[Halld-cal] FCAL CW bases

[Fernando J. Barbosa]

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
>
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