Preface

 

 

 

 

Solder paste is one of the important materials indispensable for the progress of modern science and technology. It is used to solder electronic parts to circuit boards, giving us the opportunity to enjoy ever-changing electronic products.

 

It was also because of the invention of solder paste that it indirectly contributed to the miniaturization of electronic assembly technology, allowing the Big Brother Black King Kong mobile phone that was originally as big as a brick to be transformed into a smart phone that can fit in a pocket and has many functions.

 

The reason why solder paste is called "paste" is because it is very similar in shape to the toothpaste we use to brush our teeth every day. Before the molten tin is soldered, its "paste" shape can be used to stick and place. The electronic components on the surface of the circuit board prevent these components from shifting even under slight vibrations; of course, its biggest role is to weld the electronic components to the circuit board to achieve the purpose of connecting electronic signals.

 

 

 

But do you know that in addition to "tin", the main component of the solder paste you usually see is also what other things are hidden in it?

 

 

 

The main components of solder paste: flux and tin powder

 

 

 

 

The ingredients of solder paste are mainly composed of flux and powder, which are completely mixed together. The following is a brief description of the contents and precautions of these two ingredients:

 

Tin powder (Powder)

 

 

 

 

 

Tin powder is also a metal alloy. Its main purpose is to reflect the weldability and welding strength. Its main components include the following metals:

 

§ Sn(tin)

 

§ Ag(silver)

 

§ Cu(copper)

 

§ Bi(bismuth)

 

Tin powder will have different ingredients and ratios due to different solder paste numbers. However, even if it is the same number but different brands, the ingredients will be slightly different. Some may be to avoid patents, while others It's my own secret recipe.

 

 

 

Taking SAC305, which is currently used by the most people, as an example, it uses a solder paste with a ratio of tin (Sn, 96.5%), silver (Ag, 3%), and copper (Cu, 0.5%); while SAC0307 uses tin (Sn, 99%), silver (Ag, 0.3%), copper (Cu, 0.7%) ratio solder paste.

 

Secondly, there are low-temperature solder pastes using SnBi or SnBiAg alloy. The SnBi alloy has a melting point of tin (Sn, 42%) and bismuth (Bi, 58%) at 138°C, while the SnBiAg alloy has a melting point of (Sn, 64%), bismuth (Bi, 35%), silver (Ag, 1 %) melting point is 178°C.

 

 

 

In addition, for copper-based circuit boards (OSP, bare copper boards), some solder paste manufacturers will add a small amount of [nickel (Ni)] to suppress and slow down the IMC that originally generates benign Cu5Sn6 and gradually transforms into inferior Cu3Sn over time. IMC.

 

In addition to the different compositions of tin powder, there are also different particle sizes of tin powder. Tin powder numbers are given according to different tin powder sizes. However, to be honest, the number of tin powder sizes is not completely unified. Most brands probably use the tacit understanding that the smaller the number, the larger the particles.

 

 

 

The number and diameter size of the solder powder are roughly as follows. The following table is based on the [IPC J-STD-006A] specification. However, the B version of the document has since canceled the diameter specification of this solder paste. Please refer to [IPC J-STD-005] instead. The table below may not be the most accurate and is for reference only.

 

In addition, various solder paste suppliers actually have other solder powder numbers besides industrial standards, such as Type 4.5 or Type 7 or Type 8 in order to meet the special needs of customers. The specifications of these companies outside the standard specifications may vary depending on the specifications. Each solder paste manufacturer is different and has slight differences. For detailed specifications, you may have to ask the respective solder paste supplier.

 

No. None Larger than Less than 1% Larger than 80% Minimum Between 10% Maximum Less Than

1 160um 150um 75～150um 20um

2 80um 75um 45～75um 20um

3 50um 45um 25～45um 20um

4 40um 38um 20～38um 20um

5 30um 25um 15～25um 15um

6 20um 15um 5～15um 5um

 

 

 

 

The smaller the diameter of the tin powder, the better the amount of tin falling. Because smaller particles are easier to roll down the aperture of the steel plate, which means they are easier to print on the circuit board through the steel plate, and are less likely to remain on the edge of the opening on the steel plate, which helps improve the printing ability of fine-pitch parts. It is also easier to obtain consistent solder paste printing volume; and smaller tin powder can also improve its collapse resistance and better wetting effect.

 

 

 

However, the smaller the tin powder, the easier it is to oxidize. It may be necessary to supplement it with nitrogen (N2) to reduce the oxidation speed to achieve a good tin eating effect, especially when using parts with large solder pads (such as shielding frames) with size 5 or above. This is because the smaller the tin powder, the larger its contact area with air, so it is easier to oxidize. Therefore, when selecting solder paste, it is not that the smaller the particles of the solder powder, the better, but it depends on the needs of the product, and the uniformity of the solder paste particles must also be specified.

 

 

 

Generally, No. 3 tin powder is used for SMT bonding, while No. 4 tin powder is used for fine pitch or small pad bonding. Type5 and Type6 solder powder are usually used on [Flip Chip] or [CSP] bumps. The smaller the tin powder, the more expensive it is because it is harder to produce.

 

 

 

Although we do not want the solder paste to be oxidized, no matter how the solder paste is processed, there will still be some oxidation residues, especially the slight oxidation on the surface of the metal particles. Fortunately, we can use these micro-oxidations to prevent the solder paste from remaining. They fuse together before being formally printed on the circuit board. Because the same pure metals placed together will cause problems with mutual fusion. This is called "birds of a feather flock together."

 

 

 

Oxidation basically has three elements: temperature, air, and water.

 

 

 

There are two shapes of tin powder: spherical and elliptical. The spherical shape has a wide range of printing applications, a small surface area, low oxidation degree, and bright solder joints; the elliptical shape is worse.

 

 

 

 

 

 

 

Flux

 

 

 

Flux is actually the biggest promoter of turning solder paste into a paste, because the solvent contained in the flux can mix all the substances together into a paste.

 

 

 

 

 

The purpose and main function of "flux" is to remove oxides and dirt on the metal surface. It can also form a thin film on the metal surface to isolate air during high-temperature operations, making the solder paste less likely to oxidize. Its composition mainly includes the following four components: Ingredients:

 

 

 

§ Resin rosin: 40~50%. It can be divided into natural resin (Rosin) or synthetic rosin (Resin). Usually lead solder paste uses Rosin, while lead-free solder paste uses Resin. Rosin can form a protective layer on the surface of the metal being soldered to isolate the air, so it can Prevents oxidation, is sticky, and has a slight ability to clean metal surfaces.

 

 

 

§ Activator: 2~5%. The main components are organic acids and halogens. It has the ability to strongly clean metal surfaces. It is often used as a cleaning agent in the reflow soldering process. It can dissolve oxides on the metal surface and improve the welding effect. Halogen is highly toxic. In order to meet today's environmental protection needs, halogen-free solder paste has become a trend. However, it has strong deoxidizing ability and is cheap, so it is still often used in some solder pastes.

 

 

 

§ Solvent: 30%. Contains ethanol and other ingredients. These solvents will evaporate during the preheating process of the solder paste, so they will not affect the solderability of the entire solder paste. They can help dissolve and mix different chemicals in the flux, so that the flux can be more evenly distributed. Coated in tin powder, it improves the effectiveness of the flux. It also makes the flux easier to control and can be used to control the viscosity and fluidity of the solder paste. If the reflow preheating temperature rises too quickly, these solvents may boil immediately and cause problems with solder paste splashing.

 

 

 

§ Thickener (rheology modifier): 5%. Provide thixotropy or thixotropy to control the viscosity of the solder paste to achieve a paste-like consistency, enhance the printability and collapse resistance of the solder paste, and allow the solder paste to maintain its original texture after being printed on the circuit board. The shape will not collapse and cause a solder short circuit.

 

 

 

In addition, one thing I must remind you is that if the ratio of solder paste is calculated by weight, the ratio of tin powder to flux is about 90%:10%, because tin powder is relatively heavy; but if the ratio is calculated by volume, The ratio of tin powder to flux is about 50%:50%, which will affect the calculation of the amount of solder paste after soldering.