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Improved innerlayer bonding for sequential lamination: multiple lamination cycles add cumulative thermal stress to the innerlayer bonds.

Article Excerpt
Industry requirements to meet the demands of lead-free applications have challenged many areas of PCB fabrication and design. These demands have initiated a rapid influx of new dielectric materials, which, despite increased thermal stress, must remain effectively bonded to copper innerlayers. Delamination failures in advanced multilayer and sequential-build products, caused by the conversion to lead-free soldering, have also driven the need for improved oxide replacement technology.

Such products must resist higher peak temperatures, some 30[degrees]C greater than eutectic applications. Based on statistically designed experiments, a new process has been developed that delivers a more resilient copper conversion coating. The process has shown improved bonding performance and greater stability at higher temperatures, thus improving its capability for more thermal excursions (assembly-reflow cycles at >260[degrees]C) without failure. This article focuses on the product development criteria and performance evaluation required to develop a process that is suitable for this challenging environment. The testing and qualification will include data on peel strength, solder float and IR-reflow testing.

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Over the past decade, traditional alkaline black oxide (or brown oxide) bonding processes needed replacement due to a range of technical and environmental issues as well as heightened cost pressures. The acidic peroxide-sulfuric (oxide-replacement) processes quickly found favor due to lower cost and more environmentally friendly chemistry. These peroxide-sulfuric technologies, which use a primary organic additive to enhance the surface texturing, were shown to deliver excellent bond integrity in the pre-RoHS assembly setting.

However, it comes as no surprise that continuing advances in PCB fabrication are now pushing the performance requirements of these processes as well. These include:

* Advanced board constructions / sequential lamination / microvia capable build-up processes

* New halogen free, high Tg and filled phenolic cured dielectric materials

* Demanding, high temperature lead-free assembly conditions

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The Challenges

The current peroxide-sulfuric oxide replacement process work based on a "controlled etch" mechanism that typically removes 45 to 75 microinches (1.1 to 1.9 [micro]m) of copper. Innerlayers are first cleaned and then conditioned in an alkaline medium, rinsed, and then are processed in the peroxidesulfuric micro-etching bath. The copper saturation capacity of this bath is limited and has historically been on the order of 18 to 22 g/L. Over the past three years, this technology has improved, allowing a higher solution capacity of 50 g/L in high-volume production environments. This capacity increase has been largely the result of improved copper solubility coupled with greater process stability.

The requirements for the high-yield production of controlled impedance and fine line innerlayers also call for...