08/01/2009, Irvine, California

 

What’s Going on in There?

When it comes to die attach thermal performance, this is a question that has kept materials developers up at night. Historically, when evaluating the thermal performance of a die attach material, a well-known method called laser flash is what has commonly been employed. And, while this test technique reveals the bulk thermal conductivity for a given material, it lacks meaningful data regarding how the die attach system is really going to perform in package.

Understanding how the material will interface with the other parts of the electronic package from a heat flow perspective is critical to predicting device performance in the field and is data that, until now, hasn't been readily available from materials suppliers.

To be fair, I should point out that such precise thermal conductivity information probably wasn’t as necessary five years ago. But, with the proliferation of smaller, faster and, therefore, hotter devices, the ability to look inside the package to evaluate the different elements – and not just measure the thermal performance of the composite of all of the layers – will be essential to development of robust materials that offer optimized thermal performance as dictated by the specific characteristics of the package in which they will be used. This is precisely Henkel’s belief and it is why our company has pushed ahead with design of a method to evaluate the role of the interface in thermal performance.

After much analysis, we selected a test method and a test die. Because Henkel’s lab facilities are equipped with all the necessary systems required to build packages, we are manufacturing the test package – a 7 mm x 7 mm QFN -- and corresponding tooling in- house. For the initial evaluation, we analyzed several different metallizations on the leadframe as well as different die surface finishes including NiPdAu, Spot Ag and Double Ring Ag. Not surprisingly, results have proven our initial suspicion that the die attach material interface reacts differently from a thermal point of view depending on the other elements – lead finish, die surface finish, etc. – of the package. In fact, findings suggest that the interface plays a much larger role with thermal performance than originally anticipated; in some cases, we’ve seen close to 80% of the overall value coming from the interface. Of course, this is just for this one package and these values will vary with different package types, metallizations and finishes.

The advantage of this type of transient testing, as opposed to traditional steady state thermal analysis, is that we can get substantive information about what’s going on inside the package and utilize this data when designing materials from the ground up, thus ensuring that die attach materials deliver optimal thermal performance for specific applications. Instead of just adding a little more silver – which has been a common industry approach in the past for improving thermal performance – we can systematically design materials that are going to interface with specific metallizations or die finishes to yield the best thermal performance. Transient testing is, indeed, a more complex technical approach than steady state analysis, but it offers the ability to look deep inside the package to see what the Henkel material is contributing in the way of thermal performance. And, this kind of data is invaluable.

While Henkel’s use of this test methodology is relatively new, we have begun to incorporate it into the performance analysis of sintering materials as well as evaluation of the impact of silver morphology on the interface. Certainly, these initial efforts are only the beginning in what Henkel views as a huge step forward for materials development initiatives. It’s just further proof of our ongoing commitment to design next-generation materials that are proven in-package before customers even place an order.

So, while many will still have to wonder about “what’s going on in there” when it comes to true die attach thermal performance, Henkel will have the answer. For more information on this latest advance, contact the materials experts at Henkel via phone at 949-789-2500 or send an e-mail to: dan.maslyk@us.henkel.com

Acknowledgements: The author would like to thank Weikun Jimmy He of Mentor Graphics for his assistance with this work.