Printed Circuit Design & Fab - January 2008 - (Page 17)

FIGURE 1. Hot tear creates a crack in through-hole component joints. ered with one ounce of copper. However, in an analog PCB application designed for higher current, a two ounce copper foil may be required to insure the capacity for proper current flow. The choice of the correct internal copper foil is an important consideration in PCB long-term, reliability. The copper deposited on the hole wall through the plating process is also critical in the PTH reliability equation. If insufficient copper is deposited on the via wall long-term reliability can be affected. The problems created by thin copper can include fractures or cracks in the barrel of the via, leading to poor solder joints and intermittent or broken continuity in the current flow. Recently the IPC reduced it’s the PTH thickness requirement to a minimum of 0.8 mils. In some cases this may not be sufficient copper for a particular design. If designers require more copper in the PTH they will need to specify it on the drawing or the fabricator can use the IPC guideline. Connectors and sockets require special design attention. Performing the right PTH calculations are particularly critical because connectors and sockets demand greater PTH precision, especially for press fit connectors. If drill sizes and annular rings are calculated poorly, there is the possibility of creating a through-hole that is too small to accept the connector. And while it may be possible to maneuver a component lead into a too-small plated-through hole, such actions often increase the probability of cracked vias in the middle of the wall, creating an unreliable connection. This problem can either immediately surface during test, or become a latent or intermittent defect, causing reliability issues for products in the field. Reliability becomes a major issue when via-in-pad technology is used. In some manufacturing schemes, via-in-pad uses different materials and processes than are used in conventional through-hole via. In some cases, a conductive material such as epoxy or silver is used to fill the via. For via-in-pad, a PCB fabricator must fully understand how these materials will differ from the conventional via creation to maintain the board within its own specific tolerances. This is a challenge because the technology is still new, there are few fabricators that have the necessary trained staff, experience, and equipment to provide the high level of PTH reliability required today from this technology type. JANUARY 2008 FIGURE 2. Using arbor press, a press fit connector is installed on a PCB. Plated through holes using SMA connectors in highspeed designs require simulation modeling also known as via modeling. This software assures there are no reflections or discontinuities along along the electrical signal paths to assure certain transition points are smooth. This is especially important when there is a cable or connector used in the board. Therefore, the optimal design requires that through hole components and vias are designed with proper transition points, eliminating reflections, discontinuities, or sudden changes in signal impedance requirements. Rework cycles should also be considered at PTH design. This is especially critical to press fit connector reliability, as the connector will not tolerate warp or movement of the board (see FIGURE 2) and is more prone to defects during rework cycles. Extremely accurate PTH design is required for these connectors, as after a mistake in the PTH process is made, reworking a press-fit connection is extremely difficult. If the via is too small, the connector won’t insert into the board; if too big, an intermittent connection is created. One of the few rework options involves cutting and adding jumpers. As shown in FIGURE 3, an incorrect PTH calculation resulted in a too-small PTH. The via broke off the board, and an unsightly jumper wire is used to recreate the connection. Besides the aesthetic considerations, this repair method lacks the level of reliability of a properly designed via. At Fabrication When considering drill and pad sizes, a ± 3 mils tolerance is typical for vias up to a 10 mil size. Some are tighter, where tolerances could be +3 and -0 for a 7 mil or smaller sized via. There are specific guidelines for drill hole and pad sizes. For instance, a 10 mil drilled hole must have at least a 22 mil annular ring, allowing at least a 6 mil annular ring on each side. When space permits, a larger annual ring, such as 26 to 30 mil would be preferable, to create a more solid pad joint able to withstand several component extractions and placements, as well as being able to undergo multiple reflow cycles without incurring problems. PRINTED CIRCUIT DESIGN & FAB 17

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Printed Circuit Design & Fab - January 2008 Contents Our Line Market Watch Around the World Happenings ROI PTH Reliability: Designing to Improve PTH Reliability EMC for the Real World PTH Reliability: The Survival and Long-Term Reliability of Lead-Free PCBs iNEMI's Emerging Technologies: Research Priorities for the Electronics Industry Printed Circuit Design & Fab Annual Buyers Guide Special Suppliers Section Guide to Products and Services Off the Shelf: Designcon Preview Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - January 2008

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