[SI-LIST] Re: Basic question on power plane capacitance
- From: David <dpalmer@xxxxxxxxxxxxx>
- To: Istvan Novak <istvan.novak@xxxxxxx>
- Date: Fri, 10 Sep 2010 17:40:34 +0200
Are there SMT pads for the ground pins of components on layer 1?
Istvan Novak wrote:
> While all the various tolerances mentioned on this thread are possible
> contributors to
> the difference you experience, I would not rule out measurement problem
> either. You
> said you used a simple low-frequency instrument; probably something like
> a hand-held
> DVM with capacitance measurement capability. Those can sometimes give
> very stable,
> repeatable, but COMPLTELY WRONG readings. I recently lost a few hours
> by debugging
> a similar scenario, just to find out that the stable reading of the
> capacitance meter was
> 3x off. A simple test would be to take a 470pF or 1nF capacitor and
> measure it with the
> same instrument. Alternately, if you have a TDR instrument on hand,
> stick it into the
> planes and record the risetime after the sharp fall caused by the plane
> capacitance.
> The time constant of the rise is 50 Ohms times the (unknown) capacitance.
>
> Regards,
>
> Istvan Novak
> Oracle
>
>
> On 9/9/2010 4:14 PM, Mikhail Matusov wrote:
>> Steve,
>>
>> I dug out the manufacturer's report (the card is a few years old). The
>> dielectric was supposed to be prepreg 3.9 mil thick with Dk of 4.31. The
>> material was supposedly PCLFR370HR. Although, it seems that we had moved a
>> couple of layers around last minute, so this might be not the latest
>> report...
>>
>>
>> /Mikhail
>>
>>
>>
>> ----- Original Message -----
>> From: "steve weir"<weirsi@xxxxxxxxxx>
>> To: "Mikhail Matusov"<matusov@xxxxxxxxxxxx>
>> Cc:<wolfgang.maichen@xxxxxxxxxxxx>;<si-list@xxxxxxxxxxxxx>
>> Sent: Thursday, September 09, 2010 3:29 PM
>> Subject: [SI-LIST] Re: Basic question on power plane capacitance
>>
>>
>>> If there isn't much copper connected together on the outer layer, the
>>> difference is likely that what you think is 5mil dielectric is more like
>>> 3mil and is almost all glass. I am a bit surprised because the PWR1 /
>>> GND1, and GND4 / PWR2 layers s/b core making them well controlled.
>>>
>>> Steve.
>>> Mikhail Matusov wrote:
>>>> Steve,
>>>>
>>>> The stackup is 14-layer, 63 mil thickness:
>>>>
>>>> Top
>>>> PWR1
>>>> GND1
>>>> signal1
>>>> signal2
>>>> GND2
>>>> signal3
>>>> signal4
>>>> GND3
>>>> signal5
>>>> signal6
>>>> GND4
>>>> PWR2
>>>> Bottom
>>>>
>>>> Some of the GND planes actually contain polygons with other voltages.
>>>>
>>>> The actual polygon in question is not a simple rectangle, it kind of
>>>> looks
>>>> like the letter F.
>>>>
>>>> /Mikhail
>>>>
>>>>
>>>>
>>>>
>>>> ----- Original Message -----
>>>> From: "steve weir"<weirsi@xxxxxxxxxx>
>>>> To: "Mikhail Matusov"<matusov@xxxxxxxxxxxx>
>>>> Cc:<wolfgang.maichen@xxxxxxxxxxxx>;<si-list@xxxxxxxxxxxxx>
>>>> Sent: Thursday, September 09, 2010 2:47 PM
>>>> Subject: [SI-LIST] Re: Basic question on power plane capacitance
>>>>
>>>>
>>>>
>>>>> Mikhail, your dimensions sound like 13.5mm x 104.5mm give or take. That
>>>>> is still more than 100X your height on the short axis. So I do not
>>>>> suspect fringing. What is your actual stack-up?
>>>>>
>>>>> Steve
>>>>>
>>>>>
>>>>>
>>>>> Mikhail Matusov wrote:
>>>>>
>>>>>> Thanks Wolfgang and all,
>>>>>> Yes, indeed I haven't thought of fringe capacitance. I don't know if
>>>>>> there is an easy analytical way to find out what it will be. The area
>>>>>> of
>>>>>> the polygon is 1400 sq.mm. The perimeter is 236 mm. The dielectric is
>>>>>> FR-4, and the thikness is about 5 mil (0.127mm). The top layer is not a
>>>>>> plane and there is not much copper in it, mostly pads and vias. The gap
>>>>>> around this polygon separating it from the rest of the plane is 15 mil
>>>>>> (0.381mm) wide. The board dimensions are 233x160 mm.
>>>>>>
>>>>>> Thanks again,
>>>>>> /Mikhail
>>>>>>
>>>>>>
>>>>>> ----- Original Message -----
>>>>>> From: wolfgang.maichen@xxxxxxxxxxxx
>>>>>> To: Mikhail Matusov
>>>>>> Cc: si-list@xxxxxxxxxxxxx ; si-list-bounce@xxxxxxxxxxxxx
>>>>>> Sent: Thursday, September 09, 2010 2:11 PM
>>>>>> Subject: Re: [SI-LIST] Basic question on power plane capacitance
>>>>>>
>>>>>>
>>>>>>
>>>>>> Mikhail,
>>>>>>
>>>>>> If the polygon isn't very large (~100x) compared to its distance from
>>>>>> the ground plane then fringe capacitance will contribute significantly
>>>>>> to
>>>>>> the total value - i.e. the actual capacitance will be quite a bit
>>>>>> higher
>>>>>> than what you'd expect from the simple formula for a planar capacitor.
>>>>>> In
>>>>>> addition, if your dielectric is a low-cost material (e.g. FR-4) its
>>>>>> dielctric constant can easily be off by ~10% from the specified value.
>>>>>>
>>>>>> A 2.5D or 3D field solver can tell you how much total capacitance to
>>>>>> expect. One suitable choice would be Sonnet Lite (i.e. the demo version
>>>>>> of Sonnet). If you can tell me the size/geometry of your polygon,
>>>>>> distance to the ground plane, and dielectric constant of the board
>>>>>> material I can run a quick simulation for you.
>>>>>>
>>>>>> Just to rule out an obvious mistakes, you did take into account that
>>>>>> having both a plane below and above the polygon will douple the
>>>>>> capacitance compared to just a single pair (polygon and single ground
>>>>>> plane)? Forgetting that would immediately explain a mismatch factor of
>>>>>> 2.
>>>>>>
>>>>>> Regards,
>>>>>>
>>>>>> Wolfgang
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> "Mikhail Matusov"<matusov@xxxxxxxxxxxx>
>>>>>> Sent by: si-list-bounce@xxxxxxxxxxxxx
>>>>>> 09/09/2010 10:50 AM
>>>>>> To<si-list@xxxxxxxxxxxxx>
>>>>>> cc
>>>>>> Subject [SI-LIST] Basic question on power plane
>>>>>> capacitance
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> Hi all,
>>>>>>
>>>>>> I have a small polygon in a plane layer of a multi-layer PCB. There
>>>>>> is
>>>>>> a
>>>>>> solid ground plane underneath it and the top signal layer above. I
>>>>>> have
>>>>>> calculated from the basic plane capacitance equation that the plane
>>>>>> capacitance for this polygon should be in the range of 420 pF.
>>>>>> However,
>>>>>> I am
>>>>>> measuring 890 pF with one meter and 1 nF with another. I was
>>>>>> wondering
>>>>>> what
>>>>>> am I doing wrong?
>>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>> /Mikhail
>>>>>>
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>>>>>>
>>>>> --
>>>>> Steve Weir
>>>>> IPBLOX, LLC
>>>>> 150 N. Center St. #211
>>>>> Reno, NV 89501
>>>>> www.ipblox.com
>>>>>
>>>>> (775) 299-4236 Business
>>>>> (866) 675-4630 Toll-free
>>>>> (707) 780-1951 Fax
>>>>>
>>>>>
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>>>>>
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>>>>
>>> --
>>> Steve Weir
>>> IPBLOX, LLC
>>> 150 N. Center St. #211
>>> Reno, NV 89501
>>> www.ipblox.com
>>>
>>> (775) 299-4236 Business
>>> (866) 675-4630 Toll-free
>>> (707) 780-1951 Fax
>>>
>>>
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