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Contamination of circuit boards can bring
about severe degradation of insulation resistance and dielectric strength.
Cleanliness of completed circuit boards is, therefore, of vital interest.
For those companies who have established
circuit board cleaning procedures, the MIL Spec P-28809 has been used as a
guideline for control. Now a simple "on line" test for the relative measurement
of ionic contamination has been developed.
This fast and economical method for testing
circuit board cleanliness uses a Myron L Company Ultrameter
II™ 4P or 6P, a suitable
container, and a mixture of Dl (deionized) water and alcohol. The procedure is
as follows:
-
Mix a stock quantity of solution using 25
parts by volume of Dl water and 75 parts by volume of 99% isopropyl alcohol. The
conductivity, measured with the Ultrameter II
4P or 6P should be a maximum of 0.166 micromhos/microseimens/cm.
-
Measure out an amount of the water/alcohol
mixture equal to 100 ml per 10 square inches of circuit board surface to be
tested (considering both sides of the board but not components), and add 60 ml
additional. In other words: 2(L X W) (10 ml) + 60 ml = total solution needed.
-
Fill a poly "zip-lock" bag or other
suitable plastic or glass container with the measured water/alcohol solution.
-
Using the measured water/alcohol solution
in the poly bag, rinse out the Ultrameter II's
cell cup three (3) times, discarding the rinse solution each time. Fill the
instrument cell cup a fourth time and take a meter reading. This value should be
0.166 micromhos/microseimens/cm or less and is the very clean control (or
"comparison") reading for the test.
-
Being very careful not to contaminate the
PCB, totally immerse the circuit board in the solution. Seal bag. Allow it to
soak for three (3) minutes with mild agitation.
-
At the conclusion of the soaking, pour the
solution directly into the instruments cell cup four (4) times; take the fourth
reading.
-
Compare the control reading in Step 4 with
the reading taken in Step 6. The higher the difference between the two readings,
the greater the ionic contamination. Record this final extract reading for
comparison with other boards tested in the same manner.
The level of cleanliness needed or desired
can only be determined by each individual company.
Mil Spec P-28809 can be used as a guideline,
or standards can be established based upon available data. In either event, the
comparative method using the Myron L Company's Ultrameter
II will assist in the
determination of that level of cleanliness.
|
Problem |
Solution |
Best
Myron L Models |
|
High
water, sewer use, and waste treatment costs |
Myron
L
CONTROLSTIK
II™
Rinse
Tank System will automatically monitor the quality of rinse
water... and reduce usage by as much as 80%. Myron L continuous conductivity,
Monitor/controller. |
597
758II-XXX |
|
Need
to quickly verify DI water purity or "trouble-shoot"
reverse osmosis equipment |
Portable
conductivity instruments are easy to use, yet accurate and
reliable enough for "field" testing of ultra-pure water. |
4P and 6P
Ultrameters
EP (5 microsiemen ranges;
1 resistivity range)
532T2
(3 ranges ppm/TDS) |
|
As DI cylinders are used, quality decreases, falling
below acceptable limits. This is not always noticed before cylinder can be
replaced. |
A
Myron L continuous conductivity, resistivity monitor/controller or
Aquaswitch. When DI cylinders approach exhaustion, an alarm,
solenoid valve, etc. can be activated |
758II-101
753II-11
Aquaswitch I |
Conversion Table
|
Specific
Resistance in Ohms
|
Specific
Conductance in Micromhos/ Microseimens |
Sodium
Chloride Equivalent in Micro Grams/CM² |
Sodium
Chloride Equivalent in Micro Grams/In² |
PPM
Sodium Chloride |
|
10
megohms |
0.10 |
0.31 |
2.00 |
0.021 |
|
9 megohms |
0.11 |
0.34 |
2.22 |
0.026 |
|
8 megohms |
0.12 |
0.39 |
2.50 |
0.033 |
|
7
megohms |
0.14 |
0.44 |
2.85 |
0.041 |
|
6
megohms |
0.16 |
0.52 |
3.33 |
0.052 |
|
5
megohms |
0.20 |
0.62 |
4.00 |
0.068 |
|
4
megohms |
0.25 |
0.78 |
5.00 |
0.091 |
|
3
megohms |
0.32 |
1.00 |
6.45 |
0.130 |
|
2 megohms* |
0.50* |
1.56* |
10.06* |
0.207* |
|
1
megohms |
1.00 |
3.10 |
20.00 |
0.440 |
|
775,000 |
1.29 |
4.00 |
25.80 |
0.575 |
|
620,150 |
1.61 |
5.00 |
32.25 |
0.725 |
|
517,000 |
1.94 |
6.00 |
38.71 |
0.878 |
|
443.000 |
2.26 |
7.00 |
45.16 |
1.027 |
|
388,000 |
2.58 |
8.00 |
51.61 |
1.176 |
|
345,000 |
2.90 |
9.00 |
58.06 |
1.324 |
|
310,000 |
3.22 |
10.00 |
64.52 |
1.476 |
*Equivalent to 0.01 milligram NaCl/In.2
(MIL-P-28809) maximum contamination level)
Note: Figures
approximated to the second decimal place.
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