Impact of New Reworkable Underfills on Yield, Reliability and Cost Print E-mail
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Written by Jie Bai   
Tuesday, 05 June 2012 00:01



In the world of high-volume PCB production for modern handheld and tablet devices, high yields are king. Every defect is dollars wasted, so production techniques have to approach perfection. But the requirements for handhelds and tablets are also quite demanding, which makes achieving high yield when incorporating miniaturized components such as wafer-level CSPs (WLCSPs) very challenging.

One strategy for yield maximization is the use of reworkable underfills, which allows assembly specialists to rework the faulty component (or components) on the board and avoid scrapping an entire PCBs that may only have one or two defective parts. Considering that the total hardware cost for smart phones, tablets and laptops can range from $200 to $500 and the components in newer generation phones cost anywhere from $1.50 to $48 each, reworking one or two components is a very appealing and money-saving option. 1) Historically, however, reworkable underfill materials have generally had very low glass transition (Tg) temperatures, making them incapable of delivering the high thermal cycling reliability necessary for the WLCSPs used in modern devices. On the other hand, higher Tg underfill materials didn’t have the reworkability characteristic, putting manufacturers between the proverbial rock and a hard place. New formulation technology, however, has resulted in reworkable underfill materials that resolve these issues and now enable handheld and tablet makers to have the best of both worlds: reworkability for maximum yield and high reliability.

These new materials have been designed to cope with the complex requirements of today’s high-value products and offer excellent processability. Most newer-generation reworkable underfills are also halogen-free and comply with environmental obligations while simultaneously delivering excellent reliability as compared to older-generation reworkable underfills. Some of these novel materials boast a high Tg of 100°C, delivering thermal cycling reliability for miniaturized, complex devices such as WLCSPs and PoPs. Given this latitude, manufacturers can incorporate the new materials into high-volume operations with the confidence that reliability and reworkability will deliver the yields necessary to meet ever-challenging cost and profitability metrics.

Along with the improved thermal cycling reliability for fine-pitch (0.5mm pitch and below) area array devices, modern reowrkable underfills also provide ease-of-use. Some of these materials flow fast at room temperature as compared to other high Tg underfills that require pre-heating from 80° to 100°C to enable flow. Newer formulations also cure more quickly at a lower temperature (130°C) which is below many underfills with high Tg that require temperatures in the range of 130° to 165°C for complete cure. Lower temperature curing and the elimination of any preheat requirements also lend to environmental responsibility, enabling manufacturers to reduce energy consumption rates.

But it’s not just today’s communications products that can realize cost advantages from incorporating a high-reliability reworkable underfill. Even applications that aren’t high volume, but exceptionally high value –such as those for the aerospace and server markets – now have the thermal cycling reliability needed but in a reworkable formula. When you consider that many aerospace boards can cost $5,000 and higher and some server boards can soar to as much as $10,000, the ability to rework a single faulty component does a lot for the bottom line. Traditionally, because of the reliability requirements for these products, older-generation reworkable underfills couldn’t be used and one defective component would have meant scrapping the whole board. That’s now changed with the advanced formulation technology incorporated into these high Tg materials.

Underfills with high reliability, reworkability and easy-of-use are manufacturing advantages on many fronts: they enable improved yield, environmental responsibility, resolve the thermal cycling challenges faced by older-generation reworkable underfills and save costs. These novel materials are a welcome addition for modern assembly operations.

Acknowledgments

The author would like to thank PD Manager Stanley Hu of Henkel for his valuable input.

References

1. Perenson, M.J., PC World, “In Pictures: Inside the Apple iPhone, PC World breaks apart Apple's iPhone to analyze the device's internal components”, July 5, 2007 (http://www.pcworld.com/article/133912/in_pictures_inside_the_apple_iphone.html)

Jie Bai is a chemist at Henkel Electronics Group (henkel.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Wednesday, 06 June 2012 11:45
 

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