Product category: Materials and Processing
News Release from: GSI Group, Laser Division | Subject: Lasers provide new opportunities in micro joining
Edited by the Engineeringtalk Editorial Team on 12 December 2006
Lasers provide new
opportunities in micro joining
The constant demand for increased performance and functionality is driving a trend towards ever smaller assemblies and components
In the medical sector, for example, new treatments call for increasingly complex microsurgery tools and miniature implantable devices, while in the automotive, telecoms and electronics markets, the demand is to pack more power into a smaller and smaller footprint. This is putting increasing pressure on conventional manufacturing methods.
This article was originally published on Engineeringtalk on 12 December 2006 at 8.00am (UK)
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In the area of micro joining in particular there is a need for processes that can guarantee high quality consistent welds without damaging delicate components - often at extremely high production rates.
For many of these applications, the latest generation of pulsed Nd:YAG lasers provide the best, or indeed only, way to meet these needs.
'People are starting to become more aware of what lasers can do at the micro scale, and the process window is much bigger than it used to be,' says Mark Greenwood, Technical Director of GSI Group, Laser Division.
'Four or five years ago the smallest focus spot you could achieve was around 250 microns, and you just couldn't weld the extremely fine features that we are now talking about'.
'The new generation of laser sources, such as GSI's JK pulsed Nd:YAG lasers have high quality beams that allow us to deliver much higher powers through smaller diameter fibre optics'.
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'Combined with a precise control of the shape and power of each pulse this means we can weld minute components, exotic materials and even dissimilar metals'.
'With the JK125, for example, using a 150um fibre optic we can weld material down to 20um thick with a weld spot as small as 45um in diameter - and we can deliver average powers of 120W and peak powers of over 2kW'.
'This technology is twice as good as the previous generation enabling you to do things now that just weren't possible before'.
Laser welding is a non contact process, so, compared to resistance welding, it is quicker, more flexible, and there is no force exerted on the component.
It is less costly and time consuming than electron beam welding or diffusion bonding, and in contrast to brazing or soldering it can produce hermetically sealed components without heating the whole assembly.
The high strength of the laser welds also reduces the number of welds required on the components - reducing cycle times and further reducing the chance of any thermal damage.
And the smaller you get, the greater the advantages become.
To give an example from the medical sector, one of the most effective ways of treating certain cancers, such as cancer of the prostate, is to implant a capsule containing a radioactive source next to the tumour.
This technique, known as brachytherapy, allows controlled doses of radiation to be delivered to the cancer without damaging healthy tissue.
The capsules that hold the radioactive 'seed' are only a few millimetres long, less than a millimetre in diameter and have a wall thickness of less than 150um.
The welds that join the capsule together need to produce a hermetic seal, with a smooth weld bead.
The capsules have a high intrinsic value, and as disposing of radioactive scrap creates its own problems, there must be no rejects.
Welding with a GSI JK pulsed Nd:YAG, delivered by fibre optic, and with a focused spot size as small as 45um and pulse-to-pulse stability better than 1percent, meets all these requirements in an easily controlled, versatile process that can produce finely controlled weld beads as narrow as 50um.
It is not just that the welds themselves are smaller.
Many of the fine mechanical parts, such as disk drive components or aneurism clamps, are highly accurate components where there must be no burn through or distortion.
And on components such as implanted medical devices, microwave packages or electronic assemblies there could be heat sensitive components such as electrical circuits, sensors or polymer seals immediately adjacent to the weld.
'The JK's combination of high power, precise pulse control and small spot size means that the laser energy is delivered exactly where it is needed, with very low heat input to the surrounding area,' says Dr Greenwood.
Pulse-to-pulse stability is vital for consistent weld quality - the first pulse needs to be exactly as good as the 500th and if the peak energy is programmed to be 5kW, that must be exactly what you get.
On many of these microscopic components the margin for error is virtually nil and, on a medical device or a safety critical sensor package, failure could be deadly.
The other key aspect of pulse control is the ability to shape the laser pulse to produce the right energy profile for each weld.
The pulse length of the JK lasers, for example, can be adjusted in 0.1ms increments from 0.2ms up to 20ms, and the energy profile of each pulse can be programmed in up to 20 segments with each segment as short as 0.2ms.
This ability to precisely control the energy input into each weld is exploited in the manufacture of disk drive flexures.
These move the reading drive's head over the spinning disk of the hard drive and, even at extremely high positioning speeds, the head must be kept within a matter of a few microns of the surface of the disk.
This calls for a stiff, accurate component that is generally assembled from several stainless steel parts, ranging in thickness from 20 to more than 200um, that are spot welded together.
Each weld has to be tailored to the particular combination of thicknesses and the stresses it will experience and, because these parts are produced in millions, the process has to be fast, accurate, repeatable and completely automated.
Using a JK pulsed laser, the beam is scanned across the surface using a galvanometer mirror to the precise position of each weld, which is then created with a single pulse lasting a few microseconds.
The pulse energy for each weld can be programmed individually to ensure strong, clean and distortion free joints - some smaller than 100um in diameter - are produced at a rate of 150 welds per second.
For materials such as steel and titanium, the ideal pulse form is often a flat square wave, with no initial energy spike or ramp down - and if that is what is required the JK lasers can deliver it with a pulse-to-pulse variation of less than 1percent.
On some of the more difficult applications though, changing the shape of the pulse is the key to successful welds.
Alloys of copper, gold, silver, and platinum can be difficult to weld because of their reflectivity makes it difficult to couple the laser energy with the surface, and their thermal conductivity draws the heat away from the weld.
Shaping the pulse to give an initial high energy spike provides the concentrated energy input to overcome both these problems.
On delicate components, shaping the pulse to give a ramp up profile can prevent overheating of the part and, if the material is prone to cracking, ramping down the pulse energy can control the rate at which the joint cools.
Pulse shaping is also the key to welding dissimilar metals.
This is an new application area that opens up a wealth of possibilities - joining aluminium to copper in electronics devices, stainless steel to titanium and memory alloys in the medical sector and joining steel to low-friction bronze for micro-mechanical devices, such as watch movements.
Different combinations of metals can be joined using different physio-chemical mechanisms.
If the two materials can create a solid solution, then a combined weld pool can be formed.
Alternatively, only one of the metals may be fully fused to effectively braze itself to the other metal.
Precise shaping of the pulse can create the critical heating and cooling profile that can take into account factors such as different melting points, the degree of inter alloying required, the formation of brittle inter metallic compounds and the prevention of thermal cracking.
'We are now routinely making welds that would have been considered impossible just a couple of years ago'.
'This rate of development means that many design and production engineers just aren't aware of what can now be done - there must be thousands of new applications for laser micro welding just waiting to be found', says Mark Greenwood.
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