Minutes of the Tracker Meeting

August 26, 2004


Action Items:

  1. Arthur: send the ASTM F22-02 to Sandro.
  2. Jeff: send Nusil mechanical properties to Sandro (done).
  3. Robert: write a note on the pros and cons of eliminating the encapsulation.


Sandro reported that in Pisa they put 3 bare trays (with Kapton) and 4 completed trays (the ones with HV shorts on one side) into the thermal chamber to receive about 15 cycles before Monday.


Sandro also reported on recent inspections of Tungsten tiles as they were received at Plyform from the bead-blasting vendor in Bologna.  They were not clean and had clearly been touched by bare fingers.  The Plyform cleaning procedure cannot remove the finger prints and other contaminants from the roughened surface.  Pisa will try acetone in an ultrasonic cleaner tomorrow (this didn’t remove the stains, either).  While this is clearly a problem with the process, Sandro did not think it could explain all the debonding issues that were seen.  The initial set of tiles was done in Bologna by the lead engineer while Sandro was there and were not contaminated in this way.  The company did not flow the cleanliness requirements down to the production staff. 


Ben noted that the Plyform cleaning procedure (wiping with MEK) probably just removes part of the contamination and smears the rest uniformly over the surface of the tile. 


Sandro asks for more information on the water break test for surface cleanliness.  Ben recommended ASTM F22-02.  Arthur will forward a copy of that to Sandro.


Sandro is writing a document to detail all the relevant Plyform procedures for the anomaly review team. 


Encapsulation issue: the damage seen after thermal cycling was not caused by the glue needle.  Instead, it is clear that the thermal stresses are delaminating the Nusil CV-2502 fill material away from both the Nusil 1142 dam and the silicon surface.   The silicone glue then pulls the wires loose and kinks them.  For some reason this only happens on the heavy trays.  The only relevant difference that anybody could think of is the fact that the heavy trays bend slightly (100 um saggitta) when cooled or heated, due to the large asymmetry from one side to the other.  On one heavy tray about 80% of the wire bonds broke during thermal cycling.


G&A would like to know the mechanical properties of the 3 Nusil adhesives: 1142, CV2500, and CV2502.  Jeff will send that information (nominally in a wide range around room temperature the CTE is 320 ppm, and the modulus is 250 psi). 


Several avenues are being explored to resolve this issue:

  1. Compare by coupon tests the adhesion of CV2500 (as used in ladder assembly) with the CV2502 used between MCMs and SSDs (because of its higher viscosity).  Maybe the CV2500 works better and could be used in both applications.
  2. Move the dam much further away from the wafer edge, to increase the surface area for adhesion of the fill material to the silicon.
  3. Eliminate the encapsulation entirely from between the MCMs and SSDs.  The only rationale ever given for the encapsulation was to decrease the change of bias shorts being caused by loose conductive particles.  Mechanically it is clearly an advantage not to have the encapsulation.  Not having it also allows us to rework wire bonds in case of damage or in case bad bonds are found by the initial tray electrical test.  Furthermore, there would be a big bonus in production rate of trays through G&A if this difficult process could be eliminated.  Robert will write a note outlining the pros and cons.


Tile grounding: Plyform proposed a solution that would ensure some electrical contact between the tungsten tiles and the tray structure.  They would spread the adhesive on the tray as usual and then place by hand 8 long fibers into the glue, keeping the fibers well away from the tray edge.  The tiles would then go on top.  The 100-micron fibers would ensure electrical contact and would also help maintain the bondline thickness. 


Jeff reported that the cable-fixation corner brackets will arrive tomorrow and should ship to Pisa by Monday.  The lifting fixture will ship tomorrow, and the inner container should ship by Monday, along with the fasteners.  Two EGSE systems shipped today.


Jerry said that there will be a meeting with Parlex tomorrow regarding the flex-circuit cable coupon failures.  A large fraction of the “flight” cables are being rejected day by day due to incomplete connection between the via plating and the internal copper planes.


The review of the new pitch-adapter design will take place on Monday.


Albert reported that several non-flight MCMs were inspected for pitch-adapter cracks and will now go into a thermal chamber for 200 cycles -30C to +85C to see if the cracks open up or if new cracks are formed.


Jerry reported that the MCM boards in which bias shorts formed appear to be closely related in time during fabrication.  Investigations are continuing.  Tom Himel will direct the effort.