Composition of solder paste

Solder powder, also known as tin powder, is mainly composed of tin lead, tin bismuth, and tin silver copper alloys, with a general ratio of SN63/PB37, SN42BI58, SN96.5CU0.5AG3.0, and SN99CU0.7AG0.3. In summary, the relevant characteristics and quality requirements of tin powder are as follows:

A. The particle morphology of tin powder has a significant impact on the working performance of solder paste:

A-1. It is important to ensure that the size distribution of tin powder particles is uniform. Here, we will discuss the issue of the proportion of tin powder particle size distribution; In domestic solder powder or solder paste manufacturers, the distribution ratio is often used to measure the uniformity of solder powder: taking 25-45 μ m solder powder as an example, the particle size ratio of about 35 μ m is usually required to be about 60%, and the parts below and above 35 μ m each account for about 20%;

A-2. Additionally, it is required that the shape of the tin powder particles be relatively regular; According to the "General Specification for Tin Lead Solder Paste in the Electronic Industry of the People's Republic of China" (SJ/T 11186-1998), the relevant provisions are as follows: "The shape of the alloy powder should be spherical, but near spherical powders with a maximum ratio of 1.5 between the long and short axes are allowed. If the user and the manufacturer reach an agreement, other shapes of alloy powder can also be used." In practical work, it is usually required that the ratio of the long and short axes of the tin powder particles is generally below 1.2.

A-3、 If the requirements of A-1 and A-2 above cannot meet the basic requirements mentioned above, it is likely to affect the printing, marking, and soldering effects of the solder paste during use.

B、 The ratio of tin powder to flux in various solder pastes is also different. When choosing solder paste, different solder pastes should be selected based on the products produced, production processes, precision of soldering components, and requirements for soldering effects;

B-1、 According to the relevant provisions of the "General Specification for Tin Lead Paste Solder" (SJ/T 11186-1998) in the electronic industry standard of the People's Republic of China, the percentage (mass) content of alloy powder in solder paste should be 65% -96%, and the measured value of the percentage (mass) content of alloy powder should not deviate from the predetermined value on the purchase order by more than ± 1%; Usually in practical use, the selected solder paste has a tin powder content of about 90%, which means the ratio of tin powder to flux is approximately 90:10;

B-2、 Ordinary printing standards often use solder paste with a tin powder content of 89-91.5%;

B-3、 When using needle injection technology, solder paste with a tin powder content of 84-87% is often selected;

B-4、 Reflow soldering requires firm soldering of device pins, full and smooth solder joints, and a solder climb of 1/3 to 2/3 in the height direction of the device (resistive capacitive device) end. The content of metal alloy in the solder paste has a certain impact on the thickness of the solder after reflow soldering (i.e. the fullness of the solder joint); In order to confirm the existence of this problem, relevant experts have conducted relevant experiments. The final experimental results are summarized in the following table for reference: (Table temporarily missing)

From the above table, it can be seen that as the metal content decreases, the thickness of the solder after reflow soldering decreases. In order to meet the requirements for the solder amount of the solder joint, a solder paste with a content of 85% to 92% is usually selected.

C、 The "low oxygen degree" of tin powder is also a very important quality requirement, which should be noted during the production or storage process of tin powder; If this issue is not addressed, solder paste made from high oxidation tin powder will seriously affect the quality of the welding process.