Reliability Performance of Very Thin Boards

Thomas Krivec
Gerhard Schmid, Martin Fischeneder, Gerhard Stoiber
AT&S Austria Technologie und Systemtechnik Aktiengesellschaft
Leoben, Austria

The next generation of smart phones will demand very thin multi-layer boards to reduce the product thickness again. This paper shows three different manufacturing approaches, which can be used for very thin any-layer build-ups. The technological approaches are compared on reliability level - the any-layer copper filled micro-via technology which is to be considered as state of the art technology for high end phones and the ALIVH-C/G technology that is well established in Japan.

A test vehicle design featuring test coupons for comprehensive reliability test series has been defined as target application for investigation. The applied test vehicle build-ups comprise an 8 layers build-up with total board thickness below 500 m. The first test vehicle is based on an any-layer HDI build-up including copper filled stacked micro via structures, the second test vehicle features an 1+6+1 ALIVH-C build-up comprising an outer HDI prepreg layer while the third test vehicle is built in ALIVH-G technology featuring a full ALIVH build-up.

The influence of the applied manufacturing technology on the reliability performance of thin PCBs is evaluated based on these three test vehicle build-ups. To cover the behavior during SMD component assembly the produced samples are subjected to reflow sensitivity testing applying a lead free reflow profile with a peak temperature of +260°C. Failure occurrence and the observed failure modes are evaluated and compared.

In parallel a temperature cycling test is conducted on the test vehicles in a temperature range between -40°C and +125°C in order to evaluate the thermo mechanical reliability of the test vehicles with regard to the manufacturing technology. In order to characterize the reliability aspects influenced by electrochemical migration phenomenon the different samples are subjected to a HAST test at +130°C with 85% humidity level.

The results obtained from reliability testing are summarized and compared within this paper. The identified relations between manufacturing technology and the reliability performance of the test vehicles are shown; strengths as well as weaknesses of the applied any-layer technologies are identified and summarized.

The smart phone market drives the producers of PCB for mobile systems more and more to reduce the board thickness. For an 8 layer board for a modern mobile phone an any-layer design is considered as almost mandatory, while the thickness target is currently at 600 m for this kind of stack up and will go down to 400 um for a rigid 8 layer board in 2013. The current work showed that already with the currently materials and the available means of manufacturing it is possible to build rigid anlylayer PCBs featuring a build-up of 8 layers with a maximum thickness of less than 500 m.

To implement the boards different manufacturing approaches were successfully applied. On the one hand a high end HDI process combined with via filling step on the other hand the ALIVH technology. Additionally it was shown that also the combination of pure ALIVH with outer layer HDI, the so called ALIVH-C process could be successfully applied. Board thicknesses between 443 and 512 um were manufactured. While AT&S has available experiences from almost 20 years of HDI manufacturing in ALIVH has been licensed and introduced in 2011. Thus especially the ALIVH technology promises still additional potential for improvements in manufacturing of thin boards.

The reliability behavior of the samples built during this study is overall considered as acceptable. Currently the material situation for the HDI process can be considered as advantageous compared to the materials situation for the ALIVH process. While this is specifically true for the reflow sensitivity behavior the observed difference in performance of the three produced test vehicles was lower for the electrochemical migration in the HAST test. Finally the temperature cycling of the parts did not cause any failure during this study.

It can be concluded that all observed technologies are generally feasible for building reliable thin boards. Further work will have to focus on potential to meet functional requirements, such as the electrical performance and the economic questions that are corresponding to the manufacturing of very thin boards.