CircuiTree - February 2009 - (Page 26)

The Big Deal Over Fine Pitch Assembly Zulki Khan he role ﬁne pitch surface mount (SM) devices play in subassemblies is considerably more critical now than ever before. At 20 mils (0.5 mm) or less, ﬁne pitch demands a well thought-out and orchestrated balance involving PCB design, fabrication, assembly, placement, and careful rework on SMD pads. Otherwise, disjointed operations quickly usher in major challenges and issues. At the start, for instance, oversights at design layout can create a domino effect. That’s especially damaging in terms of excessive time and increased cost to OEMs who opt to rely on different design, fabrication, and assembly vendors with little to no coordination for this important aspect of ﬁne pitch design. T characteristics signiﬁcantly contribute to the effective use of PCB real estate and subsequent design efﬁciency. The question comes up as to why ﬁne pitch assembly demands closer attention with Figure 1 An Oscillator’s Pad Stack Showing SMD Placement, Silk Screen, and the Location of Pin #1 With the Fudicial Mark in the Middle At Design There are a number of considerations for creating a pad stack for a ﬁne pitch design. As shown in Figure 1, a pad stack is a combination of pads associated with the different layers of a printed circuit board, and these pads are used for routing the signals on the PCB. Design considerations are many, but some of the most prevalent include if the pads are deﬁned by solder mask or nonsolder mask, the silk screen, and ﬁducial marks. When implemented properly, these 26 February 2009 • circuitree.com Figure 2 Defects Like Micro-BGA Voids Result if Precise Design Layout Isn’t Applied for Fine Pitch Assembly PCB design layout taking on greater importance. The answer is PCB real estate is dramatically shrinking in today’s design, especially for small handheld devices. Fine pitch devices are used on boards that are smaller, more compact. A great number of components are placed on the top and bottom side of the board, hence a very crowded PCB. At the same time, these compact board designs call for considerably more power and, subsequently, greater amounts of heat are generated. Also, electromagnetic compatibility considerations must be taken into account, and PCB designers must be watchful for signal line losses. Rework is another major consideration during the design phase. Pad stack mistakes must be avoided; otherwise, rework becomes highly challenging simply because ﬁne pitch pads are extremely small. Applying heat to depopulate and install ﬁne pitch devices can have a devastating effect on the small pads. Because they are so small, there’s the possibility of the extreme heat causing a pad to peel off from the surface of the PCB. Also, if heat is applied multiple times, there’s a possibility of solder mask peeling off from the surface of the PCB itself. It’s also important to clearly deﬁne ﬁducial marks that pick and place machines rely on for precise component placement. Again, 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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