Evaluation of Solidification and Interfacial Reaction of Sn-Bi and Sn-Bi-In Solder Alloys in Copper and Nickel Interfaces

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The topic of the paper presented by the authors is intersting and of industrial relevenace. However, the interpretation of the results is partly premature and largely speculative. This is all the more so because the results and the discussion are very much mixed up. More obvious explanations, such as a consideration of the thermodynamic conditions of the alloy system (melting intervals, reaction sequence during solidification and in the solid state) are not taken into account in the interpretation of the results. This leads to misinterpretations. The manuscript requires a thorough revision before resubmission. In particular, the following points must be taken into account:

Page 1, line 43-44: "... In can reduce the melting temperature, which can facilitate a larger number of nucleation sites ..." What does this sentence mean? Alloy with lower "melting" (solidus- or liquidus) temperature have finder grain size?????

Page 2, line 76: "In induces lower surface tension ..." => Ref?

Page 2, line 94: a study of alloys with 12% In are mentioned, while in line 56 it is said that there are no studies on alloys with >8% In

Page 3, line 103: introduce SAC. In line 108 "CuSnIn" is mentioned (Ref?). What is the correlation of CuSnIn with SAC?

line 113: discuss the effect of high In contents on alloy cost and availability of In

line 121: typo of Ni

Chaper 2: how where the alloys manufactured? How long was the tempering time (line 125)? Was a solding jig use to solder the samples (line 137-143)? Include a drawing of the samples.

Chapter 3: the main issue with this article is the mixture of results and interpretation without a clear separation. It is highly recommended to separate results and discussion nto two different chapters.

line 159: "Bi has demonstarted strong macrosegregation" I can't see that

line 163: what is meant by "Bi particles"?

line 166-180: Describe the solidifcation path of both alloys by using the phase diagram information that is published on these alloys. What kind of "intermediate liquid" (line 169) is occuring? Which part of the microstructure is formed by reactions involving the liquid phase? Which features are due to solid state reactions? What is the lquidus and soliduc temperature of the alloy? Which non-variant reactions are occuring in both alloys? Which "transformation temperatures" (line 177) are lowered by adding In to the binary alloy? Which fractions of primary phase and euectic are expected according to the lever rule?

line 190-201: what should this paragraph say? After answering the questions above the mesage of this paragraph should become clearer.

The determination of "cell spacing", actually probably dendrite arm spacing is meant" is not reperesentative based on the shown microstructure. Instead the volume fractions of dendrites and eutectic should be discussed based on the questions above.

line 219: Z-contrast means back-scattered electron imaging?

line 219: What is menat by "institial regions"?

Fig 3: Is BiIn from from the liquid or the solid? => refer to phase diagram information. Where was the SEM-EDS dot-map taken? the composition is deviating by 3% In from the nominal composition of 10%. Discuss how this will affect the microstructure and phase fraction.

line 237: "Bi particles" here are dendrites?

line 239: a "precipitate size" has the unit of a length. Or is the phase fraction meant?

line 240: "In appears to hinder the mobility of Bi in solution" and ine 242: "facilitated diffusion in the absence of In" Make a thermodynamic consideration first and then a kinetic consideration. Show reference for the effect of In on "Bi mobility"

line 258-259: the values in Fig. 5 scatter by +/-5-10°. How meaningful is one digit for the wetting angle?

line 262: what is meant by "fluidity"? Lower vsicosity?

Fig. 5: About 50% of the images do not show the complete drop or the drops are irregular. Therefore, it is not possible to measure the contact angle. This might be the reason for tha large scattering of the data without a clear tendency for the contact angle with time. The values for the Sn-50Bi and the Sn-Bi-In are more or less the same. Is the contact angle lowered by an effect of homologous temperature or by In addition?

line 280: what is menat by "fluxes of Sn, Cu, In ... helped the nucleation of Cu6Sn5"?

line 283: No, this is no Ostwald ripening, but diffusion controlled layer growth!

Fig. 6: the Cu substrate must be visible on the image. Why is the Cu6Sn5 phase hardly visible in the BSE image, but clearly visible in Fig. 10? Cu and Sn signals are not correlating. At the highest Cu level no Sn was found. Oxide inclusion?

line 294: what is the reason for increased copper content above the substrate? Why should Cu diffuse faster into Sn and not vice versa (diffusion rate correlates with melting point)?

Fig 7: how was the thickness determined for such layers, especially in 7c)?

line 357: explain the reason for a value of 119kJ/mol in your study. Make an error calculation for Q.

line 371: How was the average max. load value calculated. A rough estimation based on Fig.9 200N and 95N for Ni and Cu substrate, respectively. How meaningful is the value for the Cu substrate?

line 385: "fast cooling assciated with Ni". Wsan't the experiment done in a furnace where the thermal conductivity of the substrate is irrelevant? What is then the reason for the different lamella spacing of the eutectic? Solubility of Cu and Ni in he Sn-Bi alloy?

Fig. 9: the beginning of some curves is missing

Fig. 11 and 12: arrage the figures of the different fracture surfaces in the same order

 

Author Response

Please see the attached file referring to the Reviewer 1.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors, please find my review of "Evaluation of solidification and interfacial reaction of Sn-Bi and Sn-Bi-In solder alloys in copper and nickel interfaces" attached. See comments below:

C1. "have not been investigated so far" - are you sure? If so, please list the other papers dealing with similar topics meticulously in CH1.

C2: The intro is weak on low-temp soldering presentation. I would present the "low temp soldering" topic in greater detail, and look for a wider scope of applications, pros and cons. Then the intro can continue with "This introduction focuses..." Also the first sentences fail to present the application. Electronics? Cabling? SMT, THT? Please clarify! Low temp soldering also contributes to sustainability. Sustainable materials (low thermal capability) are used with LTS oftentimes, e.g. degradable boards, or lowered reflow temperatures, with consequential lowered energy consumption. Please find references in these directions too. This could point potential applicability, which aspect is slightly lacking in the current form of the manuscript.

C3: Also i am very interested in the availability of Bi and In around the world as raw material ... I mean, Sn and Cu is widely used, with known sourcing. Please write a few lines about the mass transition possibility of these metals (upscaling of SnBiIn application).

C3: L121: "i were used " - ?

C4: "A heating rate of 10°C/min" it is most likely a slow profile, where IMCs could go thicker and flux can loose its effect. Please elaborate! Can you provide a thermal profile too? How is cooling handled? Later, can these information alter the discussion? (E.g. effect of cooling?)

C5: Also, please add, how the alloys were applied as solder. Paste? Flux? Type? (particle size?)

C6: Are duplicate tests enough for Fig 9? It would be better to see larger numbers.

CX: English is good, figures are nice, flow of text is satisfactory. Good job! (Only at line 335 where there is a styling error. Bcause of that L362 falls to the next page)

Author Response

Please see the attached file referring to the Reviewer 2.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

I had a hard time reviewing this and, in particular, explaining my opinion/impression. I list a few details below. Overall, this looks like a lot of explanations taken from reading literature (I don't mean plagiarized) and shown together with various measurements, as opposed to things learned from measuring and then explained based on literature. In a number of cases we are told that images show something (like fig 7), but it is not made clear how. 

Example: 'mapping results shows that In is present in the Cu6n5 reaction layer as Cu6(Sn,In)5 IMC', referring to figure 6. How do we see that? Off hand it seems more like there are Bi-In regions with little or no Sn, for example. 'A certain amount of Sn was diffused in the IMC'?

Figure 9 shows two curves for each pad finish, with the Cu results above and below the Ni. No explanation or comment

Experimental section says Sn-Bi-In was reflowed at 180C. Figure 11 says 200C with no comment

On page 6: first mention of Z-contrast with no explanation (are we talking backscattered electrons?). Not mentioned under experiment

On page 1: 'During nucleation In can reduce the alloy melting temperature, which can facilitate the formation of a larger number of nucleation sites'? That requires explanation.

The authors use results from Silva, measured on bulk samples not attached to a contact pad, to assess cooling rates from 'microstructural spacings'. No explanation of how these might apply to smaller samples on contact pads on substrates which presumably affect cooling. I am probably missing what cooling rates they are talking about here, and why these are of interest. 

Author Response

Please see the attached file referring to the Reviewer 3.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

No further comments. Accept as is

Author Response

We thanks the Reviewer for the valuable comments.

Reviewer 2 Report

Comments and Suggestions for Authors

Most points were addressed, thank you for the corrections. What i would still like to see, as a minor revision:
- sourcing of Bi as raw material (add to the paper like in the case of In)
- Figure 9 is not valid on its own, i am not seeing the well founded consequences from this measurement. Either expand the experiment, or avoid communicating "duplicate" results
- it would be nice to see further communication in strengthening the LTS / Bi-based soldering aspect, by checking a few more references, like:
https://ieeexplore.ieee.org/document/9501877
https://link.springer.com/article/10.1007/s10854-021-06820-7
https://ieeexplore.ieee.org/abstract/document/10109714
https://www.sciencedirect.com/science/article/pii/S259004982030062X
https://iopscience.iop.org/article/10.1088/1361-6528/ad66d3/pdf
https://www.mdpi.com/1996-1944/17/12/2848

Author Response

Most points were addressed, thank you for the corrections. What i would still like to see, as a minor revision:

Authors: We thank the Reviewer for these valuable comments.

- sourcing of Bi as raw material (add to the paper like in the case of In)

Authors: We introduced a brief discussion on the production and consumption of Bi, along with some sustainable solutions and the collateral benefits of using Bi in LTS applications (highlighted in blue in the manuscript).

- Figure 9 is not valid on its own, i am not seeing the well founded consequences from this measurement. Either expand the experiment, or avoid communicating "duplicate" results

Authors: We agree with the Reviewer. We have chosen to keep Figure 9 primarily to facilitate later discussions on fracture mechanisms. In the revised version of the manuscript, we have avoided using the term 'duplicate'.

- it would be nice to see further communication in strengthening the LTS / Bi-based soldering aspect, by checking a few more references, like:
https://ieeexplore.ieee.org/document/9501877
https://link.springer.com/article/10.1007/s10854-021-06820-7
https://ieeexplore.ieee.org/abstract/document/10109714
https://www.sciencedirect.com/science/article/pii/S259004982030062X
https://iopscience.iop.org/article/10.1088/1361-6528/ad66d3/pdf
https://www.mdpi.com/1996-1944/17/12/2848

Authors: We have introduced a few more references in the revised manuscript.

Reviewer 3 Report

Comments and Suggestions for Authors

This version shows significant improvement. I think sufficient to allow publication

Author Response

We thank the Reviewer for the valuable comments.