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# Calculating PCB surface area

David R

Calculating PCB surface area | 14 May, 2002

Hi. Does anyone out there know of a way to calculate the surface area of populated PCB's? We build many different types, and until now have been using a water displacement method, which I believe to be fundamentally flawed. Our technique is to drop an engineering board into a known volume of water, then measure the water displaced (in milliliters). We then multiply this by a figure of approx 0.35 (I can't remember the exact number off the top of my head). This supposedly gives us the area in square inches.

In my view, this does not work because you can have two objects of the same mass that will displace the same amount of water, but do not necessarily have the same surface area.

So, what else can we do, short of breaking the board down and working out each individual component?

This measurement is critical as it is used to determine the ionic contamination of our boards prior to coating.

CC to myself

Calculating PCB surface area | 14 May, 2002

Excuse me, but if I remember my physics lessons well, volume is what the water displacement measures, not mass. therefore, if you take the volume of water displaced by your populated PCB, divide it by the thickness of the PCB (which is easy to measure), you get the surface of the top and bottom side, since V=L x W x Thickness. converting square millimeters into square inches should not be a problem.

Scott B

Calculating PCB surface area | 14 May, 2002

I think what David was getting at was, as an example.

Take a cubic inch of gold. Surface area = 6 sq/in. Volume = 1 cu/in. Water displacement = X. Mass = Y

Roll that cube of gold out to a micron thick. Surface area = football pitch. Volume = 1 cu/in. Water displacement = X. Mass = Y

Therefore there is no direct correlation between surface area and water displacement, volume or mass.

Surface area is easy to calculate on bare PCB's but when you add all of your components how do you calculate your total assembly surface area other than getting into numerous complex calculations for each component on your PCB and adding them all together.

Calculating PCB surface area | 14 May, 2002

Claude is right, however, this may work: Dip the empty board in paint and let the access paint drip off. Measure the volume of the paint before and after. Dip a populated board and measure how much more paint was required in percent. Measure the surface area of the empty board ( all six sides). Multiply the surface area with the percent number of how much more paint was required on the populated board.

Hussman69

Calculating PCB surface area | 14 May, 2002

You guys must be managers!

Calculating PCB surface area | 14 May, 2002

I feel as though I�ve just walked in to a bar, where everyone I was supposed to meet has been drinking for 3 hours.

First, the volume of water displacement thing doesn�t make sense, as others have commented.

Second, I believe Dave Robbie wants measure the cleanliness of his board to be sure his cleaning process is in control prior to conformal coating his boards. So, he needs to know the cleanliness level is below �so & so�, before he coats the boards. The boards can be cleaner than �so & so�, but they cannot be dirtier.

As background, J-STD-001 wants boards to have a "cleanliness" level of < 1.56 micrograms / cm^2 (10.06 micrograms / inch^2) NaCl equivalence using ROSE [resistivity of solvent extract or as Eurolanders call it, Solvent Extract Conductivity (SEC).] testing. Historically, this 10.06 figure was for finished assemblies that were soldered with high solids rosin fluxes. So, it�s unclear that that number is appropriate for boards soldered with other fluxes.

So, Dave has some target [�so & so� micrograms / inch^2] that he needs to beat, but he still needs to calculate the surface area of his board, the �per inch^2� bit. * Naval Avionics Center Technical Report MRR-3-78 talked about this issue, if you can find a copy. Maybe try EMPF. * Old timers used LXWX3 as a "rule of thumb", but this was in the days when �men were men� and components were BIG. * Today, CSL [Contamination Studies Laboratory] uses LXWX2X1.1 * Even more conservative is LXWX2. If you make your �so & so� number with that, you�ve got a clean board. [The bonus here is, that none of those people in skirts (er, kilts) are going to ask you to explain how you came-up with a factor to compensate for component surface area. There is no factor.] * Contact Mike Konrad at Aqueous Technologies. He�s probably napping. He likes to talk about stuff like this. I�m sure he has a fresh point of view. * I don�t remember, and don�t have my copy handy, but as we�ve discussed before, here on SMTnet, "An In-Depth Look At Ionic Cleanliness Testing," RR0013, August 1993 is the seminal study on ROSE testing and maybe they discussed calculating surface area.

Calculating PCB surface area | 15 May, 2002

DaveF to the rescue, my favorate SMT search engine.

Calculating PCB surface area | 17 May, 2002

This conversation reminds me of the old �How Many Engineers Does It Take To�� joke. We manufacturer the ionic contamination (ROSE, SEC) testers and I am not aware of anyone that performs such exacting surface area calculations.

Most people use the very simple (L x W) x 2 formula. Then, based on pure subjective observation, they add a percentage for component population (20% � 50%). Perhaps the total surface area calculation is off a percent or two but as long as you are seeing low ionic numbers, a few percent differences in surface area will not make the difference between Pass and Fail. Keep in mind, ionic contamination testers are used to verify the performance of a cleaning process. Although current standards allow a max contamination of < 1.56 micrograms / cm^2, no one breathes a sigh of relief if the results are 1.55. As long as you�re seeing results in the 0.0 � 0.3 range, minor differences in surface area will not affect your results in any great manner. When in doubt, under estimate the area and err on the side of caution. The most important factor is to be consistent. Once you are reasonable sure of a boards surface area, stick with that number.

Mike

This message was posted the Electronics Forum @

Hussman69

Calculating PCB surface area | 17 May, 2002

A man in a hot air balloon realized he was lost. He reduced altitude and spotted a woman below. He descended a bit more and shouted, "Excuse me, can you help me? I promised a friend I would meet him an hour ago, but I don't know where I am."

The woman below replied, "You are in a hot air balloon hovering approximately 30 feet above the ground. You are between 40 and 41 degrees north latitude and between 59 and 60 degrees west longitude." "You must be an engineer", said the balloonist.

"I am", replied the woman. "How did you know?"

"Well," answered the balloonist, "everything you told me is technically correct, but I have no idea what to make of your information, and the fact is I am still lost. Frankly, you've not been much help so far." The woman below responded, "You must be in management."

"I am," replied the balloonist, "but how did you know?" "Well," said the woman, "you don't know where you are or where you are going. You have risen to where you are due to a large quantity of hot air. You made a promise, which you have no idea how to keep, and you expect people beneath you to solve your problems. The fact is, you are in exactly the same position you were in before we met, but now, somehow, it's my fault."

CC to myself

Calculating PCB surface area | 17 May, 2002

in both samples the water displacement is 1 cubic inch of water since the volume is the same. the first has a thickness of 1 inch, the second has a thickness of a few microns. if you know the volume and one of the lengh, width or thickness, you can calculate the surface. It is simple basic geometry. Even is you flatten it to a football field in surface, a volume of 1 cubic inch still displaces 1 cubic inch of water, no matter the mass.

ianchan

Calculating PCB surface area | 28 May, 2002

Great story! *grinz*

how do engineering managers get portray?