SMT solder paste knowledge sharing

- Aug 06, 2019-

1. Application of solder paste

  With the application of reflow soldering technology, solder paste has become a process material in surface mount technology (SMT), and has been rapidly developed in recent years. In reflow soldering of surface mounts, solder paste is used to implement the bonding of the leads or terminals of the surface mount component to the pads on the printed board. Solder paste coating is a key process in surface assembly technology that directly affects the quality and reliability of surface mount components. Defects caused by solder paste account for 60%-70% of the defects in SMT, so it is especially important to understand the solder paste in depth and standardize the use of solder paste.


  At normal temperature, the solder paste can initially adhere the electronic component to a predetermined position. When heated to a certain temperature, the alloy powder melts as the solvent and some additives volatilize, so that the soldered component and the pad are connected together. , cooling forms a solder joint that is permanently connected. The requirements for solder paste are various coating methods, especially good printing performance and reflow-ability and stability during storage.


2. Definition of solder paste

   Definition of solder paste: English name: SOLDER PASTE

  The solder paste (solder paste) is a paste in which a uniform solder alloy powder and a stable flux are uniformly mixed in a certain ratio. The leads or ends of the surface mount component can be alloyed to the pads on the printed board during soldering. This material is ideal for reliable soldering of surface mount automated production and is a high-tech product of the electronics industry.


  It is applied to the SMT solder paste printing work station, and the quantitative solder paste is accurately applied to the fixed spot pads (Pads) of the PCB through the carrier of the scraper, steel plate, printing machine, etc., and has good viscosity (Tacky). To complete the electrical and mechanical connection between the pad and the component foot after reflow.


   Solder paste: lead-free solder paste and lead solder paste


Three. The composition of solder paste

   The composition of the solder paste:

    Solder paste = tin powder (METAL) + flux (FLUX)

   Tin powder is usually produced by nitrogen atomization or rotary disc method, and then screened by screen. Flux is a binder (resin), solvent, active agent, thixotropic agent and other additives. It plays a vital role in the entire process from solder printing to soldering.


   In general, the weight ratio of 10% solder paste and 90% tin powder (90% tin alloy powder)


   Volume ratio of 50% solder paste to 50% tin powder


   The role of each component:

    Solder powder: conductive, keyed

    Soldering Adhesive: Prevents the separation of tin powder from FLUX to prevent tin collapse

    Solvent: Dissolve all of FLUX into a homogeneous solution to obtain a uniform flux

Active agent: Eliminates oxides on the solder surface and reduces surface tension

Anti-sagging agent: Viscosity printing ability Preventing collapse ODOR


Four important characteristics of solder paste

   Important characteristics of solder paste:

1.       Liquidity

    2. Stripping

    3. Continuous printing

    4. Stability


  Solder paste is a fluid that is fluid. The fluidity of the material can be divided into ideal, plastic, pseudoplastic, expanded and thixotropic, and the solder paste is a thixotropic fluid. The ratio of shear stress to shear rate is defined as the viscosity of the solder paste. The unit is Pa.s. The percentage of solder paste alloy, powder particle size, temperature, soldering amount and lubricity of the thixotropic agent are the viscosity of the solder paste. In practical applications, the optimum viscosity is generally determined based on the type of solder paste printing technology and the thickness printed on the PCB.


V. Classification of solder paste

   According to the reflow temperature:

1.High temperature solder paste

2.Normal temperature solder paste

3.Low temperature solder paste


   According to the metal composition:

Silver-containing solder paste (Sn62/Pb36/Ag2)

Non-silver solder paste (Sn63/Pb37)

Containing antimony paste (Bi14/Sn43/Pb43)

Lead-free solder paste (Sn96.5/Ag3.0/Cu0.5)


   Low temperature application: Sn43/Pb43/Bi14


   High temperature, lead-free, high tension: Sn96/Ag4




   High temperature, high tension, low value: Sn10/Pb90