somewhere,
>at 10 GBPS without damaging the eye diagrams.
>
>I had some bad mechanical experience recently with Rogers TMM-6.
>Bends easily and then breaks suddenly. It feels somehow like a
>rubber eraser. Then we made a multilayer with TMM6 + FR-4.
>The board house swapped the sides. The uwave synthesizer and the
>microstrips ended up on FR-4, the DC stuff on TMM-6. :-((
>
>Gerhard
>
>
>
>

We did some boards with mixed lams, microwave for the top layer and
FR4 for the rest. They were curled like potato chips.

https://www.dropbox.com/s/mw0gka15okd4qgo/Spinner.AVI?dl=0

I'm ordering some fast boards now with Isola called out for the top
dielectric and FR4 optional for the rest, but with a flatness
requirement.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

George Herold wrote:
> On Friday, July 16, 2021 at 1:25:54 PM UTC-4, Phil Hobbs wrote:
>> So we have this application that requires a circuit to maintain
>> intimate contact with a curved biological surface, namely the
>> abdomen of a pregnant woman.
>>
>> Flex is the obvious thing, but since the curvature is relatively
>> gentle, we're thinking of using thin FR4. We can get a 4-layer
>> board with overall thickness of 0.4 mm. The board needs to be about
>> 20 x 100 mm, and we can make it pretty spidery--each of the
>> individual detector/TIA pairs can be on its own paddle with a long
>> skinny "cable" attaching it to one end of the board.
>>
>> Have any of you lot tried something like that?
>>
> Nope, but a crazy idea, can you cut some slots and such in FR4 such
> as to make it more bendable at two or three places? Do you need
> bendiness on more than one axis?

The paddles where the components go can be stiff because they're quite
small--a photodiode (5 mm or 14 mm), an SC70 op amp, a SOT23 bootstrap
FET, two resistors and a couple of bypass caps. It's mostly the
survival of the 'cable' parts that I'm concerned about. If necessary I
can probably use wires and leave the FR4 to do the gross mechanical
locating.

The whole thing is going to be potted in black silicone
rubber (with the PDs visible, of course) because I'm looking at a path
loss of > 200 dB electrical--with an amp of LED drive current, we expect
to get tens of picoamps of signal photocurrent, and less with a
dark-skinned mom.

Melanin absorption causes overestimation of SpO_2, by as much as 8% @ 2
sigma at low SpO_2 values, right where it has the worst effects. The
effect mostly disappears at high SpO_2.

We're going to be using a three-wavelength system to avoid that problem,
but even without that systematic shift, the absorption still hits the
SNR pretty badly.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Sat, 17 Jul 2021 12:22:48 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>George Herold wrote:
>> On Friday, July 16, 2021 at 1:25:54 PM UTC-4, Phil Hobbs wrote:
>>> So we have this application that requires a circuit to maintain
>>> intimate contact with a curved biological surface, namely the
>>> abdomen of a pregnant woman.
>>>
>>> Flex is the obvious thing, but since the curvature is relatively
>>> gentle, we're thinking of using thin FR4. We can get a 4-layer
>>> board with overall thickness of 0.4 mm. The board needs to be about
>>> 20 x 100 mm, and we can make it pretty spidery--each of the
>>> individual detector/TIA pairs can be on its own paddle with a long
>>> skinny "cable" attaching it to one end of the board.
>>>
>>> Have any of you lot tried something like that?
>>>
>> Nope, but a crazy idea, can you cut some slots and such in FR4 such
>> as to make it more bendable at two or three places? Do you need
>> bendiness on more than one axis?
>
>The paddles where the components go can be stiff because they're quite
>small--a photodiode (5 mm or 14 mm), an SC70 op amp, a SOT23 bootstrap
>FET, two resistors and a couple of bypass caps. It's mostly the
>survival of the 'cable' parts that I'm concerned about. If necessary I
>can probably use wires and leave the FR4 to do the gross mechanical
>locating.
>
>The whole thing is going to be potted in black silicone
>rubber (with the PDs visible, of course) because I'm looking at a path
>loss of > 200 dB electrical--with an amp of LED drive current, we expect
>to get tens of picoamps of signal photocurrent, and less with a
>dark-skinned mom.
>
>Melanin absorption causes overestimation of SpO_2, by as much as 8% @ 2
>sigma at low SpO_2 values, right where it has the worst effects. The
>effect mostly disappears at high SpO_2.
>
>We're going to be using a three-wavelength system to avoid that problem,
>but even without that systematic shift, the absorption still hits the
>SNR pretty badly.
>
>Cheers
>
>Phil Hobbs

How about a thing like a giant spider with legs that droop down onto
mom's belly? Make it look more friendly, of course. Turtle or rabbit
or something.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

lørdag den 17. juli 2021 kl. 18.31.44 UTC+2 skrev jla...@highlandsniptechnology.com:
> On Sat, 17 Jul 2021 12:22:48 -0400, Phil Hobbs
> <pcdhSpamM...@electrooptical.net> wrote:
>
> >George Herold wrote:
> >> On Friday, July 16, 2021 at 1:25:54 PM UTC-4, Phil Hobbs wrote:
> >>> So we have this application that requires a circuit to maintain
> >>> intimate contact with a curved biological surface, namely the
> >>> abdomen of a pregnant woman.
> >>>
> >>> Flex is the obvious thing, but since the curvature is relatively
> >>> gentle, we're thinking of using thin FR4. We can get a 4-layer
> >>> board with overall thickness of 0.4 mm. The board needs to be about
> >>> 20 x 100 mm, and we can make it pretty spidery--each of the
> >>> individual detector/TIA pairs can be on its own paddle with a long
> >>> skinny "cable" attaching it to one end of the board.
> >>>
> >>> Have any of you lot tried something like that?
> >>>
> >> Nope, but a crazy idea, can you cut some slots and such in FR4 such
> >> as to make it more bendable at two or three places? Do you need
> >> bendiness on more than one axis?
> >
> >The paddles where the components go can be stiff because they're quite
> >small--a photodiode (5 mm or 14 mm), an SC70 op amp, a SOT23 bootstrap
> >FET, two resistors and a couple of bypass caps. It's mostly the
> >survival of the 'cable' parts that I'm concerned about. If necessary I
> >can probably use wires and leave the FR4 to do the gross mechanical
> >locating.
> >
> >The whole thing is going to be potted in black silicone
> >rubber (with the PDs visible, of course) because I'm looking at a path
> >loss of > 200 dB electrical--with an amp of LED drive current, we expect
> >to get tens of picoamps of signal photocurrent, and less with a
> >dark-skinned mom.
> >
> >Melanin absorption causes overestimation of SpO_2, by as much as 8% @ 2
> >sigma at low SpO_2 values, right where it has the worst effects. The
> >effect mostly disappears at high SpO_2.
> >
> >We're going to be using a three-wavelength system to avoid that problem,
> >but even without that systematic shift, the absorption still hits the
> >SNR pretty badly.
> >
> >Cheers
> >
> >Phil Hobbs
> How about a thing like a giant spider with legs that droop down onto
> mom's belly? Make it look more friendly, of course. Turtle or rabbit
> or something.
> --

octopus with sucking cups and everythign

On Sat, 17 Jul 2021 09:34:38 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

>lørdag den 17. juli 2021 kl. 18.31.44 UTC+2 skrev jla...@highlandsniptechnology.com:
>> On Sat, 17 Jul 2021 12:22:48 -0400, Phil Hobbs
>> <pcdhSpamM...@electrooptical.net> wrote:
>>
>> >George Herold wrote:
>> >> On Friday, July 16, 2021 at 1:25:54 PM UTC-4, Phil Hobbs wrote:
>> >>> So we have this application that requires a circuit to maintain
>> >>> intimate contact with a curved biological surface, namely the
>> >>> abdomen of a pregnant woman.
>> >>>
>> >>> Flex is the obvious thing, but since the curvature is relatively
>> >>> gentle, we're thinking of using thin FR4. We can get a 4-layer
>> >>> board with overall thickness of 0.4 mm. The board needs to be about
>> >>> 20 x 100 mm, and we can make it pretty spidery--each of the
>> >>> individual detector/TIA pairs can be on its own paddle with a long
>> >>> skinny "cable" attaching it to one end of the board.
>> >>>
>> >>> Have any of you lot tried something like that?
>> >>>
>> >> Nope, but a crazy idea, can you cut some slots and such in FR4 such
>> >> as to make it more bendable at two or three places? Do you need
>> >> bendiness on more than one axis?
>> >
>> >The paddles where the components go can be stiff because they're quite
>> >small--a photodiode (5 mm or 14 mm), an SC70 op amp, a SOT23 bootstrap
>> >FET, two resistors and a couple of bypass caps. It's mostly the
>> >survival of the 'cable' parts that I'm concerned about. If necessary I
>> >can probably use wires and leave the FR4 to do the gross mechanical
>> >locating.
>> >
>> >The whole thing is going to be potted in black silicone
>> >rubber (with the PDs visible, of course) because I'm looking at a path
>> >loss of > 200 dB electrical--with an amp of LED drive current, we expect
>> >to get tens of picoamps of signal photocurrent, and less with a
>> >dark-skinned mom.
>> >
>> >Melanin absorption causes overestimation of SpO_2, by as much as 8% @ 2
>> >sigma at low SpO_2 values, right where it has the worst effects. The
>> >effect mostly disappears at high SpO_2.
>> >
>> >We're going to be using a three-wavelength system to avoid that problem,
>> >but even without that systematic shift, the absorption still hits the
>> >SNR pretty badly.
>> >
>> >Cheers
>> >
>> >Phil Hobbs
>> How about a thing like a giant spider with legs that droop down onto
>> mom's belly? Make it look more friendly, of course. Turtle or rabbit
>> or something.
>> --
>
>octopus with sucking cups and everythign

Suction cups aren't a bad idea at all.

Given a super-flexy, maybe silicone insulated, multiwire cable, I
wonder how much vacuum could slip in between the wires. Or make a
custom thing, a silicone tube with wires loosely packed inside.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

jlarkin@highlandsniptechnology.com wrote:
> On Sat, 17 Jul 2021 12:22:48 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> George Herold wrote:
>>> On Friday, July 16, 2021 at 1:25:54 PM UTC-4, Phil Hobbs wrote:
>>>> So we have this application that requires a circuit to maintain
>>>> intimate contact with a curved biological surface, namely the
>>>> abdomen of a pregnant woman.
>>>>
>>>> Flex is the obvious thing, but since the curvature is relatively
>>>> gentle, we're thinking of using thin FR4. We can get a 4-layer
>>>> board with overall thickness of 0.4 mm. The board needs to be about
>>>> 20 x 100 mm, and we can make it pretty spidery--each of the
>>>> individual detector/TIA pairs can be on its own paddle with a long
>>>> skinny "cable" attaching it to one end of the board.
>>>>
>>>> Have any of you lot tried something like that?
>>>>
>>> Nope, but a crazy idea, can you cut some slots and such in FR4 such
>>> as to make it more bendable at two or three places? Do you need
>>> bendiness on more than one axis?
>>
>> The paddles where the components go can be stiff because they're quite
>> small--a photodiode (5 mm or 14 mm), an SC70 op amp, a SOT23 bootstrap
>> FET, two resistors and a couple of bypass caps. It's mostly the
>> survival of the 'cable' parts that I'm concerned about. If necessary I
>> can probably use wires and leave the FR4 to do the gross mechanical
>> locating.
>>
>> The whole thing is going to be potted in black silicone
>> rubber (with the PDs visible, of course) because I'm looking at a path
>> loss of > 200 dB electrical--with an amp of LED drive current, we expect
>> to get tens of picoamps of signal photocurrent, and less with a
>> dark-skinned mom.
>>
>> Melanin absorption causes overestimation of SpO_2, by as much as 8% @ 2
>> sigma at low SpO_2 values, right where it has the worst effects. The
>> effect mostly disappears at high SpO_2.
>>
>> We're going to be using a three-wavelength system to avoid that problem,
>> but even without that systematic shift, the absorption still hits the
>> SNR pretty badly.

> How about a thing like a giant spider with legs that droop down onto
> mom's belly? Make it look more friendly, of course. Turtle or rabbit
> or something.

Fun possibilities spread out before us. ;)

The idea is to keep it really low-profile, mostly for the mom's comfort.

We're looking at all sorts of ways to improve heat-sinking of the LEDs
for the same reason. A watt of dissipation in a small area right
against your skin, insulated by 0.1 W/m/K elastomer, could get old
pretty fast.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com