A Study of Copper Dissolution During Lead Free PTH Rework Using a Thermally Massive Test Vehicle
Bibliographic record
Abstract
ABSTRACT The process of reworking a lead-free (Pb-free) pin-through-hole (PTH) connector using conventional solder fountain equipment presents a number of new challenges and issues which require resolution. Most importantly, it has been determined that the operating process window for a solder fountain rework, using a Sn-Ag-Cu (SAC) based alloy is substantially smaller when compared to the traditional 63Sn-37Pb (Sn-Pb) process. The maximum allowable cumulative contact time for which a PCB is exposed to molten solder during a SAC405 PTH rework has been reduced by up to 50% when compared to a typical Sn-Pb rework. The reduction in the allowable contact time is primarily due to increased copper (Cu) dissolution rates observed with SAC based Pb-free alloys at elevated process temperatures. The objective of this study was to determine both the Cu dissolution rates as well as to establish an optimized PTH rework process window and operating parameters using a Sn-Ag-Cu alloy to rework a complex PCB, as found in server or storage applications. Further efforts (such as studying alternate Pb-free alloys) were pursued to attempt to increase the allowable process window (or maximum allowable contact time) during PTH rework. This paper includes details of the experiments performed to study the phenomenon of Cu dissolution. Testing was performed using an OEM’s actual product as the test vehicle, which is thermally massive in nature. In Phase 1, a design of experiment (DOE) was performed to identify the significant factors influencing the rate of Cu dissolution occurring during the Pb-free PTH rework process using a Sn-3.8Ag-0.7Cu (SAC405) alloy. The factors included in the DOE were preheat temperature, pot temperature and contact time. In addition to determining the significant factors, average Cu dissolution rates as well as recommended inspection/analysis techniques and visual indicators of Cu dissolution were determined. Furthermore, in Phase 2, a DOE was performed to study and compare the dissolution rates of various Pb-free alloys including, Sn-3.8Ag-0.7Cu (SAC405), Sn-3.0Ag-0.5Cu (SAC305) and Sn-0.7Cu-0.05Ni in order to potentially expand the process window. Each Pb-free alloy was compared to a 63Sn-37Pb (Sn-Pb) control alloy. Lastly, the effects of solder flow rate on Cu dissolution will also be discussed in this paper.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| 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 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 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 source (direct Gemma or distilled Codex), 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".