1 Quality control of the LHC main interconnection splices before and after consolidation C. Scheuerlein and S. Heck 2 nd LHC splice review, Acknowledgements: F. Bertinelli, Z. Charifoulline, J.-P. Tock, A. Verweij

Outline o Introduction o QC of existing main interconnection splices (before consolidation) o QC of repaired main interconnection splices o QC of shunts o QC of consolidated main interconnection splices o Rough estimation of resources o Conclusion 2

Introduction o Main interconnection splice quality assessment during LHC installation o A typical splice defect o What did we learn during the 2008/2009 shutdown concerning main interconnection splice QA and QC? o Assumptions for the QC of main interconnection splices during LS1 3

Main interconnection splice quality assessment during LHC installation o During LHC installation the LHC main splice quality assessment was based on: visual inspection according to the standard procedure IEG-C- BR-001 rev C “Contrôle visuel des brasures” numerous measurements of the current decay constant of LHC busbar cable loops with test splices at the Cryolab hundreds of tensile tests at 4.2 K and RT of test splices ultrasonic transmission tests through U-profile and wedge of some splices in the LHC recording of apparent process parameters (temperature and pressure) o Despite these visual inspections and tests it could not be detected that the stabilisation of an estimated 1500 splices is insufficient, and that the temperature regulation of the inductive soldering machines was out of control. 4

A typical defect in a splice with very high excess resistance 5 Unmolten Sn96Ag4 solder at the splice extremities (due to an insufficient splice temperature) is the reason for the excess resistance in most very high resistance splices. Gamma ray imaging courtesy J.-M. Dalin

What did we learn during the 2008/2009 shutdown concerning main interconnection splice QA and QC? o Results of a test program on 13 kA splice samples by P. Fessia and P. Thonet that was started after the 2008 incident indicated severe problems of the temperature regulation of the inductive soldering machines as they were used during the LHC installation (e.g. by using a wrong wire as a “thermocouple” to regulate the soldering temperature). o The inductive soldering machines are recording the targeted temperature cycle. The true splice temperature can differ strongly from the recorded values, e.g. when a wrong “thermocouple” is used (underheating) or when the thermocouple is not well placed or not well connected to the splice (overheating). o Test loops of 13 kA splices are not representative for splices produced in the LHC, e.g. because the strong temperature gradient across splices in the LHC is not present in the test splices. o Room temperature resistance measurements (so-called R-8/R-16), which were suggested by H. Pfeffer and B. Flora, are an excellent tool for detecting splices with insufficient stabilisation [1]. 6 [1] “Production and Quality Assurance of Main Busbar Interconnection Splices during the LHC Shutdown”, IEEE Trans. Appl. Supercond., 22(3), (2011), 1786

Assumptions for the QC of main interconnection splices during LS1 o Each of the LHC main interconnection splices needs to be controlled before consolidation and after consolidation. o Each shunt needs to be controlled separately. o QC needs to be based on quantitative acceptance criteria. o QC should contribute to a continuous improvement of the splice consolidation process. 7

QC of existing main splices o Risk of cable damage during splice disconnection o When do we need to repair main interconnection splices before application of shunts? R-8 acceptance threshold values for existing splices geometrical acceptance criterion and test splices in the segments with high 1.9 K excess resistance 8

Risk of cable damage during splice disconnection o Any splice opening invariably causes a small busbar cable degradation, and there is a risk of severe cable damage (in 2009 two severely damaged cables were found that could not be re-connected). o It is likely that problematic cables will be found in 2013 as well, which may require the removal of several magnets if the cables cannot be repaired in the tunnel. o The QC of the finished main splices at RT cannot provide any information about the quality of the cables and the cable to cable contact. o It should be avoided to unnecessarily open existing main splices! 9 NCR : Cable overheated Half of the strands of one M3 cable of MB2446 were cut.

When do we need to repair existing main interconnection splices before application of shunts? o If R-8 is too high. o If shunts cannot be applied as is. o In case of high 1.9 K excess resistance. 10

R-8 acceptance threshold values for existing splices 11 o R-8 threshold values: Redo a splice when additional R-8 exceeds 5 µΩ. Dipole splice R-8>10.6 µΩ. Quad splice R-8>14.3 µΩ. o 5 µΩ excess resistance corresponds with a non- stabilised cable length of about 4 mm. o The safe currents for a splice with 4 mm non-stabilised cable are 17.2 kA and 15.4 kA for qaudrupole and dipole splices, respectively. Courtesy of A. Verweij and D. Molnar.

Geometrical acceptance criterion and test o Geometrical splice distortions must be in acceptable limits in order to be able to put shunts without machining too much Cu from the existing busbars and splice profiles to be able to put the splice insulation on the consolidated splice (maximum misalignment over the 150 mm: horizontal ±3mm, vertical ±5mm, EDMS Nr ) o In view of the large number of splices an efficient and reliable test is needed. o As suggested by the LMF team, gauges will be used for the assessment of the splice distortions (horizontal and vertical gauge for dipole and quadrupole splices). 12

What to do with the splices in the segments for which an excess resistance at 1.9 K has been found? o The maximum 1.9 K electrical resistance measured for all main interconnection splices is 3.3 nΩ [2]. o In order to estimate the mechanical strength of main interconnection splices with 1.9 K-excess resistance, a series of electrical and mechanical tests has been performed with splices with varying intercable overlap length [3]. 13 [2] Z. Charifoulline, ”Status of 1.9K Splice Resistances in LHC Main Magnets (interconnects and internals)”, CERN EDMS No [3] S. Heck et al, “Electrical resistance and mechanical strength of LHC busbar cable splices as a function of intercable contact length”, CERN-ATS-Note , EDMS No LHC busbar cable test loops with varying intercable overlap length. LHC busbar cable splice with 3 mm overlap length (2.5 % of nominal overlap length).

Splice resistance as a function of intercable overlap length o As expected the splice resistance is nearly inversely proportional to the cable overlap length. 14 Comparison of the resistances measured with FRESCA for splices with different intercable contact lengths and the calculated resistances [3].

Mechanical strength at 4.3 K as a function of intercable contact length o A splice resistance of 3.3 nΩ (the highest resistance measured for all main interconnection splices) corresponds with an intercable contact length of about 12 mm. o The tensile strength of such a splice can exceed 3.5 kN (without the additional strength from the splice Cu profiles). 15 Force at rupture and electrical resistance R at 4.3 K of LHC busbar cable splices as a function of the intercable contact length. Each data point is an average value of 3 measurements.

Proposed strategy o MB splices: repair all 21 splices in the 8 segments with highest excess resistance (R 1.9K >0.82 nΩ). For all other splices put shunts as is. o MQ splices: at present 7 segments with 102 splices with R 1.9K >2 nΩ. Repair all splices in the 5 segments with 6/8 splices; to be decided for the 2 segments that contain 32 splices. 16 Maximum Splice Resistance in a Bus Segment. From Z. Chariffouline, EDMS No , September 2011

QC of repaired main interconnection splices (before application of shunts) o Visual inspection according to standard pocedure: IEG-C-BR-001 rev C “Contrôle visuel des brasures” No macroscopic (visible) gaps No steps between the Cu profiles >1 mm o Visual splice control should be performed already by operators producing the solder connections. o QC is mainly based on in situ R-8 results (measurement of R-16 is done as a cross check of R-8 results). o Photos 17 QBBI.A27R3-M3-corridor Gap between U-piece and busbar stabiliser (QBBI.A27R3-M3- corridor “new” before repair) Gap between U-piece and busbar tongue (QBQI.33L2-M3-QRL) Step between U-piece and busbar (QQBI.32R6-M2)

R-8 acceptance threshold values for “new” splices produced during LS1 18 o Redo “new” splices that will be produced during LS1 when: R-8 dipole >7.6 µΩ R-8 quad >12.3 µΩ.

QC of the shunts o Why RT electrical tests for the QC of the shunt to busbar contacts? o Can we conclude from RT resistance measurements on the shunt resistance at cryogenic temperatures? o What is the smallest detectable gap size? o How will we determine acceptance threshold values? o Can we conclude from R_RT-top-side results on the mechanical shunt integrity? o How many resistance measurements are needed for the QC of the main interconnection splices before and after consolidation? o R-8/R-16 and R_RT-top-side data acquisition o How do we deal with non-conform shunts? o Visual inspection of shunts, witness samples 19

Why RT electrical resistance measurements for the QC of the shunt solder contacts o In order to assess the splice quality an efficient nondestructive test is needed. o Three nondestructive testing methods have been studied: Ultrasonic testing -amplitude of the US wave reflected at the solder interfaces- (at CERN [4], EMPA [5] and BAM [6]). Active thermography –thermal conductivity of the solder contact- (at BAM [7]). Electrical resistance (at CERN [8]). o The quality criterion selected for the shunt QC is the electrical resistance at room temperature (RT) [9]. 20 [4] J.-M. Dalin, CERN EN-MME, in collaboration with Olympus, GE and ECCND. [5] J. Neuenschwander, “Feasibility study NDT of solder joints”, Swiss Federal Laboratories for Materials Testing and Research (EMPA), Dübendorf, Test Report No , (2010) [6] G. Brekow, D. Brackrock, “Ultrasonic testing of solder joints using a phased array technique with matrix arrays”, Federal Institute for Materials Research and Testing (BAM), Berlin, report reference number: part 1, LHC-CONS, (2010). [7] C. Maierhofer, M Röllig, “Feasibility study for non-destructive testing of solder joints – Results of active thermography”, BAM, report reference number: part 2, LHC-CONS, (2010). [8] S. Heck et al., “Room temperature resistance measurements for the quality control of shunt solder connections for the consolidation of the LHC main interconnection splices”, CERN TE-Note , EDMS Nr: , (2010) [9] C. Scheuerlein, “Local quality control of LHC electrical interconnections during the 2012 shutdown”, presentation at the 1 st splice review,

o The electrical resistance of the lap joints for the consolidation of the LHC main interconnection splices was measured at cryogenic temperatures in order to study the influence of the solder alloy on the overall splice resistance. o Lap joints have been produced with 3 different solders: Sn96Ag4 (existing main interconnection splices) Sn60Pb40 (for splice consolidation) Sn77.2In20Ag2.8 (solder with very high electrical resistivity; for comparison). 21 Can we conclude from the RT resistance on the resistance at cryogenic temperatures? Electrical resistivity of different solder alloys as a function of temperature [10]. [10] S. Heck et al, “Resistivity of different solder alloys at cryogenic temperatures”, CERN TE-MSC Internal Note , EDMS Nr: , (2011)

R-6 resistance ratio o R-6 is only slightly influenced by the solder resistance (at 23 K: R-6 Sn96Ag4 =49.6±5.6 nΩ, R-6 Sn60Pb40 =49.5±6.4 nΩ, R-6 Sn77.2In20Ag2.8 =61.4±6.4 nΩ). o An approximate fit of data points with Cu resistance ratios is possible. R-6 ratio of Sn96Ag4 and Sn60Pb40 splices follows the resistance ratio of Cu with a RRR between R-6 resistance ratio as a function of temperature for shunt to busbar solder connections produced with different solders. For comparison resistance ratios for pure Cu with RRR 200 and 400 are shown as well. Courtesy R. Lutum.

R-trans resistance ratio o R-trans is strongly influenced by the solder bulk resistance. o At 20 K the resistance ratio of Sn77.2Ag20In2.8 soldered splices is roughly 3 times lower than that of Sn96Ag and Sn60Pb40 soldered splices. o For Sn96Ag4 and Sn60Pb40 soldered splices R-trans at 20 K can be estimated in reasonable approximation by dividing the room temperature resistance by a factor of 100. R-trans resistance ratio as a function of temperature for shunt to busbar solder connections produced with different solders. For comparison resistance ratios for pure Cu with RRR 20, 50 and 100 are shown as well. Courtesy R. Lutum.

What is the smallest detectable gap size? o Solder defects can be detected if they cause an additional R_RT-top- side resistance >0.5 µΩ [8]. o Gaps of 10 mm or larger between the shunt solder contacts will be detected. 24 [8] EDMS Nr: , (2010) Comsol simulation results of additional R_RT-top-side as a function of the gap size between the shunt solder contacts.

Safe current as a function of the gap size between shunt solder contacts 25 Safe current as a function of gap size between both shunt contacts for one quadrupole and for one and two dipole shunts. Courtesy A. Verweij and D. Molnar.

How will we determine R_RT-top-side acceptance threshold values? o Average R_RT-top-side values will be determined for dipole and quadrupole shunts. Initially 3 complete interconnects (48 shunts) will be tested. o After R_RT-top-side tests the shunts will be peeled off to determine the delaminated solder area. o Acceptance threshold values will be set such that the R_RT-top-side test has sufficient sensitivity in order to exclude the presence of unacceptably large defects, and sufficient specifity to avoid the repair of many sound shunts. o Comsol simulations indicate that 10 mm. 26

A real case: R_RT-top-side results for test splices with large defects and shunts (SM18 test) o R_RT-top-side result (not in optimum measurement configuration) of the shunt contact on the wedge of M3-QRL-connection was 10.5 µΩ. After repair R_RT-top- side=2.1 µΩ. o Visual inspection did not detect the defect. o In the future, the size and location of all solder defects will be analysed and documented by photos. Removed shunts will be stored by QC team. 27

Can we conclude from R_RT-top-side results on the mechanical shunt integrity? o The tensile force on a shunt cannot exceed the force to induce full elongation (100 mm) of the lyra, which at cryogenic conditions is about 750 N [11]. o The shunt QC test should guarantee that the tensile strength of shunt to busbar contacts exceeds 1 kN. o In order to verify if R_RT-top-side tests can detect shunt to busbar solder connections which tensile strength is below 1 kN, connections with deliberately added solder defects have been tested by tensile tests at 4.3 K and R_RT-top-side measurements. 28 [11] P. Fessia, “Specification for the electrical consolidation of the LHC 13 ka interconnections in the continuous cryostat”

R_RT-top-side results (center defects) 29 o Center defects are most difficult to detect by resistance measurements. o A defect area of 9 × 13 mm 2, i.e. 52% of overlap area (neglecting the hole) is detectable by an additional R_RT-top-side of up to 0.7 µΩ. R_RT-top-side for different positions on samples with defect type 4 (center defect)

Fracture surfaces of shunt samples with artificial solder defects after tensile test at 4.3 K 30 Samples courtesy M. Pozzobon and LMF team.

Force at fracture at RT and 4.3 K of shunt solder contacts with different defects o Center defects (type 4) are the most difficult to detect by R_RT-top-side measurements. The force at fracture of a shunt with a solder defect area of 50 % of the entire solder area exceeds 4 kN and 7 kN at RT and at 4.3 K, respectively. 31 RT force at fracture (average of 3 measurements) for different defect types. The defect area is always 50 % of the entire contact cross section. RT and 4.3 K force at fracture (average of 3 measurements) for defect types 2 and 4. The defect area is always 50 % of the entire contact cross section. Tensile tests courtesy A. Gerardin

How to deal with non-conform shunts? o Non-conform shunts are considered as a treasure. Careful analysis of defects should help to continuously improve the production and QC process. o Non-conform shunts will be disconnected such that the shunt to busbar solder contact can be inspected, and the delaminated surface area can be measured. o All disconnected shunts will be clearly labeled, and then stored by the QC team. 32

Estimation of the number of resistance measurements needed for the QC of the main interconnection splices (preliminary) o R-8/R-16 tests of existing main interconnection splices before consolidation91530 R-8/R-16 resistance measurements (each R-8/R-16 result is the average value of 3 resistance measurements). o R_RT-top-side tests of shunts (54240 solder contacts)  R_RT-top-side resistance measurements (4 resistance measurements at different positions of the shunt). o R-8/R-16 tests of consolidated main interconnection splices 91530 R-8/R-16 resistance measurements. o In total about room temperature resistance measurements need to be done! o More than photographs need to be taken only for the main interconnection splices. 33

R-8/R-16 data acquisition o In view of the large number of resistance measurements, an efficient data acquisition and analysis is mandatory. o Resistance measurements will be performed with a Digital Low Resistance Ohmmeter DLRO 10X. Resistance values are transmitted via the RS 232 interface of the DLRO 10X. o Operator has to input all required interconnection details. o Average R-8/R-16 values are automatically plotted in a chart and compared to threshold values.

R_RT-top-side data acquisition o 4 resistance measurements per shunt. o Fast and well reproducible positioning of the voltage taps at no-through holes in the positions -3 mm, -1 mm, 1 mm, 3 mm that will be added systematically to all shunts. o Photos can be linked to the corresponding data series.

Visual inspection of shunts o There are obvious differences in the visual surface aspect of samples with artificial solder defects and defect free samples, which may be exploitable for QC. o On the surfaces of the samples with solder defect one can observe: solder spreading in extreme cases the Sn60Pb40 reservoir is completely empty and the underlying busbar surface is visible. o A procedure for the visual inspection of shunts will be prepared when more shunt to busbar connections have been analysed. o Witness samples will be produced regularly. Exact sample type and tests to be defined. 36 Samples with artificial solder defects Samples without artificial solder defects

QC of consolidated main interconnection splices o Directly after the QC of the shunts R-8/R-16 of the consolidated splices will be measured as the final main interconnection splice QC test. o R-8 acceptance threshold values for consolidated splices need to be determined when more consolidated test splices are available. Threshold resistance values should be close to: R-8 dipole =7 µΩ and R-8 quad =11 µΩ. o Photos of all consolidated main splices will be taken by the QC team. 37

Roughly estimated resources for routine QC of main interconnection splices (preliminary) o We have to be prepared to control up to 60 interconnects per week. o The routine splice QC will be performed by five polyvalent teams, each made of two persons. 1 st QC of existing main splices (R-8/R-16 and geometrical check) 60 minutes for the 6 splices per interconnect (2×1700 hours). 2 nd QC of repaired main splices (visual control, R-8/R-16) estimated 1500 splices and 30 minutes per splice. 3 rd QC of consolidated main splices (visual control, R_RT-top-side, R-8/R-16, photos) 1 hour per interconnect (2×1700 hours). 4 th QC of splice insulation, new US weld and solder connections (visual control, geometrical check, photos) 1 hour per interconnect (2×1700 hours). o Not included are special QC and QA activities: Visual inspection of cables of disconnected splices Line N QC Audits Analysis of production data All other QC and QA activities that need to be done in the context of the work of the special intervention team 38

Conclusion o The electrical continuity of the main busbar stabiliser through the interconnection splices, as well as through the shunt solder connections can be guaranteed by in situ tests (R-8/R-16 and R_RT-top-side). o Gaps of 10 mm or larger between the solder contacts will be detected by R_RT-top-side tests. o Shunts which solder contact area is so small that the tensile strength of the shunt connection is below 1 kN will be detected by R_RT-top-side tests. o During LS1, roughly resistance measurements will be required for the main interconnection splice QC. o R-8 acceptance threshold values for existing (“old”) and “new” splices have been suggested. o Exact threshold values for shunts (R_RT-top-side) and consolidated splices (R-8) will be determined when more shunted splices have been tested. 39