Browse Prior Art Database

Method and Apparatus for Detecting Discrete Component Assembly Without Fanout Vias on Printed Circuit Boards

IP.com Disclosure Number: IPCOM000073627D
Original Publication Date: 2005-Feb-22
Included in the Prior Art Database: 2005-Feb-22
Document File: 7 page(s) / 136K

Publishing Venue

IBM

Abstract

The disclosed device utilizes an input voltage step, along with specialized logic to interpret the time-domain reflections, to determine the presence of correctly placed components and trace continuity for printed circuit board (PCB) assemblies. This device enables the design of high-speed traces on PCBs without the inclusion of parasitic fanout vias, which are typically required for in-circuit testing of the PCB.

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Method and Apparatus for Detecting Discrete Component Assembly Without Fanout Vias on Printed Circuit Boards

High volume printed circuit board (PCB) manufacturers use the fanout vias of surface-mount components to test for path connectivity and component assembly. When a board is designed as a single-sided assembly, standard backside bed-of-nails/in-circuit test (ICT) fixtures cannot access all of the traces. A common standard design practice is to fanout every pin on every component with through vias in order to make the traces accessible to backside ICT fixtures and insure high yield in volume manufacturing/assembly. The alternative to fanning out vias to the backside is to produce a top-side ICT fixture which would be able to probe the component pads, but this adds cost (since it is custom to each design), and may not even be able to access the test points (depending on how densely packed the card assembly is). If the vias are not present to the backside, and the top-side fixture cannot reach the points, then the absence of testing adds risk to the assembly in the form of possible board shorts/opens, or poor component assembly. Without being able to test this, low-quality PCBs can end up in the development lab, draining valuable resources away from critical verification and onto PCB failure analysis.

     Some high-speed interfaces (SAS, etc.) utilize series AC-coupling capacitors. Because these caps are discrete components, assembly manufacturers would like to have them fanned out for test. As signal speeds continue to increase, the presence of parasitic effects caused by vias can hamper channel performance. A method of testing for component assembly which does not require vias to be used would be desirable.

     One can trivially design PCBs without fanout vias, but their absence prevents the manufacturer from easily performing ICT and component assembly testing. A method of testing trace continuity, along with component assembly, is required to enable the removal of undesirable vias in the termination components without increasing the yield and cost risk.

     This disclosure describes a method and an apparatus for testing trace continuity and component assembly for series capacitors, per the problem description above. It utilizes a step waveform to test for the presence of the discrete component, and for possible shorts/opens on the PCB trace.

     Most high-speed cabled interfaces that require series AC termination have that termination placed near the cable connector. By placing a known structure consisting of a fixed length of transmission line and a step input waveform into the connector, one can measure and interpret the resultant near-end waveform (regardless of whether the DUT is powered on or not), and determine if the PCB is assembled correctly. A further advantage of the invention is the ability to test the entire trace for open/short discontinuities, whereas a simple DC test is not able to test across a series capacitor.

     Figure 1 sh...