Electronic Grade Soldering, Its Principles and Basic Theories

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1.0. INTRODUCTION

The basic theories, on which the formation of a good (electrically conductive and mechanically strong) solder joint depends, have been described in this chapter.

It is essential to understand these theories for the designing of a reliable solder joint.

1.1. DEFINITION

Soldering, is a technique to form a metallurgical bond. It has a history of over 5000 years. It has even been established that the basic theory of soldering was known before it became an inherent part in the electronic interconnection process.

Unlike glueing, soldering is not a surface phenomenon and the joint formed by this method not only provides an electrical connection but also generates mechanical strength. It is also different from brazing, since in the latter phenomenon, the temperature requirement for joint formation is much higher (425°C – 900°C), in comparison to that required for soldering (40°C – 425°C). Ease of repair and rework, and process economy and ease of automation have also made the soldering process a unique phenomenon amongst all metallurgical, mechanical and chemical joining methods. Solder joints are also stable against vibration and the natural oxidation process. Electronic grade soldering covers only a small portion of the complete spectrum of the soldering process which also includes other areas like can seaming, filling dents in automobile bodies and torch soldering. A soft solder joint is required to fulfill the following essential functions:

(a) Provide an electrically conductive path.

(b) Connect components together mechanically.

(c) Allow heat to flow between components.

(d) Retain adequate strength at temperatures from the cryogenic level to within, approximately, 50°C of the solidus of the solder.

(e) Ideally form a tight liquid or gas seal.

To produce reliable solder joints, it is perhaps mandatory to understand its three basic theories fully, namely:

(a) Wetting, non-wetting and dewetting.

Electronic Grade Soldering, Its Principles and Basic Theories

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(b) Surface tension.

(c) Capilliary action.

1.2. WETTING, NON-WETTING AND DEWETTING

If a drop of liquid is put on a clean metallic plate, the liquid will spread completely over the metallic surface and form a thin even film. This phenomenon is called wetting. Exactly the same process happens when molten solder is applied to a perfectly clean PCB pad. The solder spreads over the pad to form intermetallic bonds. It is then said that solder has wetted the metal. However, due to the presence of impurities like grease, dirt, etc. and also due to the natural oxidation of the metallic surface, the angle of spreading, called the dihedral angle–(θ) is never equal to zero, the condition of total wetting. In reality, the dihedral angle θ may assume any of four ranges:

1. If θ < 20°, the corresponding wetting is called good wetting.

2. If 20° > θ > 75°, the corresponding wetting is called marginal wetting.

3. If 180° > θ > 90°, the corresponding wetting is called dewetting. In dewetting, solder spreads over the metal initially but recedes later, to leave a thin solder layer.

4. If θ = 180°, the condition is called non-wetting.

The above classification of wetting is depicted in Figure 1.1.

θ

θ

θ θ

θ < 20°

(Good wetting)

180° >θ > 90°

(Dewetting)

20° >θ > 75°

(Marginal wetting)

θ = 180°

(Non-wetting)

Fig. 1.1. Classification of wetting

The factors which influence the magnitude of the dihedral angle in order to realise the basic need for the formation of a good solder joint are examined below:

(a) Flux, which is used to remove the oxidation layer from the metallic surface, decreases the dihedral angle.

(b) A sudden decrease of the dihedral angle occurs beyond a certain temperature, called the critical temperature.

(c) The dihedral angle is less for a rough surface as compared to a smooth surface.

(d) Cohesive force of the molten solder opposes the decrease of the dihedral angle.

From the above discussion it is concluded that:

(a) In the case of wetting, solder becomes an integral part of the base metal.

(b) A perfectly clean base metal is essential to realise complete wetting of it by the solder.

(c) The dihedral angle, which is the interfacial angle between the solid (base metal) and liquid (molten solder), is an indication of the nature of the wetting.

(d) Flux, when activated at a higher temperature, contributes in achieving good wetting.

(e) Dewetting and non-wetting indicate either defects in the solderability of the base metal or the presence of a higher level of contamination in the solder.

Fig. 1.2. Good wetting in a single sided board

Fig. 1.3. Good wetting in a double sided, PTH board

Figures 1.2 and 1.3 are examples of good wetting in a single and double sided PWB with plated through hole (PTH), respectively. In both cases, the solder has wetted the component leads and mounting pads.

1.3. SURFACE TENSION

If mercury is dropped on a clean floor, it will break up into small spherical globules. Since amongst all possible three dimensional bodies, a sphere has the minimum surface area, and since mercury is a liquid, it may be concluded that any liquid has a tendency to acquire minimum surface area. This contractile property of a liquid is called surface tension. When solder remains in the molten state, it is also governed by the rule of surface tension and hence has an inherent tendency not to spread over the metallic surface. This process subsequently leads to conditions of partial wetting or even total non-wetting. Fortunately, however, the force of gravity and interfacial tension between the base metal and the surroundings tries to reduce the force of surface tension.

In a tin-lead solder alloy, surface tension decreases with an increase in the percentage of lead in the alloy. Fluxes also reduce surface tension. Hence, determination of surface tension is a good test in the incoming inspection level of foam fluxes and organic coatings on assembled PCBs. However, since the test is a temperature dependent test, the utmost care is required to obtain reliable test data.

Surface tension also increases the PWB cleaning solvent’s ability to penetrate into crevices and displace other materials. Hence, the determination of surface tension of a cleaning solvent is an important test when the static cleaning mechanism is used.

1.4. CAPILLIARY ACTION

If two clean metal surfaces are held together, and dipped into molten solder, the solder will wet the metal and climb up to fill the gap between the adjacent surfaces.

This is known as capilliary action. When a PCB with plated through holes (PTH) is soldered, it is this capilliary force which fills the holes, and produces a concave meniscus on the upper surface. Hence, the common belief that it is the pressure of the solder wave or the extra solder that pushes the solder up into the hole, is not correct. However, it is to be remembered that to achieve capilliary action in order to obtain complete solder joint fillet, the following factors are essential:

1. The solder must wet the PTH.

2. The annular cross-section left between the lead and the PTH must be small.

This, in turn, dictates the standardisation of lead to hole size ratio, which states that the difference between the diameter of the hole and the lead shall not be greater than 0.5 cms.

3. The PTH finish should be free from microcracks or similar processing defects.

It has been observed that molten solder spreads more easily over a rough surface than over a smooth surface. This is because, each groove of the rough surface acts like a capilliary tube. The process is known as capilliary hysteresis.

The strength of the solder joint depends critically on the annular space left between the component lead and the plated through hole (Fig. 1.4). If the space is less,

Fig. 1.4. Optimum annular space in PTH joint

trapping of flux will result in a decrease in the wetted area, while, in the case of a larger space, the capilliary force will be less. In both cases, the ultimate result will be the formation of a solder joint with less shearing strength. Hence, such solder joints will be mechanically weak.

Further, it is to be noted that the capilliary force alone is sometimes not sufficient to fill a hole in multilayer boards of more than four layers. In such cases, artificial agitation on the top surface of the solder wave is created to supplement the capilliary force so as to obtain adequate filling of the multilayer PTH.

Finally, although a complete fillet as shown in Figure 1.3, is desirable, as a result of capilliary action, it is often found that the top on the board filling has not been achieved (Figure 1.5) due to the downward action of the gravitational force. In addition any crack in the plating of a PTH also restricts capilliary rise action.

Fig. 1.5. Partial PTH filling due to gravitational force

1.5. INTERMETALLIC LAYER FORMATION

It is essential to understand the mechanism and the effect of formation of an intermetallic layer during the process of soldering. When two plates of similar or dissimilar base metals are joined together using solder, an intermetallic compound is usually formed at the joints (Figure 1.6). For example, if two copper plates are joined together using tin-lead solder, tin interacts with copper to form intermetallic compounds in the form of either C₆Sn₅ or Cu₅Sn.

For the formation of a strong mechanical joint the formation of such intermetallic compounds is necessary. These compounds are also believed to reduce surface tension, resulting in better wetting.

Due to the formation of ionic bonding, in the case of an intermetallic compound, it is harder and more brittle as compared to the alloys from which it is formed and it also possesses a different coefficient of thermal expansion. Further, although the

Solder Base metal

I

Base metal II Intermetallic

compounds

Fig. 1.6. Formation of intermetallic compound

thickness of the intermetallic compound layer is usually of the order of 1 µm, it is found to grow linearly with the square root of time, at a specified temperature (Fig. 1.7). Hence, it is desirable to form solder joints at the lowest possible temperature in the minimum possible time. This also leads to the conclusion that rework at any stage is not admissible since in each rework (touch up) action, joints are subjected to heat. Figure 1.8 depicts an example of a cracked joint which is a result of rework.

(170°C)

(25°C) (100°C) (135°C)

0 10 20 30 40

Time in days

11 22 33

Intermetalliccompound layerthickness(m)µ

Fig. 1.7. Growth of intermetallic layer due to temperature

Cracks

Fig. 1.8. Cracked joints due to improper rework

It is essential to know the possible intermetallic compounds, which may be formed by the chemical action of tin present in tin-lead solder with base metals, in order to understand a soldering problem. Table 1.1, depicts such compounds.

Table 1.1. Common intermetallic compounds of tin with base metals Base metal Common intermetallic compounds

Cu (Copper) Cu₆ Sn₅

Cu₃ Sn

Bi (Bismuth), Zn (Zinc), Cd (Cadmium), Pb (Lead) None

Au (Gold) Au Sn₂

Au Sn Au Sn₄

Ag (Silver) Ag₃ Sn

Sb (Antimony) Sb Sn

Ni (Nickel) Ni₃ Sn

Ni Sn₃

In (Indium) In Sn

In Sn₄

Fe (Iron) Fe Sn

Fe Sn₂

qqq

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2.0. INTRODUCTION

Soldering materials play a significant role in designing reliable solder joints. In this chapter, the requirements, constituents, classifications and properties of basic soldering materials, namely, solder, flux, adhesive and PCB cleaning solvents are described, with corresponding tests, wherever applicable. Special emphasis has been paid to their domain of application. The materials cover the process requirements of both through hole and SMD components.

2.1. SOLDER ALLOY, COMPOSITION, PHASE DIAGRAM

By definition, an alloy is a mixture of two or more metals having basic parameters different from its consituents. In the case of soldering, used as a common electronic interconnection method, the soldering temperature remains in the range of 70.4°C to 422.4°C. This process is called soft soldering and the alloy used for such a purpose is called a soft solder. Though the main constituent elements of soft solders are tin (Sn) and lead (Pb), other elements like bismuth (Bi), indium (In), silver (Ag) and antimony (Sb) are added to obtain different properties like lowering of the melting point, prevention of silver migration etc. It is important to note here that the basic characteristics like the melting point of an alloy change with a change in its composition. This can be depicted using a phase diagram.

As described above, a binary alloy comprising of tin and lead, is the basic solder used in electronic grade soldering. However, tin-lead solder also consists of antimony, bismuth, silver, copper, cadium, etc. as associated elements, permissible only upto a very small, limited, configuration. The most common solder specifications are described in the Americian Standard for Test and Measurement (ASTM) specification B-32 (Table 2.1) and the federal specification QQ-S-571 (Table 2.2).

Figure 2.1, represents the phase diagram of a tin-lead solder, which is comprised of three distinct states of the solder alloy, namely, liquid, plastic and solid. From the figure, it is evident that for a particular composition of the alloy, the plastic state is absent. This composition, 63% tin and 37% lead, is called a eutectic alloy and the corresponding melting point (183°C) is called the eutectic point. At the eutectic point, the solder goes directly from solid to liquid and vice-versa.

Material Requirements

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Table 2.1. Composition of various solder alloys as listed in ASTM B-32-70 Composition percent AlloyTinLeadAntimonySilverBismuthCopperIronAluminiumZincArsenic GradeDesired NominalMin.DesiredMax.Min.DesiredMax.Max.Max.Max.Max.Max.Max. 70A7030– – 0.12– – – 0.250.080.020.0050.0050.03 70B70300.20– 0.50– – – 0.250.080.020.0050.0050.03 63A6337– – 0.12– – – 0.250.080.020.0050.0050.03 63B63370.20– 0.50– – – 0.250.080.020.0050.0050.03 60A6040– – 0.12– – – 0.250.080.020.0050.0050.03 60B60400.20– 0.50– – – 0.250.080.020.0050.0050.03 50A5050– – 0.12– – – 0.250.080.020.0050.0050.025 50B50500.20– 0.50– – – 0.250.080.020.0050.0050.025 45A4555– – 0.12– – – 0.250.080.020.0050.0050.025 45B45550.20– 0.50– – – 0.250.080.020.0050.0050.025 40A4060– – 0.12– – – 0.250.080.020.0050.0050.02 40B40600.20– 0.50– – – 0.250.080.020.0050.0050.02 40C40581.82.02.4– – – 0.250.080.020.0050.0050.02 35A3565– – 0.25– – – 0.250.080.020.0050.0050.02 35B35650.20– 0.50– – – 0.250.080.020.0050.0050.02 35C3563.21.61.82.0– – – 0.250.080.020.0050.0050.02 30A3070– – 0.25– – – 0.250.080.020.0050.0050.02 30B30700.20– 0.50– – – 0.250.080.020.0050.0050.02 30C3069.41.41.61.8– – – 0.250.080.020.0050.0050.02 25A2575– – 0.25– – – 0.250.080.020.0050.0050.02 25B25750.20– 0.50– – – 0.250.080.020.0050.0050.02 25C2573.71.11.31.5– – – 0.250.080.020.0050.0050.02 (Contd...)

Table 2.1 (Contd...)Composition of various solder alloys as listed in ASTM B-32-70 Composition percent AlloyTinLeadAntimonySilverBismuthCopperIronAluminiumZincArsenic GradeDesired NominalMin.DesiredMax.Min.DesiredMax.Max.Max.Max.Max.Max.Max. 20A20800.20– 0.50– – – 0.250.080.020.0050.0050.02 20C20790.8101.2 – – – 0.250.080.020.0050.0050.02 15B15850.20– 0.50– – – 0.250.080.020.0050.0050.02 10B10900.20– 0.50– – – 0.250.080.020.0050.0050.02 5A5 95– – 0.12– – – 0.250.080.020.0050.0050.02 5B5 950.20– 0.50– – – 0.250.080.020.0050.0050.02 2A2 98– – 0.12– – – 0.250.080.020.0050.0050.02 2B2 980.20– 0.50– – – 0.250.080.020.0050.0050.02 2.550 97.5– – 0.402.32.52.70.250.080.020.0050.0050.02 1.551 97.5– – 0.401.31.51.70.250.080.020.0050.0050.02 951A950.20 max.4.55.05.5– – – 0.150.080.040.0050.0050.05 95.51S96.50.20 max.0.20– 0.503.33.53.70.150.080.020.0050.0050.05