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High reliability electronics assembly and encapsulation for extreme service environments using micro friction stir welding

Final Report Summary - MICROSTIR (High reliability electronics assembly and encapsulation for extreme service environments using micro friction stir welding)

Executive Summary:
The integrity and reliability of electronic assemblies is critical to our everyday lives whether they are used in domestic, transport, communications, industrial or security applications. The European electronics assembly industry faces many issues with achieving high quality joints at the time of manufacture and maintaining these once the assemblies are in service. Even if a high quality joint is manufactured it may degrade through a number of mechanisms over time; these negative mechanisms are accelerated by extreme service environments such as elevated and sub-zero temperatures and vibration.

The MicroStir project has developed a small scale friction stir welding (FSW) machine for high quality joining and electronics encapsulation for harsh service environments. FSW has proven to be a high integrity joining technology at a macro scale and has now been scaled down for sub-millimetre joining, termed micro friction stir welding (µFSW). The MicroStir project has developed tooling, procedures and prototype production equipment in order that industry may take advantage of micro-FSW to produce high integrity, stable joints. The results of the project provide useful information required for further understanding and industrial take-up of the FSW process. During the course of the project the physical limits of miniaturisation have also been established. These results assist in the determination of suitable applications, welding parameters and set-ups for µFSW.

Two MicroStir prototype systems were developed and implemented in the project. The spot welding prototype was developed for joining of sheet materials in the thickness range 0.1 mm - 0.7 mm and the primary field of application was electronics assembly. The spot welding prototype utilized force/position controlled linear axis technology developed by VTT, the motion providing sub-system of GIN Kolibri CNC machine and the spot welding tools designed by TWI. The seam welding prototype was developed for joining materials of 0.5 mm nominal thickness in hermetic sealing applications. The seam welding prototype was built on GIN Kolibri CNC machine and utilized commercial high frequency spindle and miniaturized seam welding tools developed in the project.

An electronics assembly production run was simulated by joining strips of copper to the contact pads on FR4, LTCC and alumina printed circuit boards. The copper strips emulated leads of the assembled components and the size of the contact pads were adjusted to accommodate the necessary tool shoulder diameter. Spot welding trials on FR4 boards resulted in unacceptable charring of the substrate. Tools required cleaning after each LTCC weld. The alumina substrate is much weaker (brittle) compared to LTCC and although mechanical joints were achieved, board damage (cracking) was visible.

A hermetic sealing production run was simulated by seam welding of a 0.5 mm thick aluminium alloy lid to a wall of an aluminium box. In the demonstration tests welds were produced without visible flaws, although the surface quality of both the welded boxes was relatively rough. Surface quality may be improved by adjusting the tool tilt angle or changing the direction of traverse motion.

The MicroStir system has required the development of new joining technology and know-how. The key outputs of the project include the use of the µFSW process for joining similar and dissimilar material combinations together with a rigorous appraisal of process capability. Key elements of the MicroStir system have been specified, designed and assembled to give a prototype capable of producing quality welds in sheet materials and encapsulation assemblies at suitable production rates.
Project Context and Objectives:
The integrity and reliability of electronic assemblies is critical to our everyday lives whether they are used in domestic, transport, communications, industrial or security applications. The European electronics assembly industry faces many issues with achieving high quality joints at the time of manufacture and maintaining these once the assemblies are in service. Even if a high quality joint is manufactured it may degrade through a number of mechanisms over time; these negative mechanisms are accelerated by extreme service environments such as elevated and sub-zero temperatures and vibration.

The MicroStir project has developed small scale friction stir welding for use in joining and electronics encapsulations seeing harsh service environments. FSW has proven to be a high integrity joining technology at a macro scale, typically in the range 1.5--25 mm for aluminium alloy, and has now been scaled down for sub-millimetre joining, termed micro friction stir welding (µFSW). The MicroStir project has developed tooling, procedures and prototype production assembly equipment in order that industry may take advantage of µFSW to produce high integrity, stable joints.

Micro friction stir welding does not require filler material (solder or braze) or generate undesirable metallic spatter or sparks which could cause electronic shorts and reliability issues later in service. The µFSW process generally results in an ultra-fine grain microstructure with minimal formation of intermetallic compounds at the interface between the two materials. These features will give performance advantages for electrical joints. The prospect of using µFSW for hermetic sealing is also of interest as friction welding techniques operate successfully in gaseous atmospheres and in some cases under liquids. FSW has also been used to make welds through liquid polymer sealants in the aircraft industry.

No commercially available FSW systems are available for welding below 1mm in thickness. Existing FSW equipment is used for welding on a macro-scale and cannot be adequately controlled at the low levels of force required for the application of µFSW. The MicroStir equipment has the necessary capability to apply much higher pressures at the spindle nose than any existing micro-machining or electronics assembly system is currently capable of achieving. While applying high pressures, the MicroStir system also maintains the ability to place the rotation tool very accurately, again this is outside the capability of any existing micro-machining or electronics assembly equipment.

The MicroStir project has developed µFSW, a high integrity micro-welding technique, for use in electronics assembly where products will be exposed to extreme or harsh service environments. In the project the µFSW technique and essential tooling has been developed from their current state (laboratory demonstration). A production viable prototype welding head and tooling has been manufactured and demonstrated in a production environment. The MicroStir process technology and equipment developed by this project will enable European industry to fully understand the benefits of µFSW and to evaluate potential applications.

The MicroStir system has required the development of new joining technology and know-how. The key outputs of the project include the use of the µFSW process for joining similar and dissimilar material combinations together with a rigorous appraisal of process capability. Key elements of the MicroStir system have been specified, designed and assembled to give a prototype capable of producing quality welds in sheet materials and encapsulation assemblies at suitable production rates.

Project Results:
Process development and assessment
* The MicroStir project seeks to develop friction stir welding of thin materials for high reliability electronics assembly and electronics encapsulation for extreme service environments. The European electronics industry has a niche focus, primarily within the aerospace, defence and Power electronics (batteries / PV cells) sectors. Activities range between printed circuit board fabrication and assembly packaging, volume batch production is typical. An initial assessment of industrial requirements for the successful application of micro FSW technology in the electronics industry has been made through the identification of prospective applications. Key technical information is available to guide the scientific development of the micro-FSW process and establish the capabilities of the welded connections under representative service environments.

* A process development study has been undertaken to develop the µFSW process and achieve good quality linear and spot welds in foil and thin sheet materials. A fundamental aspect of FSW is the selection of an appropriate tool design and suitable welding parameters (primarily rotation speed and feed rates). Tool designs have been developed which permit the welding of foils 0.1mm thickness and above. Welding trials have been undertaken on four different machines; a commercial FSW machine, a commercial CNC mill and two pieces of small scale joining/machining equipment. The outcome of these trials has been used to guide the subsequent specification and design of a prototype micro friction stir welding system.

* Potential applications for micro friction stir welding include power electronics and also encapsulation for sub-sea and other applications. Based on these applications a test programme has been proposed. On ordinary PCBs the layer thickness is often 17 or 35μm. Increasing the metal layer thickness to, for instance, 100-200μm, is an advantage in power circuits, as the conductivity of the leads is increased. Increased board thickness is advantageous for the MicroStir technology. An initial test board, compatible with the demonstrated FSW capability, has been produced and successfully welded.

Development of MicroStir prototype system elements:
* Two MicroStir prototype systems were developed and implemented in the project. The spot welding prototype was developed for joining of sheet materials in the thickness range 0.1 mm - 0.7 mm and the primary field of application was electronics assembly. The spot welding prototype utilized force/position controlled linear axis technology developed by VTT, the motion providing sub-system of GIN Kolibri CNC machine and the spot welding tools designed by TWI. The seam welding prototype was developed for joining materials of 0.5 mm nominal thickness in hermetic sealing applications. The seam welding prototype was built on GIN Kolibri CNC machine and utilized commercial high frequency spindle and miniaturized seam welding tools developed in the project.

* Technical specifications for electronics assembly equipment and FSW equipment were compared to identify the key improvements required to enable the introduction of FSW equipment. A market review of machine elements, which may be easier to integrate or more suitable for the FSW application, has been conducted. A comprehensive review and testing programme has been undertaken to determine the suitability of the identified machine elements for use in the MicroStir prototype. This has included the evaluation of spindle motors, actuators, position and force sensors for use in the welding head. A parallel activity has been undertaken to assess machine elements for the motion and control sub-systems. The process requirements for seam/encapsulation welds are different to those for spot welding and two welding head configurations have been developed and manufactured.

Prototype integration & validation:
* All developed and manufactured hardware parts and software modules off the MicroStir system have been integrated with compatible CNC equipment. An integration strategy was developed to mount the MicroStir welding head, monitoring and control systems on a commercial CNC host machine. Prototype integration activities included: a) Mechanical interface of MicroStir welding heads (spot and seam welding) with the CNC machine; b) Mechanical interface of weld monitoring and control hardware; c) Integration of weld monitoring and control software; d) Integration of MicroStir and Kolibri control systems. Validation of prototype performance was performed in the research laboratory and included spot welding, encapsulation and preliminary cable joining trials.

Prototype demonstration:
* The equipment developed during the project was demonstrated in an industrial setting at GIN’s premises in Oulu, Finland. Two µFSW prototype systems were demonstrated. A spot welding prototype was developed for joining thin sheet materials in electronics assembly applications. A separate seam welding prototype was developed for hermetic sealing applications.

* An electronics assembly production run was simulated by joining strips of copper to the contact pads on FR4, LTCC and alumina printed circuit boards. The copper strips emulated leads of the assembled components and the size of the contact pads were adjusted to accommodate the necessary tool shoulder diameter. Spot welding trials on FR4 boards resulted in unacceptable charring of the substrate. Tools required cleaning after each LTCC weld. The alumina substrate is much weaker (brittle) compared to LTCC and although mechanical joints were achieved, board damage (cracking) was visible.

* A hermetic sealing production run was simulated by seam welding of a 0.5 mm thick aluminium alloy lid to a wall of an aluminium box. In the demonstration tests welds were produced without visible flaws, although the surface quality of both the welded boxes was relatively rough. Surface quality may be improved by adjusting the tool tilt angle or changing the direction of traverse motion.

Potential Impact:
The MicroStir project has developed a small scale friction stir welding (FSW) machine for high quality joining and electronics encapsulation for harsh service environments. The MicroStir project has developed tooling, procedures and prototype production equipment in order that industry may take advantage of micro-FSW to produce high integrity, stable joints. The results of the project provide useful information required for further understanding and industrial take-up of the FSW process. During the course of the project the physical limits of miniaturisation have also been established. These results assist in the determination of suitable applications, welding parameters and set-ups for µFSW.

The MicroStir developments fill a gap in the market which has arisen due to performance limitations associated with other micro joining processes. Micro soldering, brazing and welding technologies all have failings (e.g. tin whiskers, tin pest, intermetallic formation & microstructural coarsening) when their products are placed in extreme or variable service environments. The µFSW process will enable high reliability encapsulation and joining and will give increased confidence in long term performance. Almost all sectors will benefit from the MicroStir project outputs as most machines and devices contain electronic assemblies which are integral to their correct operation. The key benefits will be seen in areas where electronics are exposed to extreme service environments, examples include: aircraft engine control electronics; automotive sensors and electronics related to power steering control and engine management systems; Fuel cell and high current density battery internal connections. A major failure in a single unit used in any one of these industries can be catastrophic.

µFSW is yet to be exploited by European industry. Although larger scale FSW was invented in Europe, the industrial development of the technique and commercial exploitation have been more significant in Asia and North America. This leaves SMEs and OEMs in Europe struggling to offer competitive manufacturing strategies and solutions for fabrication of high quality aluminium assemblies. For example, while initial FSP process development was undertaken in Europe production welding of Apple’s Book casing is undertaken in SE Asia. SMEs in Europe have been unable to invest the necessary amounts required for µFSW development and commercialisation to see rewards. The deliverables of the MicroStir project increase process and equipment knowledge enabling European SMEs to identify applications which will benefit from FSW and provide sound equipment information.

Increased use of µFSW will bring about significant economic advantages for users but also major environmental and safety benefits. Use of the MicroStir system to facilitate µFSW in place of conventional small-scale joining techniques will reduce power consumption, pollution (in terms of particulate fume and gases) and improved safety for assembly line operators and supervisors who will no longer be exposed to dangerous UV radiation and extreme high temperatures (including molten metals).

Although there are many standards in existence that control aspects of welding, these are predominantly associated with, and appropriate to, the more traditional welding techniques, in particular fusion welding. FSW and µFSW are relatively new processes, an ISO standard (ISO 25239) was published in 2011, which deals with all aspects of the FSW process for the welding of aluminium alloys. The MicroStir welding equipment has the necessary process monitoring capability to ensure compliance with ISO 25239 (FSW) and also the ISO FSSW standard under preparation. Specific testing will be required to ensure compliance with electronic product and interconnection standard before commercial use.

Exploitation and Dissemination:
* Promotion of the project and dissemination of project results began during the project and will continue as part of business development activities. A project brochure has been professionally produced and all partners have been provided with copies to distribute to interested parties. SME partner GIN attended the EMO exhibition (September 2013 Hannover, Germany) to promote their technical capability. Additionally all partners have held informal discussions with business associates. All IP has been identified and appropriate protection measures have been implemented. Consideration has also been given to the codes of practice applicable to commercial exploitation. A final plan for use and dissemination has been developed giving a clear indication of past and future dissemination and exploitation activities.

The prototype system was demonstrated at SME partner GIN at the end of the project. Mounting and testing of the two welding heads developed in the project was successfully undertaken. Although the results of the seam welding / encapsulation tests are very promising, deficiencies remain which restrict the application of project results for spot welding of electronic elements. The system is now in a status of partial proof of concept. Further academic research is required to improve technology capability prior to development of an improved/commercial system. The SME partners have identified the topics to be addressed. Additionally the market situation has to be monitored too. The key challenge is to identify the customers and technical/product applications given the demonstrated technology capability. An enhanced system concept considering the lessons learned and the feedback from interested parties will be worked out by the SMEs after the project. Opportunities for further funded research will be actively sought.

List of Websites:
A project website was set up to act as a communication port between the partners and disseminate the project http://www.microstir.eu.

Project
For general enquiries regarding the SignaStir project please contact Dr Kathryn Beamish, kathryn.beamish@twi.co.uk at TWI Ltd, Granta Park, Great Abington, Cambridge, CB21 6AL, UK. Tel: +44 (0)1223 899000, Website: www.twi.co.uk

System
All enquiries regarding the SignaStir inspection system should be addressed to Prisma Electronics SA, Dimokratis Avenue 87, 68100, Greece.