Filter optimization for X-ray inspection of surface-mounted ICs

A thin Zn filter (∼20 μm) and relatively low X-ray tube voltage (∼45 kV) is recommended for X-ray inspection of surface-mounted device solder joints on printed wiring boards (PWB). An optimal filter minimizes the Si dose that could result in cumulative damage to sensitive IC circuit nodes, yet provides good contrast for metals such as Cu traces on PWB and device solder balls. While we expect orders of magnitude Si dose reductions when effective filters are inserted, a properly chosen filter should not attenuate the portion of the white X-ray spectrum required to image Cu, Sn and Pb (solder balls). Some X-ray inspection suppliers can achieve a Si dose of as little as 0.060 rads; while other X-ray inspection suppliers, not yet optimized for minimum dose, may use as much as four orders of magnitude more dose. We used thermoluminescent detectors (TLDs) to measure the X-ray dose that IC product shipments would encounter during a shipping process (personal baggage or cargo) as minimal (≤0.050 rads).