The difference between leaded solder paste and lead-free sol

News     |      2021-10-11 10:28

Abstract: As the international community has higher and higher requirements for environmentally friendly products, developed countries such as Europe and the United States have RoHS requirements for circuit board assembly. Lead is more harmful to the human body and nature. Therefore, the environmental protection requirements of RoHS must be followed; the lead-free solder paste e process is more and more sought after in the SMT industry. In the future development trend, the replacement of leaded solder paste by lead-free solder paste is bound to become a necessity.


table of Contents:

1. Lead-free solder paste and its characteristics

2. Leaded solder paste; and features

3. The obvious difference between leaded solder paste and lead-free solder paste4


Solder paste is mainly a paste mixture composed of solder powder and flux. It is mainly used in the SMT processing industry to solder electronic components such as resistors, capacitors, and ICs on PCB boards. Solder paste is divided into leaded solder paste and lead-free solder paste.

1. Lead-free solder paste and its characteristics

Lead-free solder paste is a no-clean solder paste used in SMT production. The thixotropic mixture made of special solder paste and spherical solder powder has very low oxygen content and has continuous printability: in addition, the solder paste contained in the product has low reliability and high reliability. The activator system minimizes residues after reflow and has high reliability.

Characteristics of lead-free solder paste:

a. It has excellent welding performance, can show proper wettability in different parts, and has little residual corrosion after welding.

b. The continuous printability and tin drop performance is good. After a long time of printing, it can still maintain the same effect as the beginning of printing. There will be no solder balls and sags, and the chip components will not shift.

c. The solvent evaporates slowly and can be printed for a long time without affecting the printing viscosity of the solder paste.

d. It has excellent peeling performance during printing and is suitable for installing small pitch devices from 0.5mm/20mil to 0.3mm/12mil.

e. The product has good storage stability and can be stored at a temperature of 5°C-15°C, with a validity period of up to 7 months.

f. There are few solder balls during reflow soldering, which can effectively reduce the occurrence of short circuits. After soldering, the solder joints have good gloss, high strength, and excellent electrical conductivity.

g. There are few residues after reflow soldering, and excellent ICT probe test performance can be obtained without cleaning, and it has extremely high surface insulation resistance.


leaded solder paste


2. Leaded solder paste; and features

The lead-containing solder paste uses artificial rosin, so there are few solid residues in the solder paste after soldering. The low ion activator system is used to prevent corrosion of parts after reflow soldering. This kind of solder paste is a homogeneous mixture of no-clean flux and alloy powder. It has a high viscosity and can prevent the solder paste from settling during storage. The flux also contains some additives, such as high boiling point solvents, preservatives, and edge grinding additives, so the solder paste will not spray out during the reflow process and has good thixotropy.

The characteristics of lead solder paste

a. It has excellent welding performance and can show proper lubricity in different parts;

b. The printing has good rolling and tin dropping properties and can also be used for fine printing on pads with a pitch as low as 0.3mm;

c. In the continuous printing process, its viscosity and adhesion have little change, and its life is long. It will not dry out for more than 12 hours and still maintain a good printing effect;

d. Keep the original shape for several hours after printing, basically without slump, and the chip assembly will not shift;

e. Less residue after welding, light color, high insulation resistance, will not corrode PCB and can meet no-clean requirements.

f. It can be used for the requirements of different levels of welding equipment. It is not necessary to complete the soldering in a nitrogen-filled environment, and it can still show good soldering performance in a wide temperature range of the reflow oven. Two furnace temperature setting methods of “insulation type” can be used.

  lead-free solder paste


3. There are obvious differences between leaded solder paste and lead-free solder paste in 4 major aspects

a. The difference in appearance and smell of solder paste

The color of leaded solder paste; is gray-black, and it is packed in a white bottle; the color of lead-free solder paste is gray-white, and it is packed in a green bottle; the smell of leaded solder paste is generally larger. The composition in the leaded solder paste; contains lead, and lead itself presents a black character. The lead-free solder paste complies with the RoHS standard, and the conventional way in the industry is to use green bottles for storage so that engineers can visually distinguish them when using them.

b. The difference in the composition of the solder paste

The alloy composition of leaded solder paste; is mainly composed of tin and lead, and their ratio is: Sn: Pb=63:37; lead-free solder paste

Take the lead-free high-temperature solder paste as an example: it is mainly composed of tin, copper, and silver, and their ratio is Sn:Ag: Cu=96.5:3:0.5/99:0.3:0.7.

c. The difference in melting point temperature of solder paste

The theoretical melting point of leaded solder paste; is 183℃, and the theoretical melting point of lead-free high-temperature solder paste is between 217℃-227℃, so the lead-free soldering temperature is higher than that with lead, and the lowest peak value of lead-free soldering temperature should be at 200-205℃, the highest peak temperature is 235℃-245℃.

d. The difference in the welding process

The lead-free soldering temperature is higher than that with lead, and the soldering effect is the best at a certain temperature (200-205°C, peak value 235°C). The upper limit temperature of 235°C coincides with the maximum temperature value of most component manufacturers. In order to cater to the characteristics of lead-free solder paste, the reflow furnace temperature curve is set to 20 segments in the soldering process, and a smoother curve and slower carrying rate are used to achieve a perfect soldering effect.