A Manufacturing Method for High-Reliability Multilayer Flexible Electronics by Electrohydrodynamic Printing

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors describe their research on methods of manufacturing multilayer flexible electronics. Although they use a well-known ink-jet technology in electrohydrodynamic version, they propose to examinate nontypical multilayer system of conductive paths. They isolate the subsequential layer by spraying polyimide and connect by using through holes. The tests are carried out to assess bending stress and thermal reliability. While the research undertaken is very interesting, the presentation style and research assumptions should be significantly improved.

I wonder about the through holes in the multilayer structure. According to Figure 1(c), the through holes are implemented in the system of two external conductive layers. However, in the proposed 10-layer flexible device, only one conductive layer (top) is external, while all others are buried. What assurance is there that drilling holes does not damage the inner layers, that injecting high viscosity conductive paste connects with the small lateral surfaces of the buried layers? Electrical connection diagrams are not shown in Figure 1, so it is unknown which layers are connected by vias. How many vias are applied? Figure 1(d) is completely illegible. Is it possible to reliable short-circuit all 10 layers after drilling? What was the purpose of distributing successive elements on different layers? Of which elements the resistance is measured in tests? Please define "specific pins"; is it the same as "pads" (line 95)? Please define interlayer interconnection – is it between conductive elements printed on neighbouring layers or through x layers?

In Section 2.1, there is a description of the setup for the thermal cycling test and the method of measuring resistance and morphology of cross-sections. However, there is no description of experimental stand for the bending stress tests. Such a description is particularly necessary because during each cycle of bending change, resistance is measured. Additionally, there is a lack of information on how resistance is measured in this complex electrical connection diagram. How many different resistive paths are arranged in the circuit?

In Section 3, the first paragraph describes the relationship between the thickness of the printed lines and the printing speed, working height, and nozzle diameter set in EHD printing process. These dependencies are well-known in the literature. While this information may be reiterated in the paper, it should be somehow linked to the methodology of preparing the proposed 10-layer layout. The Authors did not explain why they specifically selected the settings of 300 μm nozzle, 100 μm working height, 0.5m/s printing speed. They did not relate this choice to the size of their printed pattern. Although certain relationships are provided in Figure 3 and described in the text, no factors determining the choice of printing settings are indicated based on them. For example, the text suggests that increasing the nozzle diameter can lead to discontinuity in printing. Then why was the largest nozzle size chosen from the analysed values?

Similarly, in Section 3, the second paragraph and Figure 4 describe the morphology of the printed conductive paths. Such behaviour of the conductive paste during the heating process is well-known. Just like before, these pieces of information can certainly be reiterated in the paper, but they should be related to the main research goal, which is testing the 10-layer structure rather than the resistance of the conductive path. Does the heating process of the first layer of conductive paths when applied to the substrate follow the same procedure as when applying the conductive paths printed on the insulate interlayer? What substrate was used for experiments with heating up to 300 degrees Celsius?

 

Some sentences are ambiguous in meaning. Authors often use words inadequately to convey the intended message, or the sentences are overly complex with ambiguous meanings. Below are a few obvious examples, although almost every sentence contains stylistically questionable statements.

- in line 31: word „structure” – what kind? Also "wear comfort"?;

- in line 33: the subject of the parent sentence is "conformal ability" and it cannot be connected to the action "attach"; similar mistake there is in line 92/93;

- in line 49: a linking verb connecting the subject is missing;

- in line 54/55/56: there is the lack of a clear connection between the actions described in each clause;

- line 182: What did the authors mean in the sentence “In order to improve the bending resistance, the upper layer of the circuit obtained thicker deposits by gradually increasing the number of print times”; please define “thicker deposits” and “number of print times” as well as “printing time” (in next sentence) according to the upper layer implementation? Define exactly what do you mean writing “upper layer”? In Figure 6(a) I can distinguish two different thicknesses of layer but in Figure 5(c) I can see only one thickness. The cross-section in Figure (b) show another distribution of layer thicknesses. I also judge that there is the conductive layer on the bottom side of the PI substrate but it was not mentioned in the text. How does the successive increase in thickness manifest itself? Does it apply only to the last four layers? Does it have any practical significance?

- line 183: “After testing…” What tests are being referred to? Tests do not lead to the achievement of any physical properties; at most, they can be used to characterize those properties.

-line 185: What does mean “The flexible multi-layer circuit was attached on a wing shaped curved substrate”. According to the description the multilayer circuit was produced on PI. So, what kind of another substrate it was attached to? What about the “warpage or delamination”- is it between these two substrates or between PI and conductive and insulation printouts?

 

Other remarks:

- space is missing in line 58 "printing[14, 17]";

- in line 82:  The type (trade name) of silver ink should be provided;

- Figure 2(b) is completely illegible;

- was the multilayer nature of the conductive structure taken into account in the model shown in Figure 2(a)? Were the via structures considered – the disturbance of surface equality and the weakening of the substrate structure?

- Figure 4 (e): the scale of the vertical axis should be more detailed described;

- Figure 6 (g)-(i) needs more explanation;

- the definition of the “standing time” and “bending prestress” should be provided;

- the definition of “isolated” and  “insulated” should be provided with the tested flexible multilayer electronic circuit point of view;

- Figure 9(c): is the measured resistance temperature-dependent? If such dependence exists, it would be more appropriate to present changes in the resistance in relation to both the temperature range and the number of temperature cycles (e.g., as done for Figure 7(c)). It is essential to clearly distinguish aging processes caused by temperature fluctuations from phenomena occurring in the technological process related to additional heating of the printed layers. Unfortunately, thermal testing processes are very time-consuming and I understand it. Therefore, it would be more interesting to investigate whether bending tests at a constant temperature of -25 degrees Celsius leads to faster degradation of the structure than the same experiments performed at a constant temperature of +25 or +85 degrees Celsius.

Comments on the Quality of English Language

Some sentences are ambiguous in meaning. Extensive editing of English language required.

Author Response

Thank you very much for your comments and suggestions on our work. We are very glad to hear your encouraging evaluation and so many helpful comments. We have carefully considered those advices and modified the manuscript. Revised portions are marked with yellow color in the manuscript. The detailed responses to the reviewer’s comments are listed in the text.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This Paper shows that a manufacturing method for multilayer flexible electronics by EHD Printing process. The circuit was designed ten-layers and used specific pins to interconnection each layer. For the bending test of the multilayer flexible electronics, using a self-developed testing platform, and thermal cycling test was using a thermal cycling test chamber.

 

There are some comments.

 

1.     There is no verification about Numerical Simulation. Due to lack of data, it is difficult to confirm whether the numerical analysis was performed well. It would be good to present data that compares the author's experimental data with theoretical values.

 

2.     Flow rate is a key factor in determining the amount of ink deposition per unit time. Therefore, you need to add the applied flow rate to explain for what parameters affect the amount of ink.

 

3.     In Fig. 3(a), there appears to be an error in the printing speed unit. mm/s is an excessively slow value. Please recheck the print speed unit in Figure 3(a).

 

Overall, it is not easy to understand the main target to explain in this paper. The main topic to be explained in the title and abstract appears to be the manufacturing method for multilayer flexible electronics. However, there is not enough information about the manufacturing method in the text.

The manuscript is insufficient for publication in its current state. It is recommended to recheck and modify the entire thing rather than modifying just a part of it.

Author Response

Thank you very much for your comments and suggestions on our work. We are very glad to hear your encouraging evaluation and so many helpful comments. We have carefully considered those advices and modified the manuscript. Revised portions are marked with yellow color in the manuscript. The detailed responses to the reviewer’s comments are listed in the text.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I have carefully studied the response to my comments. The Authors indeed have improved the manuscript and responded to all remarks. I have no further questions. I believe that the manuscript meets the high editorial requirements of the Coatings Journal. The paper can be accepted for publication after a standard proofreading by the Editorial Board of the MDPI Journal.

Comments on the Quality of English Language

Minor editing of English language required

Reviewer 2 Report

Comments and Suggestions for Authors

Because it has been well revised, it is recommended to publish in this journal.