The area of spread solderability test use for roughness influence assessment

978–1–4244–7850–7/2010/$26.00 ©2010 IEEE 160 33rd Int. Spring Seminar on Electronics Technology

The Area of Spread Solderability Test Use for Roughness Influence Assessment

Tomáš Novák, František Steiner

Department of Technologies and Measurement, University of West Bohemia, Pilsen, Czech Republic Email address: [email protected], [email protected]

Abstract: The article deals with the influence of surface roughness on solderability. The area of spread test was used as a method for solderability measurement. For this measurement, test samples made from substrates for printed circuit boards, small solder balls, and two soldering technologies – vapor-phase and reflow soldering - were used. These technologies have been used because of similarity with the industrial process of soldering. The subjects of examination and comparison were wetting angle and expanse of melted solder on the surface. The paper presents results and conclusions from solderability testing of printed circuit boards with various surface roughnesses. Other conclusion is comparison of several types of fluxes and their impact on solder wetting on roughened surface.

1. INTRODUCTION

Soldering is one of the most important processes in electronic device production. The objective of soldering is to achieve mechanically unyielding, electrically conductive, and in the long term reliable joint. There are several tests to estimate how suitable for soldering process material is. For use in soldering, materials must have suitable solderability.

Solderability is not only an ability of solder flushing on the surface. Solderability is a complex of properties which designates how much is the material suitable for industrial soldering. These properties are for example good wetting, mechanical and chemical straining immunity during cleaning, or thermal straining immunity of Printed Circuit Boards (PCBs). Solderability is not invariable parameter. During time, it changes according to surrounding effects which influence material surface. Solderability gets worse in consequences of surface corrosive change, inception of intermetallic adducts on material surface or the way of holding in storage. The material can be kept on air, where can oxidize, or can be kept in boxes with inert atmosphere.

Wetting is nearly related to solderability, which has already been mentioned. Wetting can be explained as an ability of surface, which determines how the

surface could be wetted by molten solder. To achieve good wetting, the surface must be quit of all contaminations. Surface roughness has influence on wetting as well. To determine the influence of surface roughness on solderability, several tests of solderability on specimen of PCBs with different roughness level were performed.

During surface wetting with molten solder, physiochemical activity of surface atoms between connecting parts and molten solder occurs. Further, one interphase coupling from connecting surface and molten solder arises. Surface wetting is divided into several levels. Critical parameter of surface wetting is contact (wetting) angle between drop of molten solder and wetting surface. Contact angle θ is shown in Figure 1.

Fig. 1. Contact angle θ.

Material can be designated as “good wetting” if the contact angle θ is between 0° and 50°, when contact angle θ is between 50° and 90° the material is “poorly wetting”. Materials are “non-wetting”, when contact angle θ is above 90° [1].

wetting non-wetting

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2. SURFACE ROUGHNESS

Real surface differs from the ideal surface by various asperities. Surface roughness is geometrical asperities with relatively small spacing. These asperities arise in production or owing to production. Surface roughness is one of factors affecting soldering process. It mainly affect the process of wetting and flushing of molten solder. Surface roughness reduces effective contact angle θ+, which is related to ideal plain surface contact angle θ. It describes quadratic (1) [2]:

+θcos = θcos⋅r (1)

where r is defined as proportion of roughness of real and ideal plain surface.

As ensue from quadratic (1), contact angle θ+ is smaller than θ. It means, that solder will spread better on roughness surface than on ideal plain surface. Further, surface roughness should provide better mechanical gripping of solder on wetting surface [1],[2].

Surface roughness is characterized by two basic statistical parameters. First parameter is arithmetic mean of roughness Ra, i.e. average value of absolute values of profile deviations yi in the range of primary length L, see quadratic (2). Next parameter is quadratic mean value Rq, sometimes marked as RRMS (root mean squared). It is quadratic average value of all profile roughness deviations, see quadratic (3). Values of deviations are deducted from the middle curve of profile. Middle curve divides real profile into two parts in which the sum of areas on both sides is equal in whole range of primary length L [1],[2].

Ra = ∫L

dxxyL 0

|)(|1 (2)

Rq = ⎟⎟⎠

⎞⎜⎜⎝

⎛∫L

dxxyL 0

2 |)(|1 (3)

Surface topography is most often established by microscope. SPM (Scanning Probe Microscopy) are scanning microscopes, which create increased 3D scan of surface. There are two basic types of SPM, Atomic Force Microscope (AFM), and Scanning Tunnelling Microscope (STM) [1].

3. SOLDERABILITY TESTING

For solderability testing, several tests could be used, for example: dipping test, globular test, wetting balance test, area of spread test.

To determine the effect of surface roughness on solderability and solderability testing, the area of spread test was used. Area of spread test was chosen to simulate reflow soldering process. In reflow soldering, there is only limited amount of solder, which is for example exactly defined amount of solder paste on soldering pads. For testing, two technologies of solder reflow were used. The first method simulated process of reflow soldering by hot air. Increase in temperature causes a higher oxidation of connected materials and solder alloy during soldering in a normal atmosphere. Today, inert atmosphere are used for elimination of oxides creation. Vapor-phase soldering is the second method of solder reflow used for testing. For measurement, surfaces of tested samples were purposely roughened by various abrasive papers. For spreading test, surface roughness was not oriented. The tested sample is shown in Figure 2.

a) b)

Fig. 2. Photos of tested sample: a) sample, b) detail on solder drop.

978–1–4244–7850–7/2010/$26.00 ©2010 IEEE 162 33rd Int. Spring Seminar on Electronics Technology

3.1. Area of Spread Method

The area of spread method is often used in the laboratory. This test is used, when we want an easy and quick find out effect of wettability of the base metal by the solder alloy. The test is also often applied for the testing of fluxes efficiency with using known base metal and solder alloy. This method gives only qualitative information for base metal, which is very easily wetted. If the wetting is not so good, the solder will form shape of spherical drop. We can measure dimensions of drop. In these cases, the area of spread method is possibility for a quantitative assessment of wettability. The evaluation of the area of spread method is often expressed as a spread factor Sf or contact angle θ. Calculation of spread factor is describe in relation (4)[5].

%100⋅−

=D

hDS f (4)

Where D is defined as diameter of spherical drop made of the amount of solder after test. The h is measured height of the solder drop after spreading.

The contact angle is related to the spread factor. Relation of spread factor and contact angle is:

%100

2cot31

113 2

⋅

⎟⎠⎞

⎜⎝⎛ +

−=θ

fS (5) for 0° < θ < 180° [5].

The contact angle is possibility also express relation (6) according to Figure 3[5].

22

22sinhb

bh

Rh

hR +

=+

=θ ; bD 2= (6)

Fig. 3. Geometry of spherical drop [5].

3.2. Tested Specimens

For determination of surface roughness effect on solderability, samples of PCBs with copper surface finish, see Fig. 2, and 1,5 mm thickness were tested.

Tested samples differed in their surface roughness. Surface roughness of single sample was created by adhesive paper. Marking roughness was 120, 180, 240, 320 and 400. These numbers sign roughness of used abrasive paper. Scratches made by adhesive paper were not oriented. To determine the effect of surface roughness, samples without additional roughage were tested too. These samples are marked as NO.

4. RESULTS OF MEASUREMENT

Used surface roughnesses and measured values of roughness are shown in Tab. 1.

Roughness

Ra [μm] Rq [μm]

120 2.194 2.676

180 1.211 1.684

240 0.749 0.951

320 0.592 0.771

400 0.336 0.430

NO 0.230 0.283

Tab. 1. Surface roughness of samples.

For area of spread test, two technologies of solder recast were used. These technologies were vapor-phase soldering and reflow soldering by reflow oven. Two kinds of fluxes, which are Actiec2 and Actiec5 were used for testing. The Actiec2 has lower activity

978–1–4244–7850–7/2010/$26.00 ©2010 IEEE 163 33rd Int. Spring Seminar on Electronics Technology

than Actiec 5. Three solder balls were applied on sample. Each combination of roughness, flux and technology for recast solder were tested on sample with solder balls. The diameter of a solder ball is 0.5 mm before solder reflows. So small balls were chosen therefore we could neglect a gravitation force. The measured values from the area of spread test, which are diameter and height of spherical drops, are presented in Tab. 2 and Tab. 3. The diameters of spherical drops were measured by confocal microscope Olympus SZX10. The resultant value of diameter is an average of all measured values for each combination. The height of spreading solder balls were measured by micrometer. Comparison and evaluation of measured values is resumed in “Conclusions”. The values from Tab. 2 and Tab. 3 are present in Fig. 4 and Fig. 5.

flux Actiec2 – A2 Actiec5 – A5

height – h

diameter - D h [mm] D

[mm] h [mm] D

[mm]

roughness

120 0.288 0.688 0.190 0.881

180 0.347 0.585 0.212 0.835

240 0.391 0.506 0.263 0.738

320 0.324 0.664 0.583 0.681

400 0.360 0.529 0.237 0,824

NO 0.347 0.501 0.307 0.517

Tab. 2. Measured values of spreading solder balls – reflow soldering.

120

- A2

180

- A2

240

- A2

320

- A2

400

- A2

no -

A2

0,0

0,2

0,4

0,6

0,8

1,0

diam

eter

[mm

]

surface roughnessreflow soldering vapor-phase soldering

Fig. 4. Chart of measured values – flux Actiec2.

flux Actiec2 – A2 Actiec5 – A5

height – h

diameter - D h [mm] D

[mm] h [mm] D

[mm]

roughness

120 0.152  0.930  0.164  0.932 

180 0.167  0.929  0.171  0.896 

240 0.177  0.901  0.175  0.930 

320 0.179  0.907  0.205  0.844 

400 0.185  0.879  0.183  0.913 

NO 0.186  0.837  0.159  0.887 

Tab. 3. Measured values of spreading solder balls – vapor-phase soldering.

120

- A5

180

- A5

240

- A5

320

- A5

400

- A5

no -

A5

0,0

0,2

0,4

0,6

0,8

1,0

diam

eter

[mm

]

surface roughnessreflow soldering vapor-phase soldering

Fig. 5. Chart of measured values – flux Actiec5.

For visual demonstration, pictures of tested samples and surface roughness are shown in Fig. 6 to Fig. 14.

a) b)

Fig. 6. Solder drops after reflow soldering: a) Actiec2, b) Actiec5. Roughness 120.

978–1–4244–7850–7/2010/$26.00 ©2010 IEEE 164 33rd Int. Spring Seminar on Electronics Technology

a) b)

Fig. 7. Solder drops after vapor-phase soldering: a) Actiec2, b) Actiec5. Roughness 120.

Fig. 8. Surface roughness 120.

a) b)

Fig. 9. Solder drops after reflow soldering: a) Actiec2, b) Actiec5. Roughness 240.

a) b)

Fig. 10. Solder drops after vapor-phase soldering: a) Actiec2, b) Actiec5. Roughness 240.

Fig. 11. Surface roughness 240.

Fig. 12. Solder drops after reflow soldering: a) Actiec2,

b) Actiec5. Roughness 400.

a) b)

Fig. 13. Solder drops after vapor-phase soldering: a) Actiec2, b) Actiec5. Roughness 400.

Fig. 14. Surface roughness 400.

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Next parameters are a spread factor and a contact angle for evaluation of area of spread method. The contact angle is possible take off from chapter in Fig. 15, when we know the spread factor. The spread factor values ensue of quadratic (5) and in Tab. 4 and Tab. 5 are shown [5].

flux Actiec2 – A2 Actiec5 – A5

spread factor Sf [-] Sf [-]

120 0.581 0.784

180 0.408 0.746

240 0.227 0.644

320 0.513 0.143

400 0.319 0.713

NO 0.308 0.406

Tab. 4. Spread factor values – reflow soldering.

flux Actiec2 – A2 Actiec5 – A5

spread factor Sf [-] Sf [-]

120 0.837 0.824

180 0.821 0.809

240 0.804 0.812

320 0.803 0.757

400 0.789 0.800

NO 0.778 0.821

Tab. 5. Spread factor values – vapor-phase soldering.

Fig. 15. Relation between Sf and θ for spherical caps [5].

5. CONCLUSION

It is perceptible from measured values that activity of Actiec5 flux more assisting spreading of melted solder on surface in comparison with Actiec2 flux. The diameters values of spreading solder achieved by vapor-phase soldering were bigger than the values achieved by reflow soldering. Bigger roughness makes bigger area of spread by solder. This statement does not apply for combinations of Actiec5 flux with vapor-phase technology and each levels of surface roughness. In these cases are values of diameters and heights very similar for all levels of roughness.

ACKNOWLEDGEMENTS

This paper is part report and has been supported by the research plan of Ministry of Education, Youth and Sports of Czech Republic No. MSM4977751310 “Diagnostic of Interactive Processes in Electrical Engineering“.

REFERENCES

[1] Novak, T; Steiner, F. Surface Roughness Influence on Solderability. 32ST INTERNATIONAL SPRING SEMINAR ON ELECTRONICS TECHNOLOGY. Presented at 32ND International Spring Seminar on Electronics Technology in Brno, CZECH REPUBLIC, ISBN: 978-1-4244-4260-7, MAY 13-17, 2009.

[2] HUMPSTON, G., JACOBSON, D., M. Principles of Soldering, ASM International, ISBN 0-87170-792-6, 1,vydání, 2004

[3] Harant, P; Steiner, F. Double-sided Preheating Influence on Solderability. 31ST INTERNATIONAL SPRING SEMINAR ON ELECTRONICS TECHNOLOGY - RELIABILITY AND LIFE-TIME PREDICTION : 379-384. edited by IllyefalviVitez, Z. Presented at 31st International Spring Seminar on Electronics Technology in Budapest, HUNGARY, ISBN: 978-1-4244-3973-7, MAY 07-11, 2008.

[4] Stary, J; Leadfree soldering – materials and process compatibility in inert and inert reducing atmosphere, Disertation thesis, Brno, 2005

[5] R. J. K. WASSINK, “Soldering in Electronics” – Electrochemical publications Limited, Bristol, England, second printing, 1994.

[6] Harant, P; Steiner, F. Solderability of lead-free surface finished PCB. 30TH INTERNATIONAL SPRING SEMINAR ON ELECTRONICS TECHNOLOGY: 388-392. Presented at 30th International Spring Seminar on Electronics Technology in Cluj Napoca, ROMANIA, ISBN: 978-1-4244-1217-4, MAY 09-13, 2007.