We did a study several years ago and here is the calculation method we came
up with.  Hopes this helps.

        Solvent to Surface Area Ratio Calculation

                                The Omega Meter System is most accurate when
a ratio of 30 to 50 milliters of solution per square inch of surface area is
maintained.

                                The width of the circuit card assembly is
the smallest of the two board dimensions unless otherwise specified in the
Work Instructions.

                                The amount of solvent required to perform a
test is determined by the width of the assembly being tested.  The following
table shall be used to determine the quantity of assemblies needed to
maintain the suggested solvent to surface area ratio.

        SOLVENT RATIO   TABLE

                                Board Width     Minimum Surface Area
                                   0-2"  40 sq. in.
                                2.1 - 4"         60 sq. in.
                                4.1 - 6"        110 sq. in.
                                6.1 - 8"        150 sq. in.
                                8.1 - Up        210 sq. in.

        Calculation Examples

                                PWB is 4.2" x 7.5" and consists of mostly
surface mount components.

                                Total surface area =
                                4.2" x 7.5" x 2 (two sides) x 1.6 = 100.8
sq. in.

                                The solvent ratio table indicates a minimum
area of 110 sq. in. associated with a board width of 4.2". Therefore, two
CCAs shall be placed in the Omega Meter test cell for the test.  The total
surface area of the assemblies being tested is then 100.8 sq. in. x 2 =
201.6 sq. inches.

                                PWB is 5.5" x 10" and is approximately 50%
surface mount components.

                                Total Surface Area =
                                5.5" x 10.0" x 2 x 1.6 x .50   =  88.0
                                5.5" x 10.0" x 2 x 2.1 x .50   = 115.5
                                  Total Surface Area           = 203.5

The solvent ratio shows a minimum area of 110 sq. in. for a board width of
5.5".  Only one CCA is required for the test.

> ----------
> From:         HUCKL CHANG, P.E./SHA
> 086-520-7314449.EXT.30156[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;HUCKL CHANG, P.E./SHA
> 086-520-7314449.EXT.30156
> Sent:         Wednesday, November 04, 1998 11:41 PM
> To:   [log in to unmask]
> Subject:      [TN] FOR YOUR HELP
>
>  Hi,
>
>  Will you help me to answer the following question:
>
>  Our company produce payment-transaction products, to ensure the quality
> we
>  sampleing test its PCBA (print circuit board assembly) ionic
> contamination
>  useing OMEGA METER 600-SMD before final assemble. While wanting to define
>  the acceptance limit (unit: ug.NaCl/sq.in. the general is 14.0), we
>  encounter the problem: how to calculate its total surface? In my opinion:
>  Total surface area=level factor*2*bare board surface.
>
>  Reason:
>
>  1. high density of component of PCBA can enlarges its "contamination
>     surface", then times "level factor".
>  2. components lie on the both sides of PCB, then times "2".
>
>  FYI--my company's PCBA manufactureing process:
>
>  PCB side A print solder paste----->Side B through wave soldering.
>
>  Although we have four volumes of IPC Standards and Specifications, I
> can't
>  find the clear definition about PCBA ionic contamination test, especially
>  how to calculate the total surface. Will you help to direct the
> information
>  or offer your opinions?
>
>  Thanks with best regards,
>  Huckl Chang.
>
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