CircuiTree - February 2009 - (Page 27)

the reason is ﬁne pitch devices must be placed extremely precisely because the area of a pad is extremely small. If those markings aren’t properly deﬁned, component placement is offset, requiring solder paste to be reapplied and placement to be performed again. Therefore, it is crucial that the silkscreen and ﬁducial marks precisely comply with pad stack requirements. Plus, maintaining sufﬁcient distance from pad-to-pad must be assured because if a pad size is too large, it increases the possibility of shorting other SMD pads of the device, especially if the board is under-etched. Tolerances must also be tightly controlled when using a pad wizard or any other kind of software used to create the pad stacks within the CAD tools. Tolerances should be closely and frequently inspected when using a pad wizard so that everything is precisely deﬁned. By doing so, pads are not only properly created but routing can also be correctly performed. When it comes to ﬁne pitch micro-BGAs, it becomes even more crucial to properly deﬁne those pads. The reason is to have a proper fan-out for a dog bone structure or any one of a myriad of BGA fan-out strategies. This and other considerations targeted at the ultimate in precision are based on experienced design layout. That precision, or lack of it, ultimately has a direct bearing on ﬁne pitch device performance at fabrication and assembly. Otherwise, defects such as BGA voids are created (Figure 2). Consequently, proper placement of a ﬁne pitch SMD is not possible because the mass of the pad is out of proportion with the component lead size. This creates a mismatch between the pad on the PCB’s surface with the leads of an SMD component. A weak solder joint is thus created, and, over time, that solder joint can be damaged and cause a failure, especially if the application under- change the size of the SM pad, especially if a ﬁne pitch device is involved. So, whenever a conversion is performed, there is a possibility of inducing an error. Direct imaging eliminates one layer of translation, and the image is directly etched on the board. It is performed by expensive equipment; nonetheless, it produces a crisper image. Whatever is created on CAD gets directly etched on the board without having to go through an additional layer of transla- Fine pitch devices require extremely tight tolerance control for the correct level of etching. Hence, selecting the right and experienced fab shop is highly important. At Fabrication Fine pitch devices require extremely tight tolerance control for the correct level of etching. Hence, selecting the right and experienced fab shop is highly important. If a board is over-etched and the SMD pad size is at 20 to 30 percent less than the required pad speciﬁcation, then sufﬁcient mass is not left on the board for a stable solder joint to be performed at the assembly stage. goes mechanical vibrations or is subjected to rugged mil/aero environments. Laser direct imaging (LDI) is a relatively new, commercially viable technology to directly image circuitry on the photoresist from CAD data, eliminating the need for the traditional ﬁlm or photo tools. Plots are used to create different traces and pads, solder masks, and silk screen on different layers when fabricating the board. LDI eliminates the need for using photo plots or ﬁlms by removing an intermediate step. Plus, it offers better registration than traditional contact printing. LDI also eliminates ﬁlm related defects, as well as the handling and storage needs of photo tools. The only reason for using photo plot is to etch the image on the board. However, when that ﬁlm is developed, there is a slight probability that the development process might tion associated with photo plot or the ﬁlm. Surface ﬁnish is another major aspect of ﬁne pitch during fabrication. It is an especially important consideration to assure BGA or micro-BGA co-planarity. The board surface must be completely ﬂat so that component placement is precise and solder joints are solid without issues arising due to surface ﬂatness. Surface Finishes The surface ﬁnish of a board plays an important role in component placement for ﬁne pitch devices. One important aspect of successful ﬁne pitch assembly is for the ﬁnish to provide an extremely ﬂat surface. Therefore, it’s best to consider electroless nickel immersion gold, immersion silver, or organic solder preservative (OSP) ﬁnish for a board heavily Different Board Finishes Compared HASL Flatness Solderjoint Wirebond Cost Reflows Shelf Life NO Cn-Sn NO $ 6 18 Months OSP YES Cn-Sn NO 0.7X $ 2 6 months ENIG YES Ni-Sn Al 3X$ 6 24 months Tin YES Cn-Sn NO 0.8 X $ 2 to 3 6 months Silver YES Cn-Sn Au, Al 1.5 X $ 6 12 months Figure 3 Bridging Results From Too Much Solder Applied During the Printing Process circuitree.com • February 2009 27 Table 1 Special Attention Should Be Given to Surface Finishes for Fine Pitch Assembly http://www.circuitree.com

Table of Contents Feed for the Digital Edition of CircuiTree - February 2009

CircuiTree - February 2009 Contents Lead Wire The Wire Tech Talk Flexible Thinking Are Better PCB Design Tools and Manufacturing Enough to Stay Competitive? The Strategist Fein-Lines The Big Deal Over Fine Pitch Assembly Asian Section View From the Middle Market Outlook Technical Product Spotlights Classified Ads Upcoming Events Ad Index

CircuiTree - February 2009

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