PSAS/ news/ 2004-06-09 - Foam density revisited

Long term weight of the foam

In 2004-02-12, Glenn weighted two strips of the 0.75" wide foam. I reweighed those two strips to see if there was any long term drift in weight.

Raw data:

"#1" 0.75 x 6 in strip in weigh boat 7.3565 g
"#2" 0.75 x 6 in strip in weigh boat 7.4082 g


Assuming the 2004/02/12 "average weight" of the weigh boat of 6.8645 g:

Sample 2004/02/12 2004/06/09
#1 0.5217 g 7.3565 - 6.8645 = 0.492 g
#2 0.5889 g 7.4082 - 6.8645 = 0.543 g

Unfortunately, we'll have to throw out this part: the weight boats are pretty variable, like in the 0.1 g range, which could completely account for the difference between the two measurements. I went ahead with the measurements based on the assumption that we had the weights for the "#1" and "#2" boats, which isn't true. Oh well.

VOC boil off on wide foam

We took a 4.032 x 4.016 inch piece of the wide foam tape and as quickly as possible - about 15 seconds or so - removed it from the roll, took off the wax paper backing, and put it in the weigh boat for weighing. Here's the weight data:

Time Weight (g)
0 2.3300
0.25 2.3056
0.5 2.2852
0.75 2.2726
1 2.2612
1.25 2.2522
1.5 2.2453
1.75 2.2385
2 2.2341
2.25 2.2311
2.5 2.2292
2.75 2.2257
3 2.2220
5 2.2005
10 2.1802
15 2.1718
Removed weigh boat from scale and waived around a bit
16 2.1506

voc weight.png

So as you can see, this is pretty serious issue. Questions that need to be answered:

  1. Why doesn't the 0.75" tape exhibit this behaviour? Both tapes are 3M's 4466 which use rubber adhesives. Maybe the thinner roll was just already evaporated because it's thin?
  2. Does the VOC's eventually evaporate through the FR4? Or will it stay there?
  3. How does the rubber adhesive affect microwave frequencies?

Density measurement

The foam was 4.032 x 4.016 inches with a theoretical thickness of 0.0625 inches. But how the hell do you pick a weight?

VOC boil off on 0.75" tape check.

I cut two small pieces of the 0.75" tape from the remaining batch. One was from the beginnging of the roll, which we had taken off the main roll to give to Spencer. The other piece came from the very end of that original role. Both pieces showed no change in weight when their wax paper was removed , which may mean that they simply outgassed earlier since they're so thin.

Closeup photos of the foam's adhesive

I got my USB microsope up and running so here's a few pictures of the foam:

foam 60x.jpg
The foam at 60x: you can just barely see the adhesive

foam 200x.jpg
The foam at 200x: you can now see the adhesive as the solid white line