Influence of Electrode Surface Condition of Electronic Components on Reliability of Interconnect Soldering

The base metal materials and their characteristics used in the pins (electrodes) of various modern Electronic components, as well as the welding performance of various anti-corrosion and weldability protective coating materials that may be used on the base metal. The physical and chemical reactions occurring in the coating, the composition, density, brightness, impurity content, etc. of the coating affect the welding reliability, so as to select the coating with the best oxidation resistance, weldability, and corrosion resistance, and Obtaining the optimum process conditions for this coating is one of the important factors in ensuring the reliability of soldered interconnects.IC, LSI, and VLSI have been widely realized in modern electronic products, and the electrode materials used for them

1. From the point of view of reliability

The base metal materials and their characteristics used in the pins (electrodes) of various modern electronic components, as well as the welding performance of various anti-corrosion and weldability protective coating materials that may be used on the base metal. The physical and chemical reactions occurring in the coating, the composition, density, brightness, impurity content, etc. of the coating affect the welding reliability, so as to select the coating with the best oxidation resistance, weldability, and corrosion resistance, and Obtaining the optimum process conditions for this coating is one of the important factors in ensuring the reliability of soldered interconnects. IC, LSI and VLSI have been widely realized in modern electronic products, and more and more attention is paid to the electrode materials used in them.

For example, the resistivity, coefficient of thermal expansion, mechanical strength at high temperature, material and shape of the material must be carefully considered. The basic requirements for pin (electrode) materials used in the modern electronics industry are:

● better electrical conductivity and thermal conductivity;

●The thermal expansion coefficient should be small;

●The mechanical strength should be large;

●The processing properties such as stretching and punching are better. At present, the commonly used pin materials can be divided into two categories: Fe-Ni-based alloys and Cu-based alloys.

2. The influence of the materials used for the pins of electronic components on the reliability of welding

1. Fe-Ni-based alloys 1) Features and application range Fe-Ni-based alloys, such as Kovar, were originally developed as alloys for glass packaging. Its thermal expansion curve is similar to that of Si of an IC chip, as shown in Figure 1. In addition, it can also be used for direct welding as a welding material for Au-Si welding. Therefore, it is commonly used as a lead material in MOS series devices. The representative alloy of Fe-Ni-based alloy system is 42 alloy, which is widely used as electrode material for ceramic package chips because of its high mechanical strength and small thermal expansion coefficient.

Influence of Electrode Surface Condition of Electronic Components on Reliability of Interconnect Soldering

figure 1

2) Commonly used brand components and their characteristics The characteristics of the main Fe-Ni-based alloys are shown in Table 1. Table 1

Influence of Electrode Surface Condition of Electronic Components on Reliability of Interconnect Soldering

Since this alloy has the characteristics of high magnetic properties and high resistivity, it is not enough as a lead material. Therefore, it is dedicated to MOS-type IC devices that consume less power and generate less heat.

2. Copper-based alloys When electronic circuits enter the stage of large integration and high-density assembly, the resistance heat that occurs on their pins has become a problem that cannot be ignored. Therefore, new Cu-based alloys with good thermal conductivity, electrical conductivity and good mechanical properties at high temperatures are widely used to replace Fe-Ni-based alloys to meet the development requirements of component lead materials, which has become the focus of the electronic components industry. The problem. Because Cu-based alloys have good electrical and thermal conductivity, good heat dissipation, and have an advantage in price compared with 42 alloys, they are widely used in plastic packaged chips.

3. In order to satisfy both mechanical strength and heat dissipation, Cu-clad stainless steel lead materials are being developed in Japan using stainless steel (SUS430 series) as the core material, and then plated with oxygen-free copper on both sides at a ratio of 10/80/10. New leadframe material for cladding.

3. The influence of solderability coating of pins on soldering reliability

1. Solderability refers to the ability of the metal and its metal coating surface to wet the solder. This ability is usually evaluated by measuring the actual wetting area and the minimum wetting time of molten solder on it under the conditions of the specified flux and temperature.

2. Solderability state classification The wetting state of solder on metal and its metal coating can be divided into the following three types. (1) Wetting: The solder can form a uniform, smooth and complete thin layer of solder on the surface of the base metal, as shown in Figure 2.

Influence of Electrode Surface Condition of Electronic Components on Reliability of Interconnect Soldering

figure 2

(2) Dewetting: The brazing filler metal covers a thin layer of brazing filler metal on the surface of the base metal, leaving some irregular small particles or nodules composed of the brazing filler metal, but the base metal is not exposed. It is also referred to as “semi-wetting”, as shown in Figure 3.

Influence of Electrode Surface Condition of Electronic Components on Reliability of Interconnect Soldering

image 3

(3) Non-wetting (Non-wetting): The solder leaves only some separated, irregular strips or granular solder on the surface of the base metal, which are covered by some small-area thin layers of solder and partially exposed matrix surrounded by a metal area, as shown in Figure 4.

Influence of Electrode Surface Condition of Electronic Components on Reliability of Interconnect Soldering

Figure 4

3. The welding process of the weldable coating is a process in which the molten solder and the crystalline structure of the base metal to be welded combine the metal and the metal through an alloy reaction. Many single metals or alloys can undergo metallurgical reactions with SnPb, SnAgCu and other solders to generate IMC. In theory, they can be used as solderable coatings.

According to the different melting state during welding, it can be divided into three categories:

(1) Fusible coating: the coating metal melts at the welding temperature, such as Sn, Sn-Pb alloy coating, etc.

(2) Soluble coating: The coating metal does not melt at the soldering temperature, but it is soluble in solder alloys, such as Au, Ag, Cu, Pd, etc., as shown in Figure 5.

Influence of Electrode Surface Condition of Electronic Components on Reliability of Interconnect Soldering

Influence of Electrode Surface Condition of Electronic Components on Reliability of Interconnect Soldering

Figure 5

(3) Non-melting and insoluble coating: the coating metal will neither melt nor dissolve in the solder at the welding temperature, such as Ni, Fe, Sn-Ni, etc.

4. Solderability Evaluation of Solderable Coatings 1) Factors Affecting the Solderability of the Coating Soldering process conditions (solder and flux, soldering parameters and process methods), etc.

It can be summarized as follows.

(1) The surface of the base metal coating is oxidized.

●The lead wire is not thoroughly cleaned after coating, and there may be acid residues such as chloride ions and sulfides on the surface. These residues oxidize the surface of the coating when they come into contact with oxygen and moisture in the air. The melting point of Sn or Pb oxides is very high, for example, the melting point of PbO is 888 °C; the melting point of PbS is 1114 °C, and the melting point of SnO2 is 1127 °C. Oxides of Sn, Pb, etc. cannot be melted at normal soldering temperature, forming harmful substances covering the surface of the plating layer, thereby causing deterioration of lead solderability.

●Even if the surface-cleaned lead wire is stored in poor conditions, and it is placed in humid air or harmful gas containing acids, alkalis and other substances for a long time, the plated metal on the lead wire surface will oxidize, causing white spots or yellowing on the lead wire surface. , blackened.

(2) Poor surface treatment of lead base metal. When there are metal oxides or grease on some metal surfaces before the lead is coated, these substances will reduce the bonding force between the metal coating and the base metal, resulting in virtual soldering and desoldering.

(3) Poor lead plating. If the coating is too thin or the coating is discontinuous or loose, with pinholes, it will affect the storage performance of the leads and deteriorate the solderability. Plating Sn and SnPb alloys on the Cu surface can prevent Cu oxidation. However, due to the loose pinholes in the coating, a channel is formed between the surface of the substrate Cu and the air, resulting in the following consequences:

●Oxygen and moisture in the atmosphere contact the surface of the base metal through the pinholes in the coating to oxidize and corrode the base metal.

●Since the standard electrode potential of Sn and Pb is negative than that of Cu, it is a cathodic coating. When the moisture passes through the pinholes in the coating and contacts the surface of the base metal, a micro-battery will be formed, and the coating metal Sn or SnPb alloy will be corroded. 2) The influence of metal diffusion layer In electroplating, the Sn and SnPb alloys of the coating layer are atomically bonded to the surface of the base metal Cu, while there are Cu6Sn5 compounds between the hot dip coating Sn and the base metal Cu. This compound can make the coating Sn adhere to the base metal, but as time goes on, the diffusion between the base metal Cu and the coating metal Sn continues, and if the alloy layer grows too thick, it is possible to grow a very thin Cu3Sn compound, which will reduce the Weldability affects weld strength.

5.Influence of Lead Solderability Plating on Soldering Reliability

1) Au coating

(1) Coating characteristics. The coating has good decoration, corrosion resistance and low contact resistance, and the coating has excellent solderability and is easily soluble in solder. Corrosion resistance and solderability depend on adequate coating thickness and porosity. The porosity of the thin coating is prone to copper diffusion, which brings about oxidation problems and leads to poor solderability. And too thick coating will cause weak welding joint due to the brittleness of Au. Many companies use ENIG Ni/Au as a surface coating with success. However, when using ENIG Ni/Au coatings in combination with BGA, sometimes the results are unpredictable. Two failure modes have emerged in recent years:

The first failure mode is non-wetting or semi-wetting, a phenomenon known as “black pads”;

• The second failure mode is interlaminar cracking associated with mechanical stress.

(2) Coating thickness. The gold plating layer for soldering is 24k pure gold with a columnar structure, excellent electrical conductivity and solderability. Its thickness: 1st grade: 0.025~0.05μm; 2nd grade: 0.05~0.075μm; 3rd grade: 0.127~0.254μm.

2) Ag coating

(1) Coating characteristics. Ag has the best thermal conductivity, electrical conductivity and solderability at room temperature, and is stable in other acids except nitric acid. Ag has good polishability, strong reflective ability, small high frequency loss, and high surface conductivity. However, Ag has a very high affinity for S, and a small amount of S (H2S, SO2 or other sulfides) in the atmosphere will make it discolored, resulting in Ag2S, Ag2O and loss of solderability. Another disadvantage of Ag is that Ag ions can easily migrate along the surface and volume of insulating materials in a humid environment, deteriorating the insulating properties of the material or even short-circuiting.

(2) Electroless Ag plating. Electroless Ag layers can be both soldered and “bonded” (bonded), so they are generally valued. The electroless Ag layer is also immersion Ag in nature. The standard electrode potential of Cu is φ oCu+/Cu=0.51V, and the standard electrode potential of Ag is φ oAg+/Ag=0.799V, so Cu can replace Ag ions in the solution to form a deposited Ag layer on the surface of Cu.

3) Ni coating

(1) Coating characteristics. Ni has good corrosion resistance and is easily passivated in the air to form a dense oxide film, so its own welding performance is very poor. But it is this oxide film that gives it high corrosion resistance, strong alkali resistance, slow action with hydrochloric acid and sulfuric acid, and is only soluble in nitric acid. Ni plating of welding parts is mainly to prevent the diffusion of the underlying metal Cu to the surface Au layer. In fact, it acts as a barrier layer, so the stress of the Ni plating layer is required to be low, and the bonding force between the Cu and Au layers is better.

(2) Coating thickness. The Ni plating layer is divided into the following two types.

●Semi-bright Ni: also known as low-stress Ni or dumb Ni, low-stress Ni is suitable for welding or crimping, and is usually used as the bottom layer of gold plating on the board;

●Bright Ni: It is used as the bottom layer of gold plating of the plug, and can also be used as a surface layer according to needs. The bright Ni layer is uniform, fine and bright, but cannot be welded. The Ni-plated layer should be uniform and dense, with low porosity and good ductility. Low-stress Ni is suitable for welding and crimping. Plating thickness (prescribed by IPC-6012): not less than: 2 ~ 2.5μm. Primer: 1st grade 2.0μm; 2nd grade 2.5~5.0μm; 3rd grade ≥5.0μm. 4) Sn coating Sn is not only afraid of cold, but also afraid of heat. When the temperature is lower than 13.2 ℃, the phase transition occurs, from β phase (white tin) to α phase (gray tin), that is, the phenomenon of tin plague occurs. When the temperature is above 161 °C, the white tin is transformed into orthorhombic tin with an orthorhombic structure. Orthogonal tin is very brittle, it will break when knocked, and its malleability is very poor, so it is called “brittle tin”. White tin, gray tin, and brittle tin are three allotropes of tin.

(1) Coating characteristics. Sn plating is a cathodic coating on steel. Only when the coating has no pores can it effectively protect steel from corrosion. The welding properties of the coatings obtained by different processes are also different, as shown in Table 2. Table 2

Influence of Electrode Surface Condition of Electronic Components on Reliability of Interconnect Soldering

The appearance of the dark Sn-plated layer is matt gray-white, and its welding performance is better than that of the bright Sn-plated layer, but it cannot resist the pollution of hand sweat. After the dark-plated Sn layer is hot-melted, its solderability is the best, and the resistance to hand sweat contamination is also greatly improved. The bright Sn-plated layer has good welding performance, and has good resistance to hand sweat and other contamination during process transfer and storage. However, due to the existence of organic additives, gas will be released during heating, resulting in defects such as bubbles and cracks in the weld, affecting the reliability of the weld.

(2) Coating thickness. Sn easily forms an intermetallic compound with Cu, and this intermetallic compound has poor solderability. But a certain amount of intermetallic compounds is a sign of wetting. Therefore, a part of the Sn coating should be used for the formation of intermetallic compounds, and the surface of the coating is occupied by the oxide film, and the remaining part can be used to improve the solderability. Therefore, the thickness of the Sn plating layer is usually 8 to 10 μm.

5) Cu coating Cu is an excellent solderability coating, as long as its surface is fresh, or has taken effective protection without oxidation or corrosion. Fine-grained coatings have better solderability than coarse-grained coatings.

6) Pd plating chemical immersion Pd (palladium) is an ideal Cu-Ni protective layer for component pins, which can be soldered or “bonded” (pressure-welded). It can be directly plated on Cu. Due to the self-catalytic ability of Pd, the coating can be thickened, and its thickness can reach 0.08 to 0.2 μm. It can also be plated on chemical Ni layers. The Pd layer has high heat resistance and stability, and can withstand multiple thermal shocks. Since the price of Pd is higher than that of Au, its application is limited to a certain extent. With the improvement of IC integration and the advancement of assembly technology, electroless Pd plating will play a more effective role in chip-scale assembly (CSP).

7) SnPb coating

●SnPb alloy coating can be used as an alkaline protective layer in PCB production, and the coating requirements are uniform, dense and semi-bright.

●The melting point of SnPb alloy is lower than that of Sn and Pb, and the porosity and weldability are good. The Sn “whisker” problem can be eliminated as long as the Pb content reaches 2% to 3%.

●The SnPb alloy plating on the PCB must have sufficient thickness to provide adequate protection and good solderability. MIL-STD-27513 stipulates that the minimum thickness of SnPb alloy is 7.5μm. This regulation was proposed by NASA and recognized by the US space industry. The report provided by the British Tin Research Institute also pointed out that the thinnest thickness of SnPb alloy coating is 7.5μm.

●Ordinary SnPb alloy coating structure is flake-like, with granular dark appearance, and the coating has many pinholes. This coating is prone to discoloration during processing and affects solderability. After hot melting (infrared hot melting or hot oil (glycerol) hot melting), a bright and dense coating can be obtained, which improves corrosion resistance and prolongs life. Hot melting can also make the organic inclusions in the SnPb alloy coating escape by heat, which can reduce the generation of bubbles during wave soldering.

● During hot melting, a thin layer of intermetallic compound will be formed between Cu and Sn, which is necessary for wetting, but the amount must be appropriate to ensure good wettability. If the amount is large, it will be harmful. The higher the temperature and the longer the time, the more conducive to the growth of the intermetallic compound, the more Sn is consumed, which may cause a lead-rich phase near the solder layer near the intermetallic compound, resulting in semi-wetting.

8) SnZn coating Sn and Zn are widely used in anti-corrosion of steel, but their anti-corrosion mechanism is different. Sn is a more expensive metal than steel, so it is a cathode coating. Steel can only form corrosion microbatteries through the pores of the Sn coating, so rust occurs at the pores. Zn is a cheaper metal than steel and it protects steel by its own anodic corrosion. The SnZn alloy coating has both the advantages of Sn and Zn, and makes up for their shortcomings. The alloy coating not only has high corrosion resistance (75%Sn/25%Zn), good weldability (10%Sn/90%Zn), but also does not form “whiskers”.

The coating is silver-white, with mirror luster, low cost, and can be used to replace Ag coating in electronic products.

9) The SnCe alloy tin-plating layer has the danger of growing whiskers, and its tendency increases with the increase of Sn concentration and the increase of internal stress. Sn also has structural variation, and low temperature produces tin plague. Sn and Cu have a tendency to penetrate into each other to form a Cu6Sn5 alloy diffusion layer. An excessively thick alloy layer has a high melting point and is brittle, which affects the weldability. The coating obtained by the SnCe alloy has high brightness, corrosion resistance, improved solderability, refinement of crystal grains, and improved coating. However, Ce is barely detectable in the coating. The coating can prevent the mutual diffusion of Cu and Sn in the matrix, and has good chemical stability, strong oxidation resistance and stable solderability.

10) Other fluorine-free and Pb-free Sn-based alloys Pb-free solderable coatings have been put into production with Sn/Cu (Cu0.3%), which can be used for electronic lead electroplating to obtain bright and semi-bright coatings. The performance comparison of several Pb-free coatings is shown in Table 3. table 3

Influence of Electrode Surface Condition of Electronic Components on Reliability of Interconnect Soldering

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