SMTA International Conference Proceedings


Author: Mumtaz Y. Bora
Company: Peregrine Semiconductor
Date Published: 10/24/2010   Conference: SMTA International

Abstract: The handheld wireless product market place demands products that are small, thin, low-cost and lightweight and improved user interfaces. In addition, the convergence of handheld wireless phones with palmtop computers and Internet appliances is accelerating the need for functional circuits designed with smallest, low-cost technology.

BTC (Bottom terminated components) is one class of packages that meets this need of light weight and low cost. These components commonly include, QFN (Quad Flat Pack-No lead), SON (small outline-No lead), LGA (Land Grid Array) and MLF (Micro lead frame) packages whose external connections consist of metalized terminations that form an integral part of the component.

Quad Flat No Leads (Fans) are thermally enhanced plastic packages that use conventional copper lead frames. These leadless components provide an advanced packaging solution that reduces board real estate, with improved electrical and thermal performance over traditional leaded packages.

The QFN devices are attractive due to their low cost per I/O, performance and low profile, they are also a challenge for assembly due to their low to zero standoff height .Successful assembly yields and solder joint reliability requires careful selection of substrate materials, component plating finishes, controlled reflow processes and flatness of package and PWB.

This challenge is enhanced with the transition to lead free reflow as the higher peak reflow temperatures results in more thermal and CTE mismatch between package and PWB. QFNs terminations are typically plated with 100% Matte tin for lead free reflow. The concern with tin plating is the issues with tin whisker formation, oxidation of tin in long term storage conditions and impact to solderability .Tin oxidation can also result in poor wetting during lead-free reflow.

An alternative lead free compatible plating used in industry for components is Nickel/Palladium /Gold.[1].The plating provides a solderable surface for both eutectic and lead-free solders, is less amenable to oxidation and provides long term storage and stable solderability over time.

The paper reviews the qualification efforts to convert leadframe plating from silver/copper to Ni/Pd/Au. evaluation The paper summarizes the qualification of NiPdGold finish component leads and solderability evaluations. The solderability results were also compared to conventional silver plated copper leadframe and two different PCB surface finishes. Visual inspection, X-sectional analysis, SEM/EDX, solderability tests, bond pull and reliability test results for thermal shock and temp /humidity tests.

Key words: QFN, NiPdGold Plating, Leadframe Qualification.

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