07/06/2009, Irvine, California

 

Innovative Formulation Provides High Reliability for High-Temp Applications

 

New Lead-Free Alloy that Takes Under-the-Hood Heat in Stride

While lead-free processes now seem like yesterday’s news and the electronics industry at large has generally accepted and commonly uses the SnAgCu (SAC) alloy, there are still market sectors that have struggled with prevailing SAC formulations. While the SAC alloy has proven to be a worthy lead-free material for the production of numerous devices, those applications that require high operating temperature performance and extremely high reliability have not had tremendous success with traditional SAC materials.

In particular, under-the-hood automotive applications demand thermal cycling reliability levels (>125°C) beyond those available with current SAC materials, and have struggled to marry their requirements with the performance of commercialized lead-free solders. Traditional tin-lead solders are subject to melting points that are too low for these applications and, of course, they face increasing use restrictions. Though SAC alloys do have higher melting points, their reliability in high operating temperature environments is inferior to that of tin-lead.

Given these challenges, a group of electronics industry specialists from the user, academic and materials supplier communities set out to develop an alloy that could meet or exceed the high-temperature, high-reliability requirements necessary for automotive applications, yet still be solderable at a reasonable temperature.    The goals for the alloy were as follows:

    •    Material must be lead-free
    •    Work in an operating temperature of up to 150°C
    •    Solder joints should survive 1,000 cycles at -55°C to 150°C
    •    Reflow at 230°C or below
    •    Meet RoHS standards and be cost-competitive

The development team agreed that the desired properties could not be achieved with a standard three-component alloy and that a multi-element approach was warranted. After extensive analysis, it was decided that the base alloy would be SnAg3.8Cu0.7 (SAC387) and its properties modified by adding additional elements to the mix. A variety of alloying elements were analyzed and the three that were selected for their properties, ability to raise creep resistance and maintain an acceptable melting temperature were bismuth (Bi), antimony (Sb) and nickel (Ni). Combining all the elements in proper balance was the challenge, but was ultimately met and the alloy – called Innolot – development was complete. Following its successful formulation, numerous standard industry tests were carried out on Innolot alloy and solder paste, and included spread, solder balling, thermal cycling reliability, shear strength, vibration and voiding analyses. Results from all of these tests confirmed Innolot’s equal or superior performance to that of SAC 387. Of course, thermal cycling performance of Innolot was arguably one of themost critical capabilities and the alloy did not disappoint: it proved superior thermal cycling results as compared to SAC387 and even to traditional SnPb37.

To further validate Innolot’s capabilities, more extensive evaluation was conducted that extended the parameters of many of the original tests in addition to adding more reliability and performance analyses. The subsequent evaluations, carried out during a German cooperative project called ‘LIVE’, included thermal shock, vibration testing, drop testing, aging impact and ductility and have given a more broad understanding of Innolot’s overall reliability performance.

Thermal shock testing results suggest that Innolot significantly outperforms both traditional SAC and SnPb alloys under harsh conditions (four different temperature ranges from 80°C to 190°C with dwell times of 30 minutes). Likewise, additional vibration testing performed as part of an in-depth thesis study at an automotive manufacturer, clearly indicated that Innolot has vibration reliability comparable with that of traditional SnPb solder and better than other lead-free solders. (2)

Though the Innolot alloy has been designed specifically for applications in which mechanical shock is not necessarily a large factor, it is less ductile than SnPb and SAC alloys. With the increasing number of leadless and area array packages being utilized today, drop test performance is critical and, therefore, drop analysis was performed. Again, Innolot confirmed its comparability – and in some cases superiority – to SAC.

These are promising developments, indeed, as automotive and other high reliability manufacturers can now confidently integrate a robust, lead-free alloy into production ofhigh-reliability, high operating temperature applications. However, it is important to note that while the alloy is a very important step forward for the industry, it is only part of the equation. Ensuring that the Innolot-based solder paste materials deliver the application- specific performance required will take flux chemistry expertise and materials formulation know-how. As one of the development partners and patent holders, Henkel has the foundation and outstanding technical skills needed to deliver the Innolot-based solder materials and superb technical support today’s sophisticated electronics firms demand. Currently available in the well-known Multicore formulations of Multicore LF318 and Multicore LF620, Henkel’s Innolot alloy solder materials provide the excellent performance and ease of use customers have come to expect from Henkel.

For more information on Henkel’s new Innolot alloy solder paste products or any of our advanced electronics materials, log onto www.henkel.com/electronics or call the company headquarters at 949-789-2500.