App. Note 44 - SCSI Removable SCA Drives, Caddies and connections
SCSI drives are used both in multi-channel sound recording (where they
replace tape drives) and in digital recording as used in animation.
In either case, they allow the digital recording to be removed. At a later
time, it can be edited or copies made, edited and assembled. One possible
problem occurs as the SCSI drives themselves are in removable caddies.
At one time, SCSI 2 was most frequently employed (50 pin connectors) but
with speed and resolution increases, the need for SCSI 3 (with 68 on connectors)
is now almost standard.
Now, even SCSI 2 and SCSI 3, may have die cast bodies. The need for
larger hard disk storage mandated the use of SCSI 3, as twelve SCSI drives
can be employed. As these connectors are smaller than the SCSI 1, their
pin spacing is smaller. But where larger arrays are involved, these are
usually plugged into a back plane by using a single unified 80 pin connector.
This technique is termed SCA connection. With a pin density this extreme,
cable and pin connection become very dependent on anything that can cause
a conductivity error. Rectification of RF signals where the signal voltages
are low and the circuit impedance is high can result in the proliferation
It is rare that connector faults show up immediately. But when the
68 pin caddies or removable SCA 80 pin drives are used, errors can accumulate until "best two out of three" becomes one "out of three" This situation
is rare; which drive is correct?
Note that the use of the RAID technique allows for "hot swapping". In
essence this means that defective SCA drive can be replaced without powering
down the system.
As the larger storage systems use error correction (best two out of
three) in a RAID configuration (Redundant Arrays or Inexpensive Drives),
it isn't until two of the records are faulty that a major problem becomes
apparent. But in some cases they may often be of an intermittent nature
and thus difficult to trace. Stabilants can be used to prevent or
solve these problems. We'll explain the use of this material in some detail.
SCSI 1, SCSI 2 and SCSI 3
On some connectors, the metal may be tin plated; with several
sets of gold flashings applied after the tin plated connector is formed. However, when a connector's stock is tin plated before it is formed one
must remember that some tin alloys resist high-speed forming and can have
areas of latent stress. Whenever this type of connector is subject to thermal
stress and/or cycling of humidity, the pre-stressed tin may "flake" away.
These "flakes" are so small that micro-techniques have to be used even
to see them. We have encountered instances where connectors passed quality
control testing and were approved for shipping. Often these connectors
were bulk shipped in cartons made from sulfide stock. In other words these
we're not manufactured from acid-free stock. In time, the
tin plating started to "micro-flake", and these connectors became erratic.
We recommend the use of Stabilants on these connectors. As it
is available as the concentrate (Stabilant 22) or in a partially
filled bottle (Stabilant 22S - for Short fill) - where the
diluant is added to the concentrate and mixed on site, there are less problems
when it has to be shipped by air and the diluant added in the field.
Removal and cleaning of back plane boards
Disassembly can be somewhat tricky because of the potential for damage
of components by static electricity.
Obviously any hard drives will have to be removed from the plug-in SCA
carriers. It is advisable to make a note of the name (part # or serial
#) for each hard drives or SCA's as they are disassembled. What might seem
obvious at the time may not be so easy to remember later, especially when
several faults occur when time is critical. While this may seen pedantic
the use of any Quality Assurance Program (such as one of the ISO's) requires
This caution also applies to any wiring harness connectors in the system.
Wires can be tagged using surgical tape until such time as a listing can
be made. A rough sketch of each connector location can often save hours
of work later on.
Most caddies that are used with large storage capacity have hard disk
drives are fan cooled. If the power supply is deficient in it's voltage
output and/or regulation, the needed cooling fans won't work. This can
lead to premature failure of the hard disk. The problem might be related
to the switching-mode power supply. These contain aluminum electrolytic
capacitors. As the efficiency and regulation of the supply can be dependent
on the operating frequency, if the capacitor develops resistance, it will
both lower the Q of the circuit but also cause a build up of heat. This,
in turn, can cause the supply to fail and so on!
Where fan-cooled SCA's are employed, regulation is generally
less of a concern. The investment is so large that the type of regulation
problems encountered in less expensive ($3,500 and lower) computing systems
should not be a problem.
Other types of equipment.
The Stabilants can also be used on connectors, the switches
in mixers, and in computing equipment. It can even be used in the consolidation
of cables. This applies to connectors in general, irrespective of whether
they are microphone connectors, patch bays, screw terminals, tab connectors
or other types.
What is Stabilant 22?
Stabilant 22 is an initially non-conductive block polymer
which when used in a thin film between metal contacts becomes conductive
the effect of an electric field. This occurs at an electric field gradient
such that the material will remain non-conductive between adjacent
contacts in a multiple pin environment. In addition Stabilant 22 exhibits
surfactant action as well as lubrication ability providing a simple component
resident solution to virtually all contact problems.
When applied to electromechanical contacts, Stabilant 22 can provide
the connection reliability of a soldered joint without bonding the contact
Where can Stabilant 22 be used?
Stabilant 22 can be used in all types of connectors, at frequencies
from to DC to several Gigahertz, on faders or potentiometers, or in non-inductive
(non-arcing power-interrupt switches. The number of uses are almost limitless.
Is Stabilant 22 just another contact cleaner?
No, Stabilant 22 is a resident potentially electrically active
which through synergistic combination of effects enhances conductivity
within a contact without causing leakage between adjacent contacts. Thus
large quantities of the material d not have to be "hosed" on as is the
case with cleaners.
Is Stabilant 22 cost effective?
As Stabilant 22 can be quickly applied to all contacts and connectors
in a system the often difficult diagnostic determination as to which one
of many contacts are erratic, can often be eliminated. This can significantly
reduce service time in the field and in many cases eliminates the need
to return boards for shop service or re-manufacturing. As any service manager
knows, the diagnosis of electronic problems especially where intermittent
failures are concerned, is often much more difficult than the actual part
replacement; as well as requiring service personnel of exception caliber.
In many cases the use of Stabilants can thus increase the efficient(
o existing staff as well as allowing many connector harness related problems
to be handled at a much lower cost.
How can Stabilants correct electrical contact problems?
In many electronic applications demodulation (detection) of RF signals
in connectors exhibiting thin-film rectification effects can either reduce
the signal-to-noise ratio or introduce artifacts which can disrupt data
flow. Stabilants can cure these.
While Stabilants have demonstrated that they can cut the cost
of both shop and field maintenance; their use in the manufacturing of electronic
systems can speed up production as well as reducing rejections.
How does Stabilant 22 work?
Contact failure is rarely caused by a single factor. Thus, treatments
that solve only one problem don't necessarily offer a reliable long term
solution. For example, cleaners do not prevent the re-entry of contaminants
or the reformation of contaminant films; nor do they offer any lubrication.
They must be used each time a connector gets dirty. Lubricants in themselves
are rarely cleaners. Corrosion inhibitors are neither cleaners nor lubricants
and are often specific to one type of metal or plating. Unsaturated oils
used as contact treatments can cross-link under the influence of elastomer
or thermoset plastic curing agents and accelerants.
While resident in the connector, Stabilant 22 performs several
concurrent functions. Its very presence in the contact gap will prevent
the entry of outside contaminants. It has sufficient surfactant action
to lift surface contaminants and hold them in suspension. In cases where
corrosion products are present Stabilant 22 will penetrate them
and prevent rectification effects. Due to its high dielectric constant
it will act to form a capactive layer which is in parallel with whatever
residual resistance exists in the contact increasing the passage of AC
signals. Given sufficient DC bias within the gaps of the contact the thin
film of Stabilant will "switch", conducting by quantum tunneling
and thus limit the resistance of the contact to a serviceable level.
In what forms is Stabilant available?
The Stabilants are available in several forms; as a concentrate (Stabilant
22) and as an isopropyl alcohol-diluted form (Stabilant 22A).
In a similar fashion, we produce Stabilant 22E Because of the 4:1
dilution, a given size container of Stabilant 22A will cost about
one-fifth the amount of a container of Stabilant 22 as it contains
only one-fifth the amount of the concentrate.
What is the difference in use of these materials?
Stabilant 22 is most useful where the connections are out in
the open - such as card-edge connectors or where the lubricating properties
of the material are useful such as an aid to installing microprocessor
IC's or on switches. Where the connections are not too easy to get at or
where the user wishes to apply the material to something such as a socketed
IC (without removing the IC from its socket) it is easier to use the alcohol
diluted form (Stabilant 22A or Stabilant 22E). The isopropyl
alcohol diluant serves ONLY to carry the concentrate into the connector.
The Stabilants have proven to be very effective in improving
the reliability of connectors in general and are developing a reputation
for ease and speed of use under field conditions. Not only are many OEM's pre-treating
sensor connectors during manufacture, many are providing the
to their service technicians either as Standard-Store items, or recommending
them for field procurement.
The Stabilants are presently used in applications ranging from
Avionics through Process control, including such critical fields as Bio-medical
electronics, Air Traffic Control, Police & Emergencies (such as communications
and the like).
How are the Stabilants applied?
The application of the Stabilants is exceptionally easy. Just
use a drop of two o Stabilant 22A on one of the electrical or electronic
sensor connectors, including an other in-signal-path connector in the wiring
harness, and reconnect the system.
What packaging is available?
We can supply the concentrate (Stabilant 22) in 5 mL, 15mL,
5OmL, lOOmL, 250m 5OOmL and 1 Liter bottles. The dilute (Stabilant 22A)
available in 5OmL, 100m 25OmL and 5OOmL containers. We do not have a 1
liter container of the dilute as 500 mL is the largest size bottle that
can be shipped by air (in single or multiple packages) without additional
restrictions. We maintain our stock in depth, and ship most orders the
same day they are received. The 15 mL and 50 mL sizes are in dropper bottles
and these are available on request for the ) 0.5 mL sizes as well.
Is it available in a spray can?
No. Why waste the material? We would like to think we are environmentally
responsible and safety conscious. We use no CFCs, HCFC's or any other ODC
(Ozone Depleting Chemical) either in or in the manufacture of Stabilants.
In addition even Stabilant 22A has only about 1/200th the solvent
impact as conventional contact cleaning solvents over a three year time
span. As Stabilant 22 contains no solvent it has absolutely minimal environmental
impact and is, therefor becoming the treatment of choice for many service
Just how much should be used?
Normally, a final film thickness of from 0.5 mils to 1 mils of the
concentrate is all that is necessary. In other words, you want just enough
to fill up the interstices between the contact's faces. When using Stabilant
22A, use enough so that once the alcohol evaporates the desired 1/2
to 1 mil film of Stabilant 22 remains.
In applications to moving surfaces, such as in slip-rings or potentiometers,
film thickness should be minimized to the point where "hydroplaning" won't
What is the 15mL service kit?
This was made up at the request of several manufacturers who wanted
a standard kit that they could issue to their service personnel. It consists
of a 15mL dropper bottle of Stabilant 22A and some applicators,
all in a small capped cardboard tub that can be tossed into a tool box
Does the action of Stabilant deteriorate with age?
In some field trial applications lasting over ten years Stabilant
22 has shown no sign of reduced effectiveness. With a high molecular
weight and a very low vapor pressure, almost none of the material will
be lost by evaporation. Unlike some other contact protection oils, Stabilant
22 will not cross-link when exposed to free-machining materials such
as high sulfur brass, or when used on contacts where agents used to promote
cross-linking of thermosets or elastomers are present in the environment
or in the actual connector components. Thus Stabilant 22 does not
form a scum or "varnish".