PCBA Tech - Causes of solder joint defects in SMT reflow soldering

1) Reasons for the formation of SMT cold welding

(1) Due to too low reflow temperature or too short reflow time, the PCB solder is not sufficiently melted;

(2) Due to the vibration of the conveyor belt, it is affected by external forces during cooling and solidification, which causes the solder joints to be disturbed, and the surface of the solder joints presents uneven shapes;

(3) Surface contamination on and around PCB pads or pins will inhibit the flux capability and cause incomplete reflow. Sometimes unmelted solder powder can be observed on the surface of the solder joint. At the same time, insufficient flux capacity will also result in the incomplete removal of metal oxides, which will subsequently lead to incomplete condensation;

(4) Solder metal powder is of poor quality, most of which are encapsulated by highly oxidized powder particles.

2) Causes of poor wetting

(1) Time, temperature and return gas have a great influence on wettability. Too short time or too low temperature will cause insufficient heat, resulting in incomplete flux reaction and incomplete metallurgical wetting reaction resulting in poor wetting. In addition, before the solder melts, the excessive heat not only oxidizes the metal transition of the pads and pins, but also consumes more flux, which will eventually lead to poor wetting;

(2) The solder alloy is of poor quality, and impurities such as aluminum and arsenic can also cause poor wetting. The quality of the solder powder is not good, the metal powder in the solder paste contains high oxygen content, and the flux activity in the solder paste is poor;

(3) The solder ends, pins, and printed circuit board pads of the components are contaminated or oxidized, or the printed circuit board is damp, resulting in poor metal wettability;

3) Reasons for wicking

It is mainly caused by the temperature difference between the pins and the printed circuit board and the surface tension of the molten solder. During reflow, the lead of the component reaches the melting temperature of the solder before the PCB pad, so that the solder rises along the lead, forming a wicking phenomenon.

4) Causes of solder cracks

(1) The peak temperature is too high, causing the solder joints to suddenly cool down, and the solder cracks are caused by excessive thermal stress caused by chilling;

(2) The quality of the solder itself;

5) Reasons for the formation of the monument

(1) There is a problem with the design of the component arrangement direction. Once the solder paste reaches the melting point, it will melt immediately. One end of the chip rectangular component reaches the melting point first, and the solder paste melts first and completely wets the metal surface of the component. It has liquid surface tension, while the other end does not reach the liquidus temperature. The paste is not melted, and there is only a bonding force far less than the surface tension, which will make the component end of the unmelted end stand upright;

(2) The quality of the pad design is not good. If a pair of pads of a chip component are of different sizes or asymmetrical, the solder paste on the small pads will melt quickly, causing surface tension to make the component stand up. If the width or gap of the pad is too large, one end of the component cannot fully contact the pad, resulting in tombstone phenomenon;

(3) The leakage of the template is blocked by solder paste or the opening is small, which will cause the amount of solder paste to be missed to be inconsistent, the surface tension at both ends of the PCB pad will be unbalanced, and the components will stand up;

(4) The temperature is not uniform. Uneven heat distribution or shadow effect of nearby components will produce a large temperature gradient;

(5) The placement position is offset, or the thickness of the component is set incorrectly, or the height of the Z-axis of the placement head is too high, caused by the component falling from a height during placement; or the placement pressure is too small, the solder end of the component or the lead The foot floats on the surface of the solder paste, the solder paste cannot stick to the components, and the position moves during transfer and reflow soldering.

(6) The welding end of the component is contaminated or oxidized, or the adhesion of the electrode at the end of the component is not good, so that the two ends of the component are prone to unbalanced force and tombstone phenomenon;

6) Reasons for offset formation

Inaccurate position of printed solder paste, uneven thickness of printed solder paste, improper placement of components, uneven heat transfer, poor solderability of pads or pins, insufficient flux activity, and pads larger than pins Too much, too much air volume, conveyor belt vibration, etc. may cause component deviation, and even tombstones may even be produced when the situation is serious, especially for light-weight components.

7) Causes of solder ball formation

(1) Improper setting of reflux temperature curve. If the temperature in the preheating zone rises too fast, causing the moisture and solvent to evaporate violently, the metal powder will splash with the solvent vapor to form solder balls; if the temperature in the preheating zone is too low, the moisture and solvent in the solder paste cannot be completely volatilized, It is easier to produce solder balls if it suddenly enters the reflow zone;

(2) The solder paste itself is of poor quality. If the metal powder content is too high or the oxygen content of the metal powder is too high, the metal powder will spatter as the solvent evaporates during reflow soldering, forming solder balls; if the viscosity of the solder paste is too low or the thixotropy of the solder paste is not good, after printing The solder paste will collapse, causing adhesion in severe cases, and solder balls will also be formed during reflow soldering;

(3) Improper use of solder paste. If the solder paste is taken out of the low temperature cabinet and used without reheating, the temperature of the solder paste is lower than room temperature, and the water vapor condenses on the solder paste, and the solder paste absorbs moisture. After stirring, a large amount of water is mixed in the solder paste, and the reflow soldering heats up. When the water vapor evaporates to bring out the metal powder, and at the same time, the water vapor will oxidize the metal powder at high temperature, so that the splash of the metal powder will accumulate and form a solder ball;

(4) Improper structure of metal template design. Improper template thickness and opening size, non-parallel or gap between template and printed board surface, or template opening size corrosion accuracy is not up to the requirements, resulting in unclear contours of the missing solder paste, bridging each other, and will cause problems after reflow soldering. A large number of solder balls are generated between the pins;

(5) The influence of PCB printing process. Excessive squeegee pressure and poor quality of the template will cause the solder paste pattern to stick during printing, or the residual solder paste at the bottom of the template will not be wiped off in time. The solder paste will contaminate the place other than the pad during printing, and solder balls will be generated after reflow. ;

(6) The mounting pressure is too high, and the solder paste is squeezed out too much, which is the pattern adhesion.

8) Reasons for bridging formation

(1) The amount of solder is too much. Improper template thickness and opening size; imbalance or gap between the template and the printed circuit board surface;

(2) The viscosity of the solder paste is too low, the thixotropy is not good, and the edges collapse after printing, causing the solder paste graphics to stick;

(3) The printing quality is not good, causing the solder paste graphics to stick;

(4) The placement of the patch is offset;

(5) The patch pressure is too high, and the solder paste is squeezed out too much, causing the graphics to stick;

(6) The placement of the patch is offset, and the solder paste graphics will stick after manual correction;

(7) The production of fine-pitch pads on the printed circuit board is defective, or the pad spacing is too narrow.

9) Causes of void formation

(1) The influence of materials. The solder paste is damp, the metal powder in the solder paste has high oxygen content, the use of recycled solder paste, the component pins or the pads of the printed circuit board substrate are oxidized or contaminated, and the printed circuit board is damp;

(2) PCB soldering process influence: the preheating temperature is too low and the preheating time is too short, so that the solvent in the solder paste cannot escape in time before hardening, and bubbles are generated in the reflow area.

10) Reasons for the formation of popcorn phenomenon

(1) The management, storage and use of the components make the chip damp due to the humidity in the environment, and the gas inside the chip or the substrate or pins will expand during reflow;

(2) The reflux temperature curve is wrong, the heating speed is too fast, and the peak temperature is too high.