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New developments in high performance solder products for power die assemblies

Indium Corporation : 18 March, 2011  (Technical Article)
The use of special purpose solder pastes in power die attach is well-established offering low voiding and reliable bonding in volume manufacturing. However these materials are designed around high lead alloys and applied by dispensing. IGBT circuits are made by printing high tin alloys to multiple die sites, placing die and reflowing in a process more similar to conventional PCB or hybrid thick film assembly.
This paper describes how the opportunity was taken to make use of the latest developments in Pb-free SMT flux technology and re-optimize them to the different requirements of IGBT die attach.

IGBT (Insulated gate bipolar transistor) modules are fast becoming one of the most important electronic power switching tools in a variety of industries from trains to military vehicles to trucks. They are being adopted because they allow control of large currents from small actuator currents, and also exhibit increased reliability over analogue switching units such as relays and knife-switches.

In the ideal IGBT die, the high conductivity at high current density seen with bipolar gate junction transistors (BJT’s) is combined with the high speed switching capability of metal oxide semiconductor FET’s (MOSFETs).

Die attach requirements

Significant amounts of I2R heat are generated during operation of the module. Any materials used in the assembly of the module must therefore have high bulk (material) thermal conductivity and high electrical conductivity. Processes used for assembly must also ensure that the measured thermal and electrical conductivities are maintained as close as possible to the bulk properties. This means that voiding (bubbles in the finished solder joint, usually found using x-ray inspection) must be reduced to as low a level as possible. Standard solder-based power die-attach processes may generate up to 5% or even 10% total voiding, however IGBT die have a much higher current density (2 to 2.5 times that of standard power MOSFETs), so voiding reduction is crucial. Typically, less than 1.0% or even 0.5% voiding may be desired.

To ensure low voiding using a solder paste die attach process it is necessary to optimise the paste chemistry, metal loading; reflow profile and preferably vacuum processing.

Bond line thickness has to be controlled to between typically 250 – 375 μm (0.010-0.015in). The need for a higher thermal conductance using a thinner solder layer is balanced by the requirement for a sufficiently high stand-off to reduce strain on the solder joints induced by the CTE mismatch between the substrate ceramic (typically alumina Al2O3, or, more commonly, aluminium nitride, AlN) and its copper pad.

Wire bonding

Wire bond strength is also central to reliable performance. Cracks in the bond, bond lift-offs, wire corrosion and coarsening of the aluminium microstructure have been identified as major causes of wire bond failure.

Aluminium bonding wire is used instead of the higher-conductivity gold, to keep costs to a minimum. Typically there are three or four bonding wires diameter 500 μm (0.020in) or ribbon of equivalent area, for each die. As with all components and interfaces in the entire module, the resistivity must be kept to a minimum, so in some assemblies each wire or ribbon is stitch bonded (multiple bond sights) to the die surface

It is well known that contaminants such as organic materials or ionic compounds on the surface of the die can cause a decrease in the pull-strength of the final wire bond. A bond that may pass the pull-strength test, yet entrap a small amount of oxide or similar material that acts as a dielectric, creating a small capacitance that, together with the known inductance problems of the wire bonding wire itself, may increase the duration of a transient (LC) voltage spike.

In many assembly processes using fluxes and solder paste, the trend for the last 15 years has been towards the use of no-clean materials. However, with the notable recent exception of clip-bonding assembly, most Power Semiconductor assembly processes, particularly those using wire bonding, still require cleaning. It is crucial that all contaminants are removed not just to ensure good wire bonding but also to ensure proper adhesion and curing of encapsulants.

The flux cleaning process is a complex function of a variety of factors to ensure safe removal of all residue components without damage to materials of construction. It is usually assessed and controlled by monitoring ionic residues, as they are the principal cause of flux related in-field failures. They are also the easiest to detect and monitor in (near) real time on a production line using ROSE (Resistance of Solvent Extract) testing. In power die assembly this is insufficient as absence of ionics is not necessarily absence of all residues. Non-ionic residues may not be harmful in the service life of the product (the usual concern), but could interfere with subsequent process such as wire bonding, or damage integrity of encapsulants or their adhesion. More comprehensive evaluation is required.

Solder Paste Selection and optimisation

A survey of users throughout Europe showed that most users screen print solder paste, a smaller number stencil print. The most widely used solder alloy is 96.5Sn/3.5Ag and post solder cleaning is virtually entirely semi-aqueous. Thus our objectives were to develop a paste capable of being screen or stencil printed through 200μm emulsion 80 mesh, and then batched for vacuum reflow and cleaning.

Project Criteria
• Product in combination with process will produce less than 1% voiding
• A minimum 8 hours print/open life
• Good response to pause (RtP)
• Post solder cleanliness levels able to routinely meet or exceed established industry criteria for wire bonding
• Capable of being used in existing standard production equipment.
The project emphasis was on reflow and cleaning as these areas have the greatest impact on yield and in-service performance.

Candidate formulations

The recent imposition of Pb-free soldering in electronics assemblies has not been without its issues and these have been thoroughly aired at virtually every industry conference for at least thelast 5 years. On a positive note this lead to a very large amount of research into flux and paste technology using high tin content solders in SMT. The increased knowledge from this work illuminated the development of the differently specialised LDA (Large Die Attach) pastes used in IGBT soldering.

After review two material types were identified and went forward for development

Paste A, New technology Rosin/resin. Paste B Print variant of high Pb die attach type
Paste A is formulated to be minimum process change material. As an established die attach material, Paste B acted as a control and as a low residue material also offered possibilities of simplifying the cleaning process.

Printing trials

Modifications made to the pastes were not foreseen to have impact on print characteristics, nevertheless, candidate types were tested at Dek’s UK facility using the range of metal percents to be investigated in the reflow, voiding and cleaning trials. These initial print trials were done to establish that materials would print and that response to pause was unaffected. All performed with no measurable variation from their existing properties.
The print pattern used was designed specially for this project. It is a generic pincushion representative of that used by many companies.

The screen material was SD 245/65 mesh at 22 deg, plus 200 um emulsion. Print method was off contact with 2mm gap /snap off.

Reflow and voiding Study

Reflow and voiding studies were carried out in cooperation with Pink, using a Pink VADU100 oven. Materials used were plain, un-patterned copper DBC tiles. Die were either Infineon 12 x 12 mm or ABB 14 x 14 mm.

Paste was printed using the screen and print parameters previously described. Candidate materials were reflowed in air, nitrogen or vacuum with metal loadings ranging from 84.5 to 90 % and evaluated visually and by X-ray.

It was found that different atmospheres did not significantly affect flux spatter. There was some correlation of spatter with metal loading (higher loading marginally lower spatter) on flux types A, but this was not strong enough to be able to draw any conclusions or utilise. Flux B showed no spatter variation with metal loading but gave unacceptable discolouration in surrounding area. This is attributed to the lower process temperatures of Sn/Ag soldering compared to high Pb alloys for which the material was originally formulated.

Cleaning Study

Post solder flux removal or “cleaning” is a complex function of a number of inter-related factors. As discussed in the Introduction post cleaning of die circuitry has to be carried out to a higher standard than is normal in electronics assembly and different criteria are needed to monitor its success. Most cleaning control procedures are centred on measuring residual ionics, or monitoring conductivity of final rinse water. Non ionics are of equal concern in die attach assemblies; non ionic residues can impact on subsequent processes such as wire bonding or encapsulation. Pastes were printed using the projects screen described above using further die and substrates as used in the voiding study. Test assemblies were then reflowed under nitrogen with ~500ppm oxygen. Profile was a straight ramp to a peak temperature of 255C, time above solidus 70 seconds. Nitrogen reflow is “similar to worse” in terms of its effects or compared to vacuum, Two cleaning processes were used for the trials to represent standard industrial processes in common use. Cleaning cycles 1 and 2 were used with pastes A and B.

Paste type A meets the requirements of large power die attachment (LDA) in IGBT module manufacturing processes. The paste has excellent print and handling characteristics and returns less than 0.5% voiding under large die over a wide range of vacuum reflow conditions.

The authors gratefully acknowledge the provision of facilities, assistance and advice by Dage, Dek, Pink and Zestron
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