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After switching from No-clean flux to Water soluble (Kester)we encountered voids problem (X-ray and microsectioning). Reducing the apperture sizes made the voids smaller but did not eliminate them. So we had to go back to No-clean. Any idea why voids occur? Will appreciate any answer.

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| After switching from No-clean flux to Water soluble (Kester)we encountered voids problem (X-ray and microsectioning). Reducing the apperture sizes made the voids smaller but did not eliminate them. So we had to go back to No-clean. Any idea why voids occur? | Will appreciate any answer. | Voiding occurs with almost all solder paste applications. It sounds like you know some serious stuff as you are capable of both X-Ray and X-Sectional analysis and apparently have serious reliability concerns as well. Therefore, I'm sure you also know voiding isn't always a bad thing.

First, how much voiding? Second, how many voids in what locations? If you're in the 20% arena, you are doing pretty well according to some in industry and I agree with most of what is said regarding this percentage.

If you can do the analysis, are you correlating results with shake and bake and cycles to failure? I have seen very high cycle numbers with and without paste. Fewer at times without paste as there are no voids formed.

Earl Moon

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| After switching from No-clean flux to Water soluble (Kester)we encountered voids problem (X-ray and microsectioning). Reducing the apperture sizes made the voids smaller but did not eliminate them. So we had to go back to No-clean. Any idea why voids occur? | Will appreciate any answer. | Grevald, one of the resons for solder voids is having a hotter profile. Now there is a big difference in the recommended profile from the no-clean solder to the water-clean.

are you using the same receipe for both paste?

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| After switching from No-clean flux to Water soluble (Kester)we encountered voids problem (X-ray and microsectioning). Reducing the apperture sizes made the voids smaller but did not eliminate them. So we had to go back to No-clean. Any idea why voids occur? | Will appreciate any answer. | Grevard: He got them ball voiding blues eh?

1. There's no spec, but a large or many small voids at interface is bad. 2. See EP&P 10/98, which says some thing like: 2a. Number of small and number of large voids correlate. 2b. Reflow time (and temperature) highly influences void formation. Soak time is not a factor. 2b1. Increasing reflow temperature increases voiding. Temperature influences void formation 8.4X greater than reflow time. 2b2. Increasing reflow time decreases voiding 2c. Reducing small voids near the base (top) of the ball, increase reflow time (60 -> 100 sec) and decrease reflow temp to 205C.

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