Printed Circuit Design & Fab - May 2008 - (Page 41)

FIGURE 2. PCBs designed with components too close to the edge can create pick-and-place problems, leading to manual placement and high probability of resulting defects. FIGURE 3. Fabrication notes call out such information as layer stackup, board thickness and impedance calculations to eliminate fabrication-related questions. protected when through-hole components undergo reflow. There is nothing wrong with placing BGAs bottom-side, but if this PCB design and placement were done correctly, it would have eliminated defects and avoided creating the basis for future failures. In cases like this, when only one side of the board is used for BGA placement, the need for using epoxy for reflow is eliminated, as is the need for a wave solder fixture. An assortment of other defects also arises from machine limitations, incorrectly designed in tolerances, and board spacings. DfA takes into consideration physical, height, length, and package-related limitations. If a board is highly complex with 0201 or 01005 packages, and tight assembly processes are not specified, multiple board defects can result, such as tombstoning, skewing of components, and inactivation of flux with solder paste. A designer also has to know the limitations of various fabrication and assembly process machines. Those include pick-an-place, AOI, reflow, wave solder and test systems, among others. The board should be designed with mechanical and other tolerances in mind, such as the size of the board and, in case of smaller boards, the panel size, as each machine has physical limitations. Also, there are some height-related restrictions, especially in the case of a flying probe tester, whereby using taller components might hinder and restrict movement of the flying probes. Components can be designed and spaced too close to each other or to the edge of the board (FIGURE 2). Consequently, this error or oversight poses limitations for the pick-and-place machine during placement. Parts must then be placed by hand, creating reliability issues and likely leading to defects. An alternative is to redesign the board, which involves a costly and time-consuming re-fabrication. Insofar as component placement, a 0.004˝ pitch BGA, for example, requires considerably tighter placement tolerance than a 0.007˝ or 0.008˝ pitch BGA; thus, it demands more placement precision (and generally, more expensive equipment). Even then, at 0.004˝ pitch, reliability and repeatability can continue to be issues unless proper design measures are taken. Fabrication. First-article is a critical step to verify virtually every aspect of PCB fabrication, including hole size verification, cutouts, slots, and all physical and mechanical dimension calculations. Eliminating defects at fabrication also takes into MAY 2008 account well-documented fabrication notes and an extensive checklist of key factors. If any one or more factors are overlooked or blatantly omitted, chances of defects increase. Here, planning at the CAM stage calls for checking, double-checking, and even triple- checking such factors as layer stackup structures’ thicknesses and impedance control calculations to make sure they are precisely correct and are within specified tolerances. The fabrication description chart (FIGURE 3) shows a four-layer stackup, with all the key elements precisely described. A board’s innerlayers also can be a breeding ground for defects. Proper design uses correct prepreg materials and December 2-4, 2008 Orlando Metropolitan Resort Orlando, FL www.pcbshows.com/orlando PRINTED CIRCUIT DESIGN & FAB 41 http://www.pcbshows.com/orlando http://www.pcbshows.com/orlando

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - May 2008

Printed Circuit Design & Fab - May 2008 Contents Our Line Market Watch Around the World Happenings ROI EMC For the Real World PCB East Conference Brochure Positive Plating Don't Let your Signals Stub Their Toes Improve PCB Layout With Skill Utility Programs The Next Generation Design Tool Challenge Thermally Conductive Microwave Materials PCB Dielectric Degradation in Lead-Free Assembly Applications A Tale of Two Trade Shows Eliminating Board Defects Off the Shelf Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - May 2008

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