The document summarizes experiments done to investigate gain failures in production units. Tests showed that warpage in die-cast parts was not the cause, as compression patterns were similar between machined and die-cast units. However, the input detector board was thicker than specified, preventing even compression. Adding extra gasketing between boards in problem areas improved results. Further examination found the detector board lacked edge grounding vias, allowing radio frequency signals to leak. To resolve issues, recommendations include adjusting the die-cast tool to equalize board compression, and using gaskets with a wider compression range to accommodate manufacturing variations.

1. MEMO
Prepared
S Stockbridge
BWPS/OM:0051
Approved
Date
Travis XXXXX
01/15/2004
1(10)
Document Number
Rev
File
A
Dist:
Subject: Taz Gain Failures
1.
General
Production began using an investment cast chassis and machined inner lid with the inner lid M3
machine screws being torqued to 10 in-lbs. This approach worked well in both the Alpha and
Beta units that were built, tested and qualified. Three out of the first five die-cast units that we
built at the CM with the inner lid M3 thread forming taptite screws torqued to 10 in-lbs were not
working correctly. It was later determined that the reason they were not working was due to
isolation problems between the output section of the Main Amp board and the input detector on
the DPD board. After this discovery a gain test was implemented into production.
We initially solved the gain problem by increasing the inner lid screw torque to 12 in-lbs, but as
production continued we saw some units fail this new gain test. So we increased the screw
torque to 12.5 in-lbs, but once again as production continued we saw some units continue to fail
gain and we increased the screw torque to a finial 13.5 in-lbs. It was believed that the increase
torque on the inner lid screws was required to take the inherent warpage out of the inner lid. The
corner-to-corner flatness of a machined inner lid is +/- .005” vs. +/- .026” for the die-cast inner lid.
Unfortunately, tighter tolerances cannot be expected from a die cast part. As a result, we have
seen fewer gain failures, but have not completely resolved this issue. In fact, these higher screw
torques have resulted in ceramic caps developing stress cracks and more and more stripped M3
core holes.
We have performed various experiments with machined and cast parts in order to determine the
root cause of the problem. This document serves to summarize our findings and make
recommendations for corrective action.
2.
Experiments: The first sets of experiments were done to look at the compression comparison
between machined and die-cast units. The following compression tests were performed using
Pressurex pressure indicating film with a pressure range of 350 to 1,400 PSI. The EMI gasket is
Gore-Shield GS5200, which requires >15% compression (< 100psi) to achieve good shielding
levels.

2. MEMO
Prepared
S Stockbridge
BWPS/OM:0051
Approved
Date
Travis XXXXX
01/15/2004
2(10)
Document Number
2.1.
Rev
File
A
Investment cast chassis and machined inner lid with boards installed and screws torqued to 10 inlbs and at 13.5 in-lbs. The measured flatness for the chassis and inner lid are shown below.
There was a maximum gap between the inner lid and chassis of .010”.
10 in-lbs results in 3,715lbs of assumed uniformed load.
13.5 in-lb results in 5,015lbs of assumed uniformed load.

3. MEMO
Prepared
S Stockbridge
BWPS/OM:0051
Approved
Date
Travis XXXXX
01/15/2004
3(10)
Document Number
Rev
File
A
The pictures below show the flatness of the investment chassis and machined inner lid that were
used in this test.
2.2.
Investment cast chassis and machined inner lid unit without boards installed and screws torqued
to 13.5 in-lbs.

4. MEMO
Prepared
S Stockbridge
BWPS/OM:0051
Approved
Date
Travis XXXXX
01/15/2004
4(10)
Document Number
2.3.
Rev
File
A
Die-cast chassis and inner lid with boards installed and screws torqued to 10 in-lbs and at 13.5 inlbs. The measured flatness for the chassis and inner lid are shown below. There was a
maximum gap between the inner lid and chassis of .039”.
10 in-lbs results in 3,715lbs of assumed uniformed load.
13 in-lbs results in 5,015lbs of assumed uniformed load.

5. MEMO
Prepared
S Stockbridge
BWPS/OM:0051
Approved
Date
Travis XXXXX
01/15/2004
5(10)
Document Number
Rev
File
A
The pictures below show the flatness of the die cast chassis and inner lid that were used in this test.
2.4.
Die Cast Units without boards installed and screws torqued to 13.5 in-lbs

6. MEMO
Prepared
S Stockbridge
BWPS/OM:0051
Approved
Date
Travis XXXXX
01/15/2004
6(10)
Document Number
3.
Rev
File
A
Conclusion of compression tests
The tests show that the compression patterns are the same for machined and die-cast units.
Therefore, the inherent warpage in the die-cast unit is not causing the gain failures as assumed.
What was noted in the tests was the DPD gasket was being compressed more than on the main
amp side. We measure several DPD boards and found them to be on the high side of the
tolerance, where as the main amp board was at its nominal thickness. As a result, the inner lid
center section is not coming into contact with the chassis floor. One would conclude that this is
the root cause of the gain failures. Gore states that < 100psi is required to achieve good
shielding levels and the pressure film that was used measures pressures at 350 to 1,400 PSI. All
of the compression patterns show an impression of each gasket on both boards, which means we
have a minimum of 350psi, so why are we having this problem? How is the RF signal getting
out? These questions lead us to the following experiments.
4.
Additional Experiments
By talking to the RF engineers we knew the general area where the RF signal was coming from.
For our first experiment we used an amplifier that had failed the gain test and had the inner lid
screws torqued to 13.5 in-lbs. We removed the inner lid and added a very compressible EMI
gasket between the two boards where the center section of the inner lid comes into contact. We
then reinstalled the inner lid and torqued the screws to 10in-lbs twice and retested the unit. The
unit greatly improved and passed the gain test. (See data below)
Test results with screws torqued to 13.5in-lbs
Test results with screws torqued to 10in-lbs and EMI gasket between the boards.

7. MEMO
Prepared
S Stockbridge
BWPS/OM:0051
Approved
Date
Travis XXXXX
01/15/2004
7(10)
Document Number
Rev
File
A
This experiment proved that the RF signal must be leaking between the board layers, so we took
a closer look at the two boards. As you can see from these photos the DPD board does not have
any ground vias on the edge of the board facing the main amp board and the main amp board
has some but are spaced far apart.
Top of DPD Board
Bottom of DPD Board
Bottom of Main Amp Board

8. MEMO
Prepared
S Stockbridge
BWPS/OM:0051
Approved
Date
Travis XXXXX
01/15/2004
8(10)
Document Number
Rev
File
A
With this discovery we conducted another test by adding copper tape to the boards in these areas
only and installed the inner lid and torqued the screws to 10in-lbs. We were unable to torque all
the screws to 10in-lbs because 13 cored holes were stripped. When we tested this unit it had the
best results compared to the other two. (See data below)

9. MEMO
Prepared
S Stockbridge
BWPS/OM:0051
Approved
Date
Travis XXXXX
01/15/2004
9(10)
Document Number
Rev
File
A
This is the inner lid that shows the location of the screws that were stripped. A black
marked next to the hole shows the locations.
This is the test result data

10. MEMO
Prepared
Document Number
S Stockbridge
BWPS/OM:0051
Approved
Date
Travis XXXXX
01/15/2004
10(10
)
5.
Rev
File
A
Conclusion and Recommendations
As result of these tests we now know that the missing edge vias are the root cause for the gain
failures.
To equalize the compression on both boards we recommend changing the die-cast tool on the
DPD side by .009.
The current Gore gasket does not have enough compression range to accommodate for
manufacturing variations. In the future it would be preferred to use Gore’s new SuperSoft gasket
or form-in-place gasket. Both of these have a larger compression range and require less
compression force.