Laddered-UBM Structure: Bump Reliability Improvement through Distribution of Load Concentration Points
Bibliographic record
Abstract
Flip-chip mounting schemes have been used for decades to mount semiconductor chips to substrates. In flip-chip process, a solder bump is metallurgically bonded to the under-bump-metallurgy, also known as UBM, on a given pad of the semiconductor chip and a pre-solder is metallurgically bonded to a corresponding pad of the substrate. Thereafter the solder bump and the pre-solder are brought into proximity and metallurgically bonded using reflow. Flip-chip solder joints are subjected to mechanical stresses from a variety of sources, such as coefficient of thermal expansion mismatches, ductility differences and circuit board warping. Such stresses can subject the conventional UBM structure to bending moments specially during the flip-chip reflow solder solidification stage where the bump is still unprotected by the underfill. The effect is somewhat directional in that the stresses tend to be greatest nearer the die edges and corners and fall off with increasing proximity to the die center. The bending moments associated with this so-called edge effect can impose stresses on the dielectric film beneath the UBM structure that, if large enough, can produce fracture. This paper will discuss the load distribution on a conventional UBM structure due to the bending moments and how the Laddered-UBM structure attempts to overcome or reduce the effects of these bending moments. Contrary to conventional methods where stress concentration points are eliminated, the Laddered-UBM is designed to strategically increase the number of load concentration points along the UBM structure. With the increased number of load concentration points, the stress along the UBM is distributed more evenly which effectively reduces the stress at any given point thus preventing a single large enough stress to cause dielectric fracture. Theoretical analysis and experimental data including reliability results on both the conventional UBM structure and the Laddered-UBM structure will be presented and discussed in this paper.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".