PCB soldering is poor, reflow soldering
SMT products mostly use integral heating reflow welding, rich in far-infrared/hot air reflow welding and vapor phase reflow welding (VPS), etc. The main influencing factor of reflow welding on SMC/SMD is the thermal shock during welding. A reliable heating process must be set during operation to reduce thermal shock stress. For the vapor phase welding method, fluorine-based inert organic solvents are mainly used as the heat-carrying medium. For chip electromechanical components and structural components without a sealed structure, if the solvent penetrates into the components, its performance and functions will be damaged.. In addition, if the welding process setting and the quality of each pre-process control fail to meet the specified requirements, it will cause the SMC/SMD “Manhattan” phenomenon (also known as “uplifting” or “tombstone”), positional deviation and The occurrence of undesirable phenomena such as lateral oscillation.
The defects that occur during the reflow process can be roughly divided into two categories:
(1) Related to metallurgical phenomena, including: cold welding, non-wetting, poor wetting, etc.;
(2) Related to abnormal solder joint shapes, including: tombstones, bridges, wicking, solder balls, voids, etc.
2, Troubleshooting countermeasures for reflow welding faults
The main causes of reflow welding failures have been analyzed in Chapter 3. Table 5.10 lists the main causes and solutions for common welding defects.
No. Defect Reason Solution
1 Displacement of components (1) Wrong placement
(2) Insufficient amount of solder paste or insufficient positioning pressure
(3) The flux content in the solder paste is too high, and the flow of the flux during the reflow process causes component displacement (1) Calibration positioning coordinates
(2) Increase the amount of solder paste and increase the pressure of placing components
(3) Reduce the content of flux in solder paste
2 Solder powder can no longer flow, it is scattered in powder form and remains on the pad (1) The heating temperature is inappropriate
(2) Deterioration of solder paste
(3) Excessive preheating, too long time or too high temperature (1) Improve heating equipment and adjust reflow soldering temperature curve
(2) Pay attention to refrigerating the solder paste, and discard the hard or dry part of the solder paste
(3) Improve preheating conditions
3 Insufficient tin in solder joints (1) Insufficient solder paste
(2) Poor welding performance of pads and components
(3) Short reflow soldering time
(2) Switch to solder paste or re-impregnate components
(3) Lengthen the reflow time
No. Defect Reason Solution
4 Too much tin in solder joints
(2) The viscosity of solder paste is small (3) Reduce the hole diameter of the screen or leakage plate
(4) Increase the viscosity of solder paste
5 The components stand firmly and appear
“Manhattan” phenomenon
(“Still standing”) (1) Shift of placement
(2) The flux in the solder paste makes the components float
(3) The thickness of the printed solder paste is not enough
(4) The heating speed is too fast and uneven
(5) Unreasonable pad design
(6) Use Sn63/Pb37 solder paste
(7) Poor solderability of components (1) Adjust printer parameters
(2) Use solder paste with low flux content
(3) Increase printing thickness
(4) Adjust the reflow soldering temperature curve
(5) Strictly carry out pad design according to specifications
(6) Switch to solder paste containing Ag or Bi
(7) Choose solder paste with good solderability
6 Solder ball (1) Heating speed is too fast
(2) The solder paste has absorbed moisture
(3) The solder paste is oxidized
(4) PCB pad contamination
(5) The component placement pressure is too large
(6) Too much solder paste (1) Adjust the reflow soldering temperature curve
(2) Reduce environmental humidity
(3) Use new solder paste to shorten the preheating time
(4) Change PCB or increase solder paste activity
(5) Reduce stress
(6) Reduce the aperture and reduce the pressure of the scraper
7 Welding (1) The solderability of pads and components is poor
(2) The printing parameters are incorrect
(3) Improper reflow soldering temperature and temperature speed (1) Strengthen the screening of PCB and components
(2) Reduce the viscosity of the solder paste and check the pressure and speed of the squeegee
(3) Adjust the reflow soldering temperature curve
8 Bridging (1) Solder paste collapse
(2) Too much solder paste
(3) Print multiple times on the pad
(4) The heating speed is too fast (1) Increase the metal content or viscosity of the solder paste, and change the solder paste
(2) Reduce the aperture of the screen or the leakage plate, and reduce the pressure of the scraper
(3) Use other printing methods
(4) Adjust the reflow soldering temperature curve
9 Slump (1) Solder paste has low viscosity and poor thixotropy
(2) High ambient temperature (1) Choose suitable solder paste
(2) Control the ambient temperature