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Report of Root Cause Analysis of RF Amplifier Gain Failures.
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MEMO
1(10)
Prepared Document Number
S Stockbridge BWPS/OM:0051 Approved Date Rev File
Travis XXXXX 01/15/2004 A
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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.
MEMO
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Prepared Document Number
S Stockbridge BWPS/OM:0051 Approved Date Rev File
Travis XXXXX 01/15/2004 A
2.1. Investment cast chassis and machined inner lid with boards installed and screws torqued to 10 in-
lbs 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.
MEMO
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Prepared Document Number
S Stockbridge BWPS/OM:0051 Approved Date Rev File
Travis XXXXX 01/15/2004 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.
MEMO
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Prepared Document Number
S Stockbridge BWPS/OM:0051 Approved Date Rev File
Travis XXXXX 01/15/2004 A
2.3. Die-cast chassis and inner lid with boards installed and screws torqued to 10 in-lbs 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 .039”.
10 in-lbs results in 3,715lbs of assumed uniformed load.
13 in-lbs results in 5,015lbs of assumed uniformed load.
MEMO
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Prepared Document Number
S Stockbridge BWPS/OM:0051 Approved Date Rev File
Travis XXXXX 01/15/2004 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
MEMO
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Prepared Document Number
S Stockbridge BWPS/OM:0051 Approved Date Rev File
Travis XXXXX 01/15/2004 A
3. 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.
MEMO
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Prepared Document Number
S Stockbridge BWPS/OM:0051 Approved Date Rev File
Travis XXXXX 01/15/2004 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
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Prepared Document Number
S Stockbridge BWPS/OM:0051 Approved Date Rev File
Travis XXXXX 01/15/2004 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)
MEMO
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Prepared Document Number
S Stockbridge BWPS/OM:0051 Approved Date Rev File
Travis XXXXX 01/15/2004 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
MEMO
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Prepared Document Number
S Stockbridge BWPS/OM:0051 Approved Date Rev File
Travis XXXXX 01/15/2004 A
5. 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.
