Surface mount device is one of Surface Mount Technology components. In the initial stage of electronic circuit board production, the assembly is completely done manually. After the introduction of the first batch of automated machines, they can place some simple pin components, but complex components still need to be placed manually for reflow soldering. There are 3 considerations while choosing the best SMD solder paste:

Solder Paste

1) The performance of solder paste

<1>Solder paste appearance: The solder paste trademark should be marked with the manufacturer's name, product name, standard classification number, batch number, production date, flux type, tin powder particle size, solder paste viscosity, alloy percentage, shelf life, etc. There should be no hard shells or lumps on the appearance of the solder paste. The alloy powder and flux should not be layered and mixed evenly.

<2>Printability of solder paste: In PCBA processing, the solder paste is required to be smoothly and continuously transferred to the PCB through the solder paste leak plate or distributor. If the printability of the solder paste is not good, it will block the leak plate. Perforation, which caused production not to proceed properly. The reason is that the solder paste lacks a printing aid or its amount is insufficient. In addition, the poor shape of the alloy powder and the unsuitable particle size distribution will also cause a drop in printing performance.

<3>Solder paste viscosity test: Viscosity is an important basis for the selection of solder paste coating methods. Qualified companies can use Brookfield viscometer to measure. The relevant viscosity test standards of IPC-SP-819 are as follows: the test temperature is (25±0.25) ℃; before the formal measurement, the solder paste is placed at this temperature for 2h; the rotation speed is 5r/min; the viscometer probe is sunk under the solder paste 2.8cm; the diameter of the solder paste sampling bottle is> 5cm; the reading is taken once every two minutes for a total of 5 times, and the average value of the last two is taken as the reading of the sample.

<4>Slump of solder paste: It is necessary to maintain enough viscosity after solder paste is placed for a period of time (8h) after printing. It can ensure that the components are adhered to the required positions, and the movement of the components does not occur during the transmission process. Slump is a term describing whether the solder paste is printed on the PCB and maintains a good shape after a certain high temperature. The appearance of solder paste pattern interconnection on the appearance indicates that the solder paste has collapsed. This phenomenon often leads to defects such as bridging and flying beads after reflow soldering.

<5>Dryness of residue after solder paste is melted

<6>Solder ball solder ball test (wetting test): The solder ball test is to test the degree of oxidation of the solder powder or whether the solder paste is immersed in water vapor. Normally, a certain amount of solder paste melts on a non-wetting substrate and should form a large sphere, instead of entraining some additional small balls or powder, otherwise it means that the solder powder has high oxygen content or is immersed in moisture.

<7>Solder paste wettability expansion rate test: The solder paste expansion rate test is to test the degree of activity of the solder paste, and to examine the ability of the solder paste to wet and spread on the oxidized copper skin. Usually, solder paste with Ø6.0mm and 0.2mm thickness is printed on the copper skin, and the diameter should be expanded by 10%~20% after reflow soldering.

2) Solder powder inspection

<1>Solder mass percentage: solder paste with a high percentage of solder powder, the solder paste on the printed pattern is thicker, after drying and hot melting, the solder paste collapses less, which is conducive to strengthening the bonding strength between components and substrate helps to improve the fatigue strength of solder joints. <2>Solder composition determination

<3>Solder particle size distribution

<4>Solder powder shape

3) Flux testing items

The meaning of the solder acid value, halide, water soluble conductivity, copper mirror corrosion test, and insulation resistance measurement in solder paste is the same as the related testing purpose of liquid flux, only from different angle Use different methods to test the corrosion of solder paste to ensure that the solder paste used not only has good solder-ability, but also the electrical performance can meet the quality requirements. In particular, with the increase in environmental awareness, a large number of no-clean solder pastes are used, and they are no longer cleaned after soldering, so the solder pastes are required to be safe and reliable.