LASER welding Process for RF Connectors for Space Applications

(1)

International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, UGC Approved List of Recommended Journal, Volume 8, Issue 3, March 2018)

371

LASER welding Process for RF Connectors

for Space Applications

Sakaram Srinivasulu

1

, Shivendra Tripathi

2

, R.K. Hegde

3

, J K Baria

4

Space Applications Centre-ISRO Ahmedabad,

V.P.R.P.T.P. Science College, Sardar patel University

Vallabh Vidyanagar Gujarat, India

Abstract—LASER (Light Amplification by Stimulated Emission of Radiation) welding of RF (Radio Frequency) connectors & DC (Direct Corrent) feedthrough finds potential applications in aerospace and defence where environmental conditions require extremely rugged and reliable weld joints. Present paper is focused on the application of Nd: YAG (Niobdenum- Yitrium Aluminum Garnet) LASER at 1064 nm wavelength for welding of RF & DC connectors with A16061 and characterization of weld joint in terms of environmental, mechanical & RF leakage tests for possible use in aerospace hardware. Using optimized laser weld parameters, welding of RF connectors and DC feedthrough with AI 6061 flange have been completed. Leak rate of <1x10-7 Atm cc/Sec Helium has been demonstrated successfully using sniff leak test method for all the seven samples which indicates good process repeatability. The laser weld joint has withstood various Environmental tests namely Thermal shock -70o C to + 180o C with dwell time of 10 min for each extreme for total of 500 cycles and Sine & Random vibration tests and passed the post environmental leak tests which is in conformance with MIL-STD-883 requirements. The Mechanical load test for weld joint strength showed no failure even at 23.2 kgf, which indicates the robustness of the joint. The micro sectioning of the joint indicates the depth of penetration of approximately in the range 280-300 microns for the optimized laser energy levels. Further the radiography analysis has been carried out which indicates crack free and uniform weld joint.

Keywords- LASER Welding, RF Connector , DC Feedthrough, He Leak Test, Weld qualification, Thermal Cycle, Vibration, Heat Affected Zone (HAZ)

I.

INTRODUCTION

The technological approach involves application of laser beam welding to hermetically seal LASER weldable RF connector and DC Feedthrough, with AI6061 alloy.

Better control on laser weld process parameters such as pulse to pulse overlap, beam energy etc. helps in the realization of a weld joint with leak rate levels <1x10-7 Atm cc/Sec He.

Being low Heat Affected Zone (HAZ)[1], it produces a metallurgical fusion weld in a small localized area at the interface. This development eliminates the otherwise mandatory need of Gold plating inside connector holes and use of mechanical fasteners.

Laser weldable RF connector/DC feedthrough housing basically consists of an outer shell made up of A14047 alloy and the contact pins are isolated from the body by means of an insulator. The use of A14047 as an outer shell has two distinct advantages namely, first it makes the Connector body laser weld compatible with A16061 and second, it saves 30 to 50% weight over its conventional counterparts made up of Kovar or Stainless steel.

The objectives of these experiments were

 To develop the LASER weld process for Weldable Feed through & RF connector assembly on Aluminium packages for RF applications.

 LASER weld process qualification for space usage.

Literature survey has been carried out on the use of Nd-YAG laser for welding of Al alloys [2-4].

II.

LASER WELD PROCESS DEVELOPMENT &

OPTIMIZATION

(2)

International Journal of Emerging Technology and Advanced Engineering

[image:2.595.83.230.172.264.2]

for soot sucker among others. The Principle of Laser welding is shown below in Figure-1.

Figure-1 Principle of Laser welding

Here, the Glove box is continuously purged with High purity 99.999% inert Argon gas -also called as carrier gas, in order to lower moisture and oxygen ppm levels suitable for laser weld process.

Weld surface cleaning of Aluminium alloy and RF laser weldable connector and laser weldable Feedthrough are performed using a soft tissue brush, followed by Iso Propyl Alcohol (IPA) solvent cleaning to remove contamination.

[image:2.595.322.534.255.563.2]

Mechanical geometric requirements for joint configuration of weld package & outer shell interface tolerance requirements are presented in Table-1 below.

TABLE I. MECHANICAL REQUIREMENTS

Parameter/Description Optimal requirements

Package Material AI6061 alloy

Connector Outer shell AI4047 Outer shell Thickness 1 to 2.0 mm

Interface gap Range: 50 to 100

+/-10 microns uniform

throughout weld

area/contour. Interface Co-planarity Up to +70 microns Outer Shell Dimension

(DC & RF) 8- 10 mm

ø

Laser Weld Process parameter optimization-

Laser weld process parameters [6] have been optimized for Laser welding of weldable RF connector & DC feedthrough with AI6061 alloy package, by carrying out several laser weld iterations. The relation between laser energy (E), peak power (P) and pulse width (W) is -

E= P x W

[image:2.595.327.531.265.420.2]

Prior to the laser weld the packages has been subjected to the demoisturisation cycle in a vacuum oven for specified temperature and duration. High purity Argon gas has been used to purge the Glove box. The laser weld process has been carried out in a Glove box. Smooth weld was achieved under optimal values (as per table-2) without any spiking/spattering defects . Laser welded RF connector image and weld geometry are given in figures 2 & 3 respectively.

Figure-2 Laser welded RF Connector

Figure-3 Weld Geometry

Optimized process parameters of the experiment are presented in Table -2 below.

TABLE-2 LASERWELDPROCESSPARAMETERS

Laser parameter Range

Peak power 1.5 – 3.0 KW

Pulse width 3 – 10 ms

Pulse repetition rate 10—20 Hz

[image:2.595.49.264.500.646.2] [image:2.595.332.529.668.777.2]

(3)

International Journal of Emerging Technology and Advanced Engineering

Typical laser weld Process sequence flow chart is shown in figure-4

Figure-4 Laser Weld Process Chart

Laser welded RF connector & Feedthrough

Figure-5 gives the visual and X-ray micrograph of the laser welded RF connector and DC feedthrough using optimized laser process parameters, under controlled glove box environmental conditions.

It shows typical weld segments as well as X-ray NDT image of welded RF connectors.

It is clear from these figures that the weld uniformity [7] is consistent which suggests good process repeatability throughout the perimeter under optimal conditions [8].

Figure-5: Visual & X Ray Micrographs

III.

PRE WELD INSPECTION

A. Visual inspection:

Pre weld joint interface dimensional requirements are as per Table-1. Proper part mating & positioning is critical for the success of the weld joint. The gap between AI6061 and connector outer shell interface is optimized at 50 to 100 µ ± 10 µ for the development of laser welding process. This is consistent with the pre-weld joint interface requirement reported in the standards [9, 10].

B. Electrical measurement:

Laser welded RF connector

Preliminary electrical checks have been carried out before laser welding of RF connectors. Resistance measurements between Connector centre pin and the outer shell of the body shows ‗open‘ connection.

Laser welded DC feedthrough

Preliminary electrical checks have been carried out before laser welding of DC feedthrough. The DC measurement indicates resistance value as ‗open‘ between the connector-centre pin and the outer shell of the body. No deviation has been found in the post weld measurements.

LASER Welding-DC feedthrough

X Ray Micrograph

LASER Welding-RF Connector

X Ray Micrograph

Leak test De-moisturisation

Laser weld Process parameters adjustment

Laser Hermetic Sealing process

No No

YES YES

YES Al 6061 package+ RF connector & DC

feedthrough

Visual inspection

Visual inspection of the Weld joint

No

YES Glove box parameters adjustment

YES

(4)

International Journal of Emerging Technology and Advanced Engineering

Further, the measured capacitance value using LCR meter bridge, was in the range of 9000-10000 pf .

IV.

POST WELD INSPECTION & SAMPLE

TESTING

Visual inspection:

Post weld joint inspection has been carried out at 40X to confirm the weld quality and absence of defects like pin hole, cracks, blow hole etc.

Electrical measurements:

Laser welded RF connector

Electrical checks have been carried out after laser welding of RF connectors. The resistance probe indicated ‗open‘ connection between the connector centre pin and the outer shell of the body.

Laser welded DC feedthrough

Electrical checks have been carried out after laser welding of DC Feedthrough. The DC measurement indicates resistance value as ‗open‘ between the connector-centre pin and the outer shell of the body & there is no change in the measured capacitance value as compared to pre weld capacitance value.

Mechanical load Tests :

The Mechanical load test on the weld joint strength showed no failure even up to 23.2 Kgf, which indicates the robustness of the joint.

Environmental tests:

Pre Environmental leak test

Sniff leak test has been carried out using Varian Leak detector of laser-welded samples as per the standard procedure. The measured pre-environmental leak rate was <2x10-8 Atm cc/sec He against the required specification of <1x10-7 Atm cc/sec He.

Post Environmental tests:

1-Thermal Cycling

Laser welded samples have been subjected to thermal cycling for 500 thermal cycles, as per the duration shown in Figure 6

Figure-6 Thermal Cycle

2-Vibration Test

Laser welded samples have been subjected to vibration levels. The dosage levels and sequence followed are given in Table -3.

TABLE-3VIBRATIONLEVELS

Sine Vibration

In plane 15g

Normal Plane 20g

Random Vibration

In plane 11.8grms

Normal Plane 17.5grms

Leak test :

Post environmental tests (thermal & vibration), Helium sniff leak rate test measurements were carried out and the results are tabulated in Table-4.

TABLE-4 LEAK RATE MEASUREMENTS

Test Event

Leak Rate

Atm cc/sec.He

RF Connector DC

Feedthrough

Pre Environmental

<2x10

-8

<2x10

-8

Post Thermal Cycling <2x10

-8

<2x10

-8

Post Vibration

<2x10

-8

<2x10

-8 Time

10 min

-700_C

1800C

(5)

International Journal of Emerging Technology and Advanced Engineering

X-ray analysis:

After the environmental tests, X ray analysis has been carried out to detect fine crack/discontinuity. The samples were found free from such defects. Fig 7 shows the x-ray image for DC Feedthrough.

Figure-7 Xray Micrograph- DC Feedthrough

Micro section Analysis:

After the qualification tests, micro sectioning of weld joint was carried out. Measured value of depth of penetration was reported to be in the range of 280-300 µ (D) and that of width (Heat Affected Zone) is approximately 817 µ (W) for the optimized laser energy levels for fiber core diameter of 600 µ.

Research survey indicates that depth of penetration is linked to the key hole weld process[11-13]. Further, the micro-section analysis has been carried out, which indicates crack free and uniform weld joint as shown in Figure 8

Figrure-8 Micro-section analysis

These have been successfully implemented in realization of GEOSAT flight packages for Ku-ALC Driver amplifiers.

V.

CONCLUSION

The major outcomes of these experiments are as follows

 Weld package-connector outer shell interface geometrical tolerance has been successfully optimized.

 Laser welding process for RF connector and DC feedthrough with A16061 package, has been successfully developed & optimized using Nd-YAG laser.

 Process repeatability has been demonstrated through qualification tests.

 Laser welded RF connector & DC feedthrough passed environmental leak test & results are meeting MIl-STD-883 standards specification < 1x10-7 Atm cc/secHe for hermeticity requirement for space applications for MMIC based RF subsystems.

Being a metallurgical fusion bonding, LASER weld technology promises potential applications [14] for the packaging & sealing of microelectronic space hardware for geostationary as well as interplanetary outer space missions, having extreme thermal excursion. The developed process had been a precursor for complex/precision part welding such as Multipin Micro D connectors on aluminum packages.

These have been successfully implemented in realization of GEOSAT flight packages for Ku-ALC Driver amplifiers.

REFERENCES

[1]V.Malin ― Study of metallurgical phenomena in the HAZ of 6061-T6 Aluminium welded joints‖ Welding Research supplement, Sept 1995 p 305-18.

[2]Siva Kumar G, Sarvanan S, Raghukandan K ―Investigation of microstructure and mechanical properties of Nd:YAG laser welded lean duplex stainless steel joints‖ Optik- International journal for Light and Electron Optics‖ Vol 131, Feb 2017, p1-10.

(6)

International Journal of Emerging Technology and Advanced Engineering

[4]A.EI-Batahgy and M.Kutsuna: ―Laser beam welding of AA5052, AA5083 and AA6061 AI Alloys‖ Advance in Materials Science and Engineering Article ID 9714182, 2009, P1-9

[5]P.S. Wei, K.C.Chuang, J.S.Ku and T.Debroy: ―Mechanisms of spiking ad humping in key hole welding‖ IEEE Transactions on Components, packaging & Manufacturing Technology, Vol 2,2012, P383-393.

[6]A.F.H. Kaplan and J.Powell: ―Spatter in laser welding‖ Journal of laser Applications, Vol23 No.3,2011, 32005-1 to 32005-7

[7]H.Zhao, D.R.White and T.DebRoy ― Current issues and problems in laser welding of automotive Aluminium

alloys‖ International Material Reviews, ASM

International, 1999.P 238-264,

[8]Wu N.Q. Cedrixxia, Ming Li, Perrusquia Scott N, Mao X, Interfacial structure and micro and Nano mechanical behaviour of Laser welded 6061 Aluminium Alloy blank. Journal of Engineering Material Technology 126,1, 2004, P 8-13.

[9] Em30, MSFC-SPEC-3679 Technical Standard ―Process Specification-Welding Aerospace Hardware‖ George C.Marshall Space Flight Centre, Alabama, NASA,Oct 11, 2012

.[10]General Motors Standard, GM4485M, 1992. ‗Weld specifications laser welds-butt joints‘

[11]Beersiek J., R. Poprawe, W. Schulz, H. Gu, R.E. Mueller and W.W Duley . ‗On-line monitoring of penetration depth in laser beam welding‘. Proceedings of ICALEO ‘97, p 30-39.

[12]Ducharme, R. K. Williams, P. Kapadia, J. Dowden, B. Steen and M. Glowacki . ‗The laser welding of thin metal sheets, an integrated keyhole and weld pool model with supporting experiments‘. Journal of Physics D: Applied Physics D: Applied Physics, Vol 27 1994, P1619-1627.

[13]Dumord, E. J.M. Jouvard and D. Grevey (1996). ‗Keyhole modeling during CW Nd:YAG laser welding‘. Proceedings of SPIE, 2789, 1996, P 213-220

[14] Yang Z.B., Tao W, Li L.Q, Chen Y.B. |Li F.Z,Zhang Y.L, Double sided laser beam welded T joints for Aluminium aircraft fuselage panels: process microsturcture and mechanical properties. Material Design, 33, 2012 P 652-658.

ACKNOWLEDGEMENTS

Authors would like to thank teams at MESA, SNPA EnTSG and SRG in Space Applications Centre, for their constant guidance and support at every stage of this activity. The activity would not have been possible without support from Microelectronics assembly division‘s support.

AUTHOR‘s BIOGRAPHY

Sakaram Srinivasulu did his M.Sc., M.Phil. , in Physics and worked on R&D projects in thin film devices at IISc., Bangalore prior to joining SAC/ISRO Ahmedabad in the year 1992. He has developed expertise

on LASER hermetic sealing

technology, which is qualified and implemented for delivery of FM hardware for all GEOSAT missions, Presently, he is working on the development of advanced Wafer level packaging technology and LTCC assembly & packaging. (

srinivasulu@sac.isro.gov.in)

Shivendra Tripathi did his B.Tech. in Electronics & communication from J.S.S. Academy, Noida. He is with Space Applications Centre (ISRO) since 2006. He is presently leading the team responsible for MIC/MMIC Assembly activities at MAPD/MEG for SAC. He has developed new fabrication processes for fine pitch, SMT & Area Array Device assembly processes which have been qualified and implemented for space-borne payloads. He has worked extensively for establishment of advance packaging & assembly facility. (shivendra@sac.isro.gov.in)

R. K. Hegde did his BE in

Electronics &

Communication Engineering from LDCE

Ahmedabad, and is

(7)

International Journal of Emerging Technology and Advanced Engineering

Dr J.K. Baria did his Ph.D. in Physics from

Sardar Patel

University, Vallabhvidyanagar. He is an Associate

Professor at