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Best VTEP sensor plate sizing

Question asked by adrianc Employee on Apr 5, 2012
_*Best VTEP sensor plate sizing*_
With BGAs being very commonly found on PCBA these days, it is important to know the best way to size your VTEP sensor plates that are used to test the BGAs to achieve the best results.

For ICs with leads (gull-wings, J-leads, bat-wings, etc), we generally size the VTEP sensor plate larger than the component body to overlap over the leads. This is so that the capacitance coupling covers a wider surface area (the wire-bond, internal IC traces, and leads) resulting in a larger capacitance detected. Consequently, it would be easier to determine a well defined PASS/FAIL result.

For BGAs, the layout of the ***** is different from ICs with leads. The ***** are found under the component body instead of at the perimeter of the component. Furthermore, the ***** are much smaller in surface area than the leads. A much more sensitive measurement method like VTEP is required compared to testing leaded ICs. However, with this added sensitivity, additional care is required to remove potential “noise” sources in order to get the best test results.

For best results, VTEP sensor plate should be sized smaller than the BGA. This reduces the coupling of surrounding components and PCB traces and therefore reduces the parasitic capacitance from these interferences. The illustration shows a sensor plate that is too large for the BGA. You can see that stray capacitance is introduced from a nearby component (C Component) on the left and from the trace (C Trace) running at the right side of the BGA under test. The final measurement of this VTEP is the sum of the BGA measurement (C Measurement), the nearby component (C Component) and the trace (C Trace).

If the VTEP sensor plate is reduced in size to about 80% of the BGA size, it will be large enough to cover just the BGA ***** (or even slightly smaller). This drastically reduces the stray capacitance from the nearby component (C Component) and from the trace (C Trace), leaving only what you desire to measure (C Measurement). 

Try this simple adjustment to the sensor plates on your BGAs and enjoy a more consistent VTEP test for BGAs.