Reliability evaluation of probe-before-bump technology

As flip chip applications gain in popularity, integration of wafer bumping and probing is becoming necessary. Wafers are usually probed after bumping, but probe-after-bump technology suffers from low throughput and high cost, and also delays wafer yield feedback to the fab. However, probe-before-bump offers excellent process integration. In this paper, the authors used electroless bumping technology to study the feasibility of probe-before-bump technology. Two probe solutions, probe on under bump metallization (UBM) or probe on Al bond pad, are investigated. Between the two methods, probe-on-Al offers better fab-to-packaging flow. However, probe damage on the UBM or bond pad may have negative impact on bump structural integrity, degrading assembly reliability performance. Three sets of experiments were carried out to study the probe process. Variables evaluated included: probe process, temperature, baking, overdrive and probe card type. Results show that probe-on-Ni/Au leaves almost undetectable probe marks on the UBM and probe-on-Al can result in heavy probe marks when a high overdrive is used. Samples from each experimental set were assembled on organic substrates and tested for AATC at -40°C to 125°C. Thermal cycling results show that neither probe-on-Al nor probe-on-Ni/Au degrades reliability performance unless there is need for a high temperature data retention bake. If the device requires a data retention bake, probe-on-Ni/Au with low overdrive provides acceptable performance. At elevated temperature, probe-on-Ni/Au using a membrane probe card provides similar reliability performance to the unprobed group