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PCB-May2017

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92 The PCB Magazine • May 2017 Flying probe testing is extremely popular in today's manufacturing theatres. The main fac- tor is cost reduction in contrast to dedicated fixtures and fixture testing. However, there are some limitations in flying probe testing when gauged against industry specifications—specifi- cally, the use of indirect vs direct testing in Test Level C. Table 4-1 of IPC-9252B outlines the methodologies allowed over the different Test Levels. This month we will be discussing Test Level C as this level raises the most questions regarding the use of direct versus indirect test methods when testing product in the Test Level C class. First, we need to define some terms used in flying probe testing. AABUS The term AABUS, or "As Agreed Between User and Supplier," is used in IPC specifications where actions within a specific requirement are allowed, but require the mutual agreement be- tween the user and manufacturer. This term is important as it amplifies the requirement for correct flow-down of customer requirements and any special allowances or deviation of the industry specifications. (See my April column regarding flow-down.) Adjacency and Adjacency Window The terms 'adjacency' and 'adjacency win- dow' are used with flying probes defining the area for which the isolation test is performed. There are two types of adjacency: horizontal (line of sight) and vertical (Z-axis). As with fix- ture testers and parametric testing, the flying probes cannot accomplish a full parametric iso- lation test as they simply do not have the hard- ware. So the industry has accepted the prac- tice of adjacency. How this works is that when a single net is interrogated for shorts it is test- ed against other nets in range or that are adja- cent to that net. That range is defined as the ad- jacency window. The adjacency window is user- definable, however the specification IPC-9252B has recommended 0.050" (1.27 mm) as a de- fault value for horizontal or line of sight adja- cency. When programming for vertical adjacen- cy, there is more information required and the window is variable. One needs the stack-up in- Flying Probe Testing vs. IPC-9252B TESTING TODD by Todd Kolmodin GARDIEN SERVICES USA

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