Printed Circuit Design & Fab - November 2007 - (Page 38)

METRIC DESIGN FIGURE 3. A 0.4-mm pitch BGA solder mask clearance. FIGURE 5. Staggered microvias. FIGURE 7. Stacked microvias. FIGURE 6. Stacked microvias. FIGURE 8. Stacked microvias. FIGURE 4. A 0.4-mm pitch BGA solder mask maximum offset. routing solutions for the 0.4-mm pitch components. One of the reasons is that the required 0.05 mm line and With 35 years of experience We make our own laminates in the U.S. • Material types in FR-4 Difunctional, Tetrafunctional and Multifunctional • Temperature ratings from 130˚C to 177˚C • Thickness ranges from .0025” to 3.000” • Coppers from 1/2 oz. to 6 oz. in stock • Black, Blue, Tan, Green, Orange & Natural • True Cryogenic G10 • Sheet sizes in 36”x48” • RoHS compliant, UL recognized and IPC approved Same Day Pricing and Lead Times Current, Inc. 30 Tyler St. Ext • East Haven, CT 06512 1-877-436-6542 • Fax 203-467-8435 www.currentcomposites.com Louis S. Owen, Director of Outside Sales 860-350-9607 • Fax 860-210-1748 38 space that would be required is not yet mainstream in the electronics industry. See FIGURE 1 and TABLE 4 for the BGA technology chart that provides through-hole via-in-land information. For all via-in-land technology, a thermal relief on the voltage and ground plane connections must be used to prevent cold solder joints. A direct viain-land connection to the plane will dissipate the heat required to melt the solder around the BGA ball, and this will result in a cold or cracked solder joint. If traces are routed between pins of the BGA land with 0.4-, 0.5- and 0.65-mm pitch, the solder mask must be a 1:1 scale to create a solder mask defined BGA land. In this way, the traces between the lands will be protected from exposure and possible short circuiting. The 0.4-mm pitch BGA via-inland through the PCB is leading-edge technology. When laser drills are capable of producing 0.125 mm hole sizes entirely through the board and PCB manufacturers can accurately fill the holes with conductive material, this technology will become mainstream. The 0.5-mm pitch BGA via-inland through the PCB is also leadingedge technology. Before it can become mainstream, laser drills must produce 0.15-mm hole sizes entirely through the board, and PCB manufacturers must accurately fill the holes with conductive material. However, this technology can be used when PCB thickness is 1.0 mm or less. The 0.65-mm pitch BGA via-inland through the PCB is mainstream technology. Mechanical 0.2-mm hole sizes entirely through the board are common, and PCB manufacturers can accurately fill the holes with conductive material. This technology can be used when PCB thickness is 1.57 mm or less. Microvia technology is the mainstream solution for 0.4- and 0.5mm pitch BGA parts when a 0.1 to 0.15 mm laser hole is drilled one or two layers deep. FIGURES 2, 3 and 4 illustrate routing solutions for microvia technology for a 0.4-mm pitch BGA. Whenever traces are routed between 0.4-, 0.5- and 0.65-mm pitch, the BGA pads on the outer layers, the solder mask size and the tolerance must be considered. It is best not to route any traces between BGA pads, but if necessary, the solder mask should be a 1:1 scale of the land size or a 0.025 mm minimum annular ring. The four PCB cross-sections shown in FIGURES 5, 6, 7 and 8 use three different via technologies: through via, blind via and buried via. Staggered microvias are used in Figure 5 and stacked microvias used in Figures 6, 7 and 8. I prefer stacked microvias since PCB design layout is easier. In this case, there are fewer visible obstacles to manage when using the CAD tool. Most PCB designers use blind, stacked microvias and buried vias as NOVEMBER 2007 PRINTED CIRCUIT DESIGN & FAB http://www.currentcomposites.com http://www.currentcomposites.com

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Printed Circuit Design & Fab - November 2007

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