[Halld-cal] FCAL CW bases

[Fernando J. Barbosa]

Hi Paul,

That's really strange that you are getting sparks at 1,500V on these 
parts across the surface. You can try cleaning with Flux-off, followed 
by a rinse in distilled water and thorough drying. I completely agree 
that it is important to find out as much as possible before committing 
to the re-design.

Yes, I got the pdf but getting dimensions from that and considering the 
component dimensions is a bit inaccurate. You could send me your file 
and I can try importing it.

Thanks and best regards,
Fernando




Paul Smith wrote:
> Hi Fernando,
>
> I have no objection to a board redesign which moves the diodes further 
> apart.
>
> I tested one of the 2225 AVX capacitors with small wires soldered to 
> the ends and just sitting in the air with no board or coating.  It was 
> soldered with water soluble flux which was then cleaned with distilled 
> hot water under pressure and then ethyl alcohol.  There was never any 
> flux on the faces or sides, only a little on the end caps where the 
> wires were soldered.  So, it should be extremely clean.  It sparks 
> somewhere between 1500 and 2000 volts.  Sparks are across the sides 
> between corners.  If you look at the end cap metallization, it is not 
> exactly smooth at the boundary where the metallization meets the 
> ceramic.  After a spark, evidently a sharp point gets burned off since 
> it will then withstand a slightly higher voltage before sparking 
> again.  Humidity was ~55%.
>
> The next thing I plan to try is epoxy-ing the diodes and humisealing 
> the capacitors on one of the existing boards.  I think it is important 
> to learn as much as possible about the problem areas before redoing 
> the board layout.  If this works we can consider avoiding the use of 
> epoxy.
>
> I sent a pdf of the present board layout a few weeks ago.  I can send 
> it again if you don't have it.  I can send the new layout when I have 
> it done.
>
> Paul
>
>
>
> On Jun 21, 2010, at 1:54 AM, Fernando J. Barbosa wrote:
>
>> Hi Paul,
>>
>> As we have discussed in the past, placing the diodes farther apart 
>> (larger than the diode's pad-to-pad length) will prevent arcing on 
>> this part of the board (the limitation will then be within each 
>> component rather than component-to-component).
>>
>> There are a number of different types of epoxies and for different 
>> applications, as you know. The problem with epoxy is that it does not 
>> allow for re-working the circuitry, over time it de-laminates or 
>> cracks (especially with high levels of radiation but this is not the 
>> case here) and it is impossible to match the CTE of epoxy to that of 
>> ceramic. Because epoxy is also slightly hygroscopic, arcing will 
>> occur shortly after de-lamination. You are correct that thick coats 
>> of epoxy are used on some components for protection from mechanical 
>> stresses and environmental factors but its purpose is not to prevent 
>> arcing, even though it may be rated for HV operation.
>>
>> The proper coating for HV should be acrylic-based (like the Humiseal 
>> you have used) or Parelene. I prefer Humiseal because it cures 
>> quickly, has very little out-gassing, is excellent in high humidity 
>> environments and it allows for re-working of the circuitry.
>>
>> The problems I have encountered that lead to arcing have usually been 
>> due to surface residues (cleanliness is the most important issue), 
>> improper pad spacing (the pad-to-pad spacing[edges] under the 
>> component should be about the same as the component length, sharp 
>> edges, component spacing and defective components, in this order. Of 
>> course, humidity is a big factor. IPC does not address HV conditions 
>> properly, in my opinion, as it is concerned primarily with assembly 
>> of PCBs for most commercial uses.
>>
>> We had some problems with HV in the FDC and the vast majority of 
>> these were due to cleanliness. It turned out that the epoxy that held 
>> the wires in place was contaminated from cleaning the solder residues 
>> which also softened the epoxy; one component was placed too close to 
>> another; residues on the PCB also caused quite a lot of problems. 
>> These problems have been resolved by proper cleaning and we have 
>> re-designed the boards more conservatively. We have also applied a 
>> conformal coating of Humiseal to HV capacitors outside of the gas 
>> volume because of humidity.
>>
>> The HV capacitors are 1825 from AVX rated at 4kV and operated at over 
>> 2kV (1825JA221KAT1A). These are smaller than the ones you mention and 
>> so I would look very carefully at the cleanliness of your assembled 
>> PCB. These SMT components should operate at the specified HV provided 
>> by the manufacturer for SMT assemblies - the disclaimer is just an 
>> acknowledgment that operation of the component may be compromised if 
>> humidity is high, for example.
>>
>> In my opinion, the CW bases should be robust by design as we consider 
>> at least 10 years of operation and the cost of re-design should not 
>> be that significant compared to production and maintenance costs.
>>
>> I will be glad to check your layout if I can import your file.
>>
>> Best regards,
>> Fernando
>>
>>
>> Paul Smith wrote:
>>> Hi Fernando,
>>>
>>> I'm not on the calorimetry email list.  Please feel free to forward 
>>> to the list if appropriate.
>>>
>>> Comments in-line below:
>>>
>>> On Jun 18, 2010, at 11:28 AM, Matthew Shepherd wrote:
>>>
>>>>
>>>> Hi Paul,
>>>>
>>>> I'm not sure you are on the calorimetry list -- see the note below 
>>>> from Fernando.
>>>>
>>>> -Matt
>>>>
>>>> Begin forwarded message:
>>>>
>>>>> From: "Fernando J. Barbosa" <barbosa at jlab.org>
>>>>> Date: June 18, 2010 11:24:28 AM EDT
>>>>> To: Matthew Shepherd <mashephe at indiana.edu>
>>>>> Cc: Hall-D Calorimetry <halld-cal at jlab.org>, "Fernando J. Barbosa" 
>>>>> <barbosa at jlab.org>
>>>>> Subject: FCAL CW bases
>>>>>
>>>>> Hi Matt,
>>>>>
>>>>> I have a few concerns about the plan for the CW bases you 
>>>>> suggested at the GlueX meeting and regarding the HV sparks 
>>>>> observed on the present design:
>>>>>
>>>>> 1. HV sparks across components - Due to close proximity of 
>>>>> components, HV sparks or leakage are observed. The present 
>>>>> proposed solution is to pot with epoxy. This seems to prevent HV 
>>>>> sparks.
>>>>>
>>>>> My concern is that this is temporary solution but not a cure for 
>>>>> the inherent problem. I really don't want to rely on potting with 
>>>>> epoxy (long term issues) and a conformal coating should only be 
>>>>> used as a preventive measure.
>>>
>>> This problem is between adjacent diodes in the CW multiplier chain.  
>>> If one follows the IPC "rule of thumb" of 500 volts for a 0.1 inch 
>>> spacing on uncoated FR4, there shouldn't be a problem.  However, 
>>> even though the pads have round corners, the diode leads have square 
>>> corners which aren't always completely covered by solder.  Even 
>>> though conformal coating is supposedly good for >1500 volts/mil, 
>>> this seems to be the case only if the electric field is transverse 
>>> to the coating.  As we reported a few weeks ago, sparks will form 
>>> under the coating in a plane parallel to the board surface, 
>>> presumably between the board and the coating.  Evidently the coating 
>>> doesn't completely bond to the FR4.  Slots, etc are possible, but at 
>>> extra cost.  This problem could be solved by moving the diodes 
>>> further apart, resulting in a longer board.
>>>
>>> What are the long term issues with epoxy you refer to?  Radiation 
>>> damage to the epoxy?  Isn't epoxy used all the time in chamber 
>>> construction, etc?  Aren't through hole HV capacitors and resistors 
>>> potted/molded in epoxy by the manufacturer?
>>>
>>>
>>>>>
>>>>> 2. HV sparks are observed across one HV capacitor - the capacitor 
>>>>> is rated for operation at 2,500V but it cannot hold 1,500V. The 
>>>>> proposed solution is to use a low viscosity epoxy to under fill 
>>>>> the capacitor and prevent sparking.
>>>>>
>>>>> A capacitor that is sold and rated at 2,500V should withstand that 
>>>>> same voltage or the manufacturer has a big problem. Most likely 
>>>>> the PCB pads to which the cap is soldered to are too close or have 
>>>>> sharp edges - this limits the maximum operating voltage. 
>>>>> Alternatively, you could choose a larger capacitor with the pads 
>>>>> spaced farther apart but this may not be necessary.
>>>
>>> This capacitor is an AVX # 2225WC103KAT1A  The data sheet states:  
>>> "may require protective surface coating to prevent external 
>>> arcing".  The length is 0.22", so the clearance is way less than the 
>>> 500 volts/0.1 inch rule of thumb.  The arcing is observed NOT 
>>> between the pads (which do have round corners) , but most definitely 
>>> between the metallization of the capacitor terminals along the sides 
>>> of the capacitor (between the ends).  Epoxy underneath the capacitor 
>>> doesn't solve the problem; it also needs the sides and top covered 
>>> with epoxy as well in order to withstand the rated 2500 volts.
>>>
>>> The previous version of the CW base used a Johanson 302H51W103KY4 
>>> for this component; this was a 3530 case size.  It was rated for 
>>> 3000 volts and would withstand 2500 volts if the humidity was 50% or 
>>> lower.  Humidity levels higher than 60% would cause sparks at 2000 
>>> volts.  Unfortunately, this capacitor is no longer made, and in fact 
>>> Johanson now has capacitors rated up 6000 volts in the 2225 case 
>>> size.  They also include the disclaimer "may require a surface 
>>> coating to prevent external arcing".
>>>
>>> AVX does make a 3640 case size capacitor; unfortunately it is 4X as 
>>> expensive as a 2225.  To my knowledge this is the only option for a 
>>> physically larger surface mount HV cap.  Several manufacturers make 
>>> 2500 volt rated capacitors in the 2225 case size.
>>>
>>>>>
>>>>> I strongly suggest that the base be re-designed to space the 
>>>>> components and pads farther apart to provide for reliable 
>>>>> operation by design, without a conformal coating. Once that is 
>>>>> verified, a conformal coating is highly recommended because of 
>>>>> moisture and dust.
>>>
>>>
>>> Assuming we can find a surface mount capacitor physically large 
>>> enough to withstand it's rated voltage without any sort of coating, 
>>> the next issue is with the 1 Gohm feedback resistors.  The previous 
>>> version of the CW base used a Victoreen (now Ohmite) MC102821007F, 
>>> in a 5025 case size.  Unfortunately, this is no longer a standard 
>>> product and is only available as a special order.  The present 
>>> version of the base uses an Ohmite HVF2512T1007FE.  This is rated at 
>>> 2500 volts in a 2512 case size, so has slightly better clearance 
>>> than the 2225 capacitor, but still well in excess of the 500 
>>> volt/0.1 inch rule.  I haven't actually seen sparks across this 
>>> resistor since they form across the capacitor first, but have also 
>>> coated it with epoxy both underneath and top and sides in order to 
>>> get it to work.  Venkel does make a 1 Gohm resistor in a 5020 size, 
>>> but it is 6X as expensive as the Ohmite 2512.
>>>
>>>
>>> To summarize, the diode clearance issue could be solved by a larger 
>>> layout.  The 2500 volt rated resistors and capacitors are difficult 
>>> to get and expensive in larger case sizes and still won't work 
>>> without a coating unless the humidity is very low.  Conformal 
>>> coating doesn't seem to be adequate for insulation parallel to the 
>>> board surface and sparking can occur underneath.  Epoxy does work, 
>>> but if you think there are long term issues, the only alternative 
>>> for producing a layout that will operate without any type of 
>>> additional insulation would be to use non surface mount components 
>>> for the parts that see the entire 2+ kV across them.  I suppose 
>>> another alternative might be to use 2 components in series so that 
>>> each only sees half the voltage.
>>>
>>> Because of the block to block spacing, it is not possible to make 
>>> the PCB any wider.  It is possible to make it longer.  This works 
>>> out fine for increasing the diode spacing, but doesn't help much 
>>> with the other parts.  It might be necessary to go to more layers to 
>>> use the longer board to advantage with larger, non surface mount 
>>> parts or doubled parts.
>>>
>>> I would point out that CW bases made by Hamamatsu (for example) are 
>>> completely potted.  Obviously, I will do whatever is required to 
>>> pass the JLab review, but changing the layout enough to allow it to 
>>> operate without additional insulating material is going to be 
>>> expensive in terms of engineering time and parts cost.  Is this 
>>> requirement going to apply to HV distribution boards on the 
>>> chambers, for example?  I know the ATLAS TRT drift tubes had to use 
>>> epoxy on their HV distribution/blocking capacitors.  The fundamental 
>>> problem seems to be that HV surface mount components have extremely 
>>> optimistic voltage ratings.  Presumably the capacitor dielectric can 
>>> withstand the rated voltage, but they must test them in an oil bath 
>>> or something other than air.
>>>
>>> I am planning to visit a new contract manufacturer next week (our 
>>> previous CM in Indianapolis went out of business).  I will discuss 
>>> the manufacturing issues involved in coating HV components with 
>>> them, whether with epoxy or some other material.
>>>
>>> In any case, your comments/suggestions are extremely useful, and I 
>>> will keep in touch,
>>>
>>> Paul
>>>
>> <barbosa.vcf>
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