Effect of Electromigration and Thermal Ageing on the Tin Whiskers’ Formation in Thin Sn–0.7Cu–0.05Ga Lead (Pb)-Free Solder Joints
, Muhammad Mahyiddin Ramli 1,6, Jitrin Chaiprapa 7, Petrica Vizureanu 1,3,* and Mohd Izrul Izwan Ramli 1,2Round 1
Reviewer 1 Report
Comments are given in the attached file.
Comments for author File: 
Comments.pdf
Author Response
Reviewer: 1
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No |
Comment |
Response |
Page/Line |
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1 |
The title would be more grammatically correct if to was replaced by on. |
The change has been made as suggested. |
Page 1/Line 2 |
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2 |
It is unclear what legs is referring to. It would be better to state either a typical length or growth rate of the whiskers. |
The explanation on the typical length of whiskers has been added and the previous explanation has been removed. |
Page 2/Line 63 |
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3 |
I would assume that cracks formed by the growing IMC appears in the interface region between the Cu and Sn rather than at the Sn surface. I therefore have a hard time understanding how whiskers can grow from such cracks. Whiskers can grow from cracks in the oxide layer on the surface, however. |
Thank you for your comment, we have amended the explanation as below: Thick interfacial IMC may create stress concentration and hence initiates cracks which may aggravate to the Sn area and eventually to the surface of Sn coating. In addition, whisker growth also involves atoms diffusing into the whisker from the area surrounding its base [17]. |
Page 2/Line 69-72 |
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4 |
I do not understand what this sentence mean. Which processes are you referring to and in what different ways are they “providing stresses”? |
We have deleted the sentence to avoid confusion. |
Line 72-74 |
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5 |
I don’t understand this sentence. Whiskers are generated as a response to compressive stress in the Sn layer. The whisker will then grow as a way to relax these stresses. This is correctly pointed out in the next sentence. I’m not sure, however, if whiskers will form if there is already some other way to relax the stresses. |
Thank you for your comment, we have deleted our previous sentence to avoid confusion. We agree on the explanation by the reviewer which was written in the manuscript. |
Line 79-82 |
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6 |
between should probably be replaced by of. |
The word has been changed as per recommended. |
Page 2, Line 89 |
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7 |
Remove in. |
The word has been removed as per suggested and the sentence was improved. |
Page 3, Line 101-102 |
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8 |
I would phrase this as under electromigration with and without thermal aging. It looks a bit strange with “electromigration and electromigration”. |
The statement has been revised accordingly as well as the other part of statement that use “electromigration and electromigration”. |
All related pages |
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9 |
Something is missing before “and cast into ingots”. |
We have improved the sentence by adding the missing word. |
Page 3, Line 115 |
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10 |
It is unclear how the Cu sheet was prepared for hot-dipping. It says that excess flux was removed by ultrasonic cleaning and that the Cu sheet was then dipped into the flux. Is this the correct procedure? If so, what is the “excess flux” that is being removed in the first step? |
Thank you for your comments, apologise for the confusion. We have revised and improved the procedure for solder coating. |
Page 3, Line 120-121 |
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11 |
Replace “connect with the temperature control” by something like controlling the temperature. |
We have corrected the sentence. |
Page 3, Line 134 |
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12 |
This formula for the IMC thickness doesn’t appear to be very accurate for scallop shaped IMC layers. It would be interesting to compare this expression to some other measures of the IMC thickness. One such measure could be the mean, or median, value of the length of several vertical lines across the IMC layer. |
The main scope of the research is to investigate the growth of Sn whiskers on the surface under electromigration and electromigration with thermal aging effects. Most studies have used this method to calculate IMC thickness and it has been proven to show the significant relation in measuring whiskers. |
Equation 2 |
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13 |
The yellow lines are very hard to see. |
The figure has been improved by changing the lines from yellow to green colour. |
Figure 2 |
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14 |
How were the Sn whiskers imaged? |
Sn whiskers were imaged using SEM (Scanning electron microscopy) which has been elaborated in the section 2.4. |
Page 4, Line 160 |
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15 |
More details about the synchrotron experiment must be given. At the very least it must be stated at what beamline the measurement was done. What elements were you looking for? How did you detect the fluorescence signal? Where was the detector placed relative to the sample? I also do not understand the last sentence in this paragraph. What does it mean that the sample “was mounted at a 2mm thickness”? And how does it ensure accuracy, and accuracy of what? |
The details of synchrotron testing with relevant information have been revised. |
Page 4- 5, Line 168-179 |
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16 |
This sentence is not phrased in a good way. |
The sentence has been rephrased. |
Page 5, Line 189-191 |
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17 |
What does it mean that “Ga tends to adsorb on the surfaces of crystal planes”? |
The addition of Ga tends to adhere on the surface and act as a solid solution strengthening. We have improved the sentence. |
Page 5, Line 191-192 |
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18 |
Leading to higher concentrations of what? |
The sentence has been rephrased. |
Page 6, Line 217-220 |
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19 |
This text is difficult to read because it is full of numbers. It would make more sense to present these results in a table. |
The data has been plotted in a graph for clearer information and was added in the manuscript. (Figure 4) |
Page 7, Line 265 |
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20 |
You claim that the IMC grows at the expense of the Cu substrate. But looking at figure 4 it seems like the IMC is growing mostly into the solder material. This needs some clarification. |
Firstly, Figure 4 claimed here has now changed to Figure 5. The higher growth rate of IMC on anode side is due to the higher diffusion of Cu from the substrate into the solder. |
Page 7, Line 251-254 |
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21 |
I don’t see how compressive stress can cause vacancies. This claim needs to be further explained or supported by references. |
The statement has been revised and more details explanation has been added in the manuscript. |
Page 7, Line 255-258 |
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22 |
These results would also be better presented in a table. |
The results have been plotted in Figure 7 which shows the trend of IMC growth at different temperature and solder alloy. |
Page 10, Section 3.3 |
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23 |
I would have expected to see a second intermetallic phase (Cu3Sn) in the thermally aged samples. Have you looked at a thermally aged sample cross-section in SEM to see if it is there? If so, that would be interesting to add to the paper. It would also affect the thickness measurements, since what you have measured is then the total thickness of the IMC layer rather than the thickness of Cu6Sn5. |
Yes, the second intermetallic phase (Cu3Sn) in the thermally aged sample may form but was rather thin and hardly to be observed in our sample. However, in this paper, we are more concerned on the formation of whiskers related to the total IMC thickness and morphology. |
Section 3.3 |
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24 |
Images g) and h) seems to be a bit unfocused. Are you certain the blurring doesn’t affect the thickness measurements? |
The measurements were measured by using threshold method and this could sharpen the IMC boundaries for accurate measurements. The reading has been properly measured and averaged. |
Figure 6 |
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25 |
How many samples of each category were measured, and how large region of each sample? These details should also be stated in the text. |
The detail explanation has been added in the manuscript. |
Figure 4, Line 151-153 |
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26 |
Which conventional analysis method are you referring to? |
Conventional analysis that has been referred was EDX analysis. We have added in the text accordingly. |
Page 10, Line 323 |
Author Response File: 
Author Response.docx
Reviewer 2 Report
This manuscript study on electromigration and thermal ageing characteristics of Sn-0.7Cu-0.05Ga solder. The authors well discussed through both experimental and theroritical approaches. This paper brings useful insights for lead-free electronics packaging, but it needs a minor revision to be acceptable for the coatings.
My major comments are as follows.
- The authors said that “the current at the anode creating a large number of electrons. (L221)”, although, the number of electrons at the anode and cathode is same in my understanding. Could you explain in more detail?
- The manuscript mentioned that Ga is eventually distributed (L317). However, micro-CRF mapping results (Figure 8) of Ga shows a red-spot around the cathode.
- The following two sentences seem to express the same meaning. If they are not, please describe them in detail. “The shear strength has improved significantly when 0.05 wt.% Ga was added to the Sn-0.7Cu solder. Furthermore, the shear strength is also increased to 16.9 MPa, which is a 14% increment relative to the Sn-0.7Cu solder. (L435-437)”
My minor comments are as follows.
- There is no space in front of the units, such as °C (Table1, L217, etc), あand A/cm2 (L217).
- Figure 5 is missing.
- “(a)”, and “(b)” is missing in Figure 8.
I hope these comments will be helpful.
Author Response
Reviewer: 2
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No |
Comment |
Correction |
Line |
|
1 |
The authors said that “the current at the anode creating a large number of electrons. (L221)”, although, the number of electrons at the anode and cathode is same in my understanding. Could you explain in more detail? |
We have revised and improved the sentence to avoid confusion. |
Page 6, Line 219-222 |
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2 |
The manuscript mentioned that Ga is eventually distributed (L317). However, micro-CRF mapping results (Figure 8) of Ga shows a red-spot around the cathode. |
Both figures show the distribution of Ga element in the sample. The sentence has been revised. |
Page 10, Line 325-326 |
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3 |
The following two sentences seem to express the same meaning. If they are not, please describe them in detail. “The shear strength has improved significantly when 0.05 wt.% Ga was added to the Sn-0.7Cu solder. Furthermore, the shear strength is also increased to 16.9 MPa, which is a 14% increment relative to the Sn-0.7Cu solder. (L435-437)” |
Both sentences are explaining the shear strength improvement. We have revised the sentence accordingly. |
Page 15, Line 443-445 |
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4 |
There is no space in front of the units, such as °C (Table1, L217, etc), あand A/cm2 (L217). |
The space has been added in front of the units |
All related pages |
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5 |
Figure 5 is missing. |
The figure numberings have been revised accordingly. |
All figures |
|
6 |
“(a)”, and “(b)” is missing in Figure 8. |
(a) and (b) have been added in the figure. |
Figure 8, Page 11 |
Author Response File: Author Response.docx
Reviewer 3 Report
The reliability of soldered joints for applications in electronics and microelectronics is a very important field of research in modern times. Therefore, the paper entitled "Effect of Electromigration and Thermal Aging to the Tin Whiskers' Formation in Thin Sn-0.7Cu-0.05Ga Lead (Pb) -free Solder Joints" is important in the field of research into the formation of Tin whiskers in solder and solder joints.
The manuscript is correctly and factually conceived and the results of the research confirm the conclusions of the authors.
However, the following comments need to be considered:
- Page 3, line 119: I assume that the dot after the word „alloy“ is a typo. Please fix it.
- Page 5, line 207: please correct the intermetallic phase designation. The numbers should be given in subscripts.
- The conclusion number 4 is not necessary. Increasing diffusion at higher temperatures is a matter of course.
I do not see more important comments that would need to be incorporated. I recommend publishing the manuscript.
Author Response
Reviewer: 3
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No |
Comment |
Response |
Line |
|
1 |
I assume that the dot after the word „alloy“ is a typo. Please fix it. |
The dot has been removed and sentence was rephrased. |
Page 3, line 115-116 |
|
2 |
Please correct the intermetallic phase designation. The numbers should be given in subscripts. |
The IMC designation has been corrected. |
Page 5, line 211 |
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3 |
The conclusion number 4 is not necessary. Increasing diffusion at higher temperatures is a matter of course. |
The conclusion number 4 has been removed as per suggested. |
Page 16 |
Author Response File: Author Response.docx
