SMT Defects Analysis and Solutions (2020 Essentials)

There are many types of defects in SMT solder joints, and their shapes are also various. Common SMT solder joint defects are:
1. (cold soldering) 2. (lcicle)
3. (pseudo soldering) 4. (void)
5. (soldering balls) 6. (open soldering)
7, (excursion) 8, (solder-off)
9, (Set component) 10, (turn)
11, (solder wrong) 12, (flux residue)
13, (solder skips) 14, (solder crazeing)
15, (reverse) 16, solder bridge)
17. (excess solder connection)
18. (insufficient solder connection)

Cold soldering: The solder at the soldering point does not reach its melting point temperature or the soldering heat is not enough, so that it is solidified before wetting or flowing, and no metal alloy layer is formed at all, so that the solder is completely or partially in the amorphous strong state. It is simply deposited on the surface of the welded metal.

Reasons: 1. The heating temperature is not suitable;
2. Deterioration of solder paste;
3. Excessive preheating, long time or high temperature;
4. Due to the surface pollution, the flux capacity is controlled;
5. Insufficient flux capacity.
solution:
1. Adjust the temperature curve of reflow soldering;
2. Replace with new solder paste;
3. Improve preheating conditions;
4. Surface contamination on and around the pads or pins will depend on the ability to control the flux and lead to incomplete reflow, which should be solved by an appropriate post-plating cleaning process;
5. Insufficient flux capacity will result in incomplete removal of metal oxides, and then incomplete coalescence, similar to surface contamination.

Virtual soldering (non-wetting or semi-wetting): Partial or all of the solder and the surface of the metal being welded do not form an alloy layer, or the metal plating of the lead / solder electrode electrode is peeled off, which causes the lead / solder end and the pad The phenomenon of unstable electrical line isolation occurs between the two, causing the electrical connection to be either on or off.

the reason:
1. Poor solderability of components and pads;
2. Poor quality of solder alloy or solder powder;
3. Poor flux activity;
4. Improper reflow temperature and heating rate;
5. The printing parameters are incorrect.
solution:
1. Strengthen the selection of PCB and components, temperature and humidity control;
2. Impurities such as aluminum, cadmium, or arsenic in the solder can cause poor wetting. The irregular shape of the solder powder also reflects a large oxide content, which consumes more flux and causes poor wetting. Obviously, poor wetting is caused by poor flux activity;
3. Adjust the temperature curve of reflow soldering (reflow time, temperature and reflow gas have a great influence on the wetting performance, or the heat is insufficient due to too short time or too low temperature, resulting in inadequate flux reaction Complete and incomplete metallurgical wetting reactions produce undesirable wetting. On the other hand, the excessive heat before the solder melts not only excessively oxidizes the pad and lead metals, but also consumes more flux;)
4. Reduce the viscosity of the solder paste, and change to increase the pressure of the scraper and slow down the speed.

Tin Bead: Excess bead solder that adheres to the substrate or components and other parts of uneven size solder joints.

the reason:
1. Generally, the poorly printed board or solder paste is mixed with moisture, which will form when the welding is heated;
2. Environmental impact, temperature (the optimal temperature for solder paste printing time is 25 ℃ ± 3 ℃, and the relative humidity should be 60%. The temperature is too high, the solder paste easily absorbs water vapor, and solder balls are generated during reflow soldering. 😉
3. Temperature curve;
4. The problem of the steel mesh template is too thick and the opening is too large.
solution:
Technology:
1. Reduce the thickness of the steel mesh template;
2. Reduce the size of the hole;
3. Use a hole design that can print less solder paste under the components;
4. Increase the interval between printed solder pastes;
5. Reduce the width of the pad so that it is narrower than the width of the component;
6. Reduce the preheating heating rate (temperature rise should not be too fast, generally should be less than 1.5 ℃ / S, too fast will easily cause splashing and form tin beads);
7. Reduce the preheating temperature;
8. Reduce the pressure of placing components;
9. Pre-bake components or PCB before use.
Materials:
1. Use flux with lower activation temperature;
2. Use a higher amount of solder paste;
3. Use coarse powder solder paste;
4. Solder paste using low oxide solder powder;
5. Use less collapsed solder paste;
6. Use solvents with appropriate vapor pressure.

Offset: It is also considered to be a drift, which is the movement of components on the horizontal plane, resulting in misalignment of components during reflow.

the reason:
1. Machine coordinate offset;
2. The components are lifted by high-density thermal fluid during reflow soldering;
3. The pad design at both ends of the chip component is unbalanced;
4. The width and area of ​​the metal layer of the components are too small;
5. Poor solderability of the metal plating of the component pins;
6. The pad is too narrow.
solution:
Process or design:
1. Correct the program coordinates;
2. Reduce the heating rate during reflow soldering;
3. The pad design at both ends of the balanced chip component, including pad size, heat distribution, heat sink connection, and shadow effect;
4. Increase the width of the pad;
5. Reduce the pollution level of components and metal layers of printed boards, and improve storage conditions;
6. Reduce the thickness of solder paste printing;
7. Pre-dry the solder paste before reflow to reduce the flux outflow rate.
Materials:
1. Use flux with lower outgassing rate;
2. Use flux with lower wetting rate;
3. Use solder paste with delayed melting characteristics, such as a solder alloy composed of tin powder and lead powder.

Vertical piece (tombstone): also known as Manhattan effect, suspension bridge effect or Stonehenge (stone suspension) effect, which is caused by unbalanced wetting of both ends of the component during reflow. (The gravity F1 of the chip component, the vertical vector F2 generated by the surface tension of the molten solder below the chip component, and the vertical vector F3 generated by the surface tension of the surface of the molten solder on the right side of the chip component; both the F1 and F2 forces are pulled down The force is used to keep the components in the proper position, but F3 presses on the corner of the chip component, it will tilt the component to a vertical position. When the force F3 exceeds the sum of F1 and F2, a tombstone occurs .)

the reason:
1. The placement position is shifted;
2. The flux in the solder paste floats the component;
3. The thickness of the printed solder paste is not enough;
4. The heating speed is too fast and uneven;
5. The pad design is unreasonable;
6. Poor solderability of components.
solution:
Process or design:
1. Adjust printing parameters and placement position;
2. Use solder paste with a small amount of solder;
3. The metal terminals under the chip components use a larger width and area;
4. Reduce the width of solder pads;
5. Minimize the uneven distribution of heat, including the connection between the pad and the heat dissipation layer;
6. Through the appropriate PCB design and reflow method selection, the shadow effect is reduced to a minimum;
7. Use organic solderability protector (OSP) or nickel / gold (Ni / Au) coating or tin SN coating on the copper pad instead of Sn-Pb coating;
8. Reduce the pollution and oxidation level of the metal layer of the component terminal or the metal layer of the PCB pad;
9. Use thinner solder paste printing thickness;
10. Improve the accuracy of component placement;
11. Use a gentle heating rate during reflow;
12. Pre-dry before reflow or use a curve with a long-term soaking zone to reduce the outgassing rate of flux;
13. Use a very slow heating curve when crossing the solder melting temperature.
material:
1. Use flux with a slower wetting rate;
2. Use flux with slower outgassing rate;
3. Use delayed-melting solder paste, such as a mixture of tin powder and lead powder or a wide paste alloy.

Misweld: It means that the components or wires welded at the welding place do not meet the design requirements (one or more of them). For example, the variety, specifications, parameters and phase of the components (referring to the reverse or misalignment of electrodes).
Cause: The cause is often the error of the mounting process, which is not detected before welding. Such as wrong materials, wrong coordinates in the program, etc.
solution:
1. Strictly follow the requirements of the program documents;
2. Hand repair parts strictly follow the hand repair program control program operation;
3. The patch coordinates of Teaching machine must be accurate, corresponding to the corresponding position of BOM.

Leak welding: where welding is required without welding (small parts)
the reason:
1. The cause is generally that the solder is not fully reached or not applied, the solder is insufficient and the equipment is the cause;
2. The steel mesh is blocked;
3. The production process is wiped out by man;
4. Mounting problems of machinery and equipment;
5. The problem of Feida’s absorption.
solution:
1. The 2D setting of the tin printing machine needs to be opened;
2. The soldering machine opens the solder paste to add an alarm setting. During the alarm, the operator needs to observe whether the amount of solder paste is enough, and shovel the solder paste on both sides of the scraper into the printing range of the scraper. (The amount of solder paste rolling is a one-dollar coin Diameter is better;)
3. The less the plate is moved by man-made during the production process, the better. If you really want to touch the plate, you should pay attention to the tightness of the sleeve to avoid wiping the sleeve with solder paste
4. The placement height of the nozzle of the placement machine is set. If the component is too high, if it is not covered by the solder paste, it will be easily vibrated or thrown away when moving; at the same time, it is necessary to pay attention to the setting of vacuum suction, (component placement After the alarm is relied on vacuum recognition and camera recognition, the vacuum setting is improper, and the lens is misrecognized because of the long-term use of the nozzle. The device will not alarm after the component is pasted, and will continue to operate, resulting in missing parts;
5. The feeder center of Feida is wrong. When the equipment is sucking materials, the suction is biased, and the components are thrown away during high-speed operation.

Reverse: Polarized components do not correspond to the polar position on the PCB
the reason:
1. The material is reversed;
2. Hand-fixed components are reversed or reversed;
3. The mounting angle or recognition angle of the machine component is set incorrectly.
solution:
1. Strictly follow the requirements of the program documents;
2. Hand repair parts strictly follow the hand repair program control program operation;
3. Normally, the identification angle of the component is set to 0. The mounting angles of the component are Panasonic and YAMAHA, which are set counterclockwise, and Kyushu Panasonic is set clockwise. Set the polarity of the component to the PCB. In the same position.

Bridging (connecting to tin or short circuit): There is solder adhesion between two or more solder joints that should not be connected, resulting in undesirable electrical connection or short circuit.

the reason:
1. The solder paste collapses;
2. Too much solder paste is too thick;
3. The heating speed is too fast;
4. Excessive component placement pressure;
5. Stains of solder paste.
solution :
1. Increase the solder paste metal content or viscosity, and replace the solder paste;
2. Use thinner steel mesh template (generally 5-6Mil) staggered hole pattern, or reduce the size of the window hole to reduce the amount of solder paste or reduce the pressure of the scraper;
3. Adjust the reflow soldering temperature curve, use a colder reflow curve or a slower heating rate;
4. Reduce the placement pressure of components (patch height);
5. Avoid stains.

Solder paste collapse: collapse
the reason:
1. Low solder paste viscosity and poor thixotropy;
2. The high ambient temperature causes the viscosity to decrease naturally.
Solution for collapse:
1. Choose the right solder paste;
2. Control the ambient temperature.

Low solder tin: The solder at the solder joint is less than the required amount, which causes one or more parts of the welded part to not be completely covered by solder, or the lack of solder between the solder joints.
the reason:
1. The solder paste is not enough;
2. Poor solderability of pads and components;
3. Less reflow time;
4. Improper hole design;
5. The hole is blocked.
solution:
1. Thicken the missing version and increase the scraper pressure;
2. Improve solderability;
3. Increase the reflow time;
4. The smooth hole wall is easy to release solder paste and not easy to cause blockage;
5. (1) The solder paste removed from the refrigerator must be thawed (2) Check whether the solder paste has expired, (3) Avoid the solder paste staying on the template for too long, (4) Avoid printing under high humidity conditions.

More solder tin: The solder at the soldering place is much more than the normal demand, which makes it impossible to see the outline of the welded part or the solder to form a piled ball.

the reason:
1. The opening of the leaky version is too large;
2. The viscosity of solder paste is small.
solution:
1. Reduce the opening of the leaked version;
2. Increase the viscosity of solder paste.

Pointing: There is solder that protrudes outward in the form of needles or thorns, but there is no electrical short circuit formed by connection or contact with other places that should not be interconnected (such as pads or wires, etc.), also called burrs and drag tail.

the reason:
1. No wiping steel mesh or wiping frequency is not enough;
2. Equipment problems, no water spray, steel mesh paper can not wipe the steel mesh;
3. Repair welding occurs.
solution:
1. Standardize the wiping frequency of the tin printing machine (generally set to 5-6 boards for a wiping cycle);
2. Adjust the equipment to a normal level;
3. Training the skills of repair welders.

Holes: Various gases in the solder are improperly or poorly discharged during soldering. After cooling, holes of various shapes are formed on the surface or inside.

Cause: During the reflow, it is caused by the outgassing of the flux trapped in the solder joint.
solution:
1. Improve the solderability of components / substrates;
2. Use flux with high activity;
3. Reduce solder powder oxide;
4. Use inert heating gas;
5. The smallest component coverage area;
6. Separate molten solder joints during welding;
7. Slow down the preheating phase before reflow to promote soldering, and use the appropriate time at peak temperature.

Desoldering: also called open welding, open circuit; the pins / solder ends of the components are all detached or deviated from their corresponding pads, and the proper welding is not performed.

the reason:
1. Along with the tombstone and fierce wicking;
2. Lifting of components and PCB.
solution:
1. Use a solution to the tombstone;
2. Strengthen the rigidity of components through packaging design to avoid local heating.

Solder joint peeling: the pins of the welded components at the welding place

Solder joint stripping: The pin / weld electrode plating of the soldered component at the soldering site is separated from its body, or the soldered pad is stripped from the substrate.
Reason: due to the shrinkage stress of solder during cooling, reduce the amount of solder appropriately.
solution:
1. Improve PCB quality;
2. Reduce the amount of solder;
3. The slope of cooling.

Turned parts: The components are turned 180 degrees, according to IPC-A-610D welding standard capacitance resistance turned parts are acceptable (of course, some customer special requirements are not accepted, subject to customer requirements), other types of turned parts are not acceptable.
Reason: Bad feeder. The pressure of the feeder is too high. When the feeder is feeding, the thrust is too large, and the inertia acts, and the component rushes out of the turning piece and sucks.
solution:
1. Reduce Feida pressure;
2. The center of the material must correspond to the center of Feida.

Flux residue: white spots (the flux residue that remains on the printed board after solder cleaning, white, yellow, gray or brown) and carbonized residue (caused by excessive heating)
the reason:
1. Avoid excessive heating and flux problems in solder paste;
2. Avoid excessive heating and prevent oxidation.
solution:
1.1 Use flux with thermally stable ingredients;
1.2 Flux using anti-oxidation stable ingredients;
1.3 Use flux that does not form insoluble metal salts;
1.4 Use properly cured solder mask and laminated PCB;
1.5 Cleaning agent with proper dissolving ability for flux residue;
1.6 Use a lower reflow temperature;
1.7 Use a shorter reflow time;
1.8 Use mechanical agitation when cleaning;
1.9 Use appropriate cleaning temperature;
2.1 Use flux with thermally stable ingredients;
2.2 Flux using anti-oxidation stable ingredients;
2.3 Use a lower reflow temperature;
2.4 Use shorter reflow time;
2.5 Use inert reflow gas.
3.If the residue is cleaned after reflow, the following three solutions will help alleviate the symptoms of carbonized residue:
(1) Use appropriate solvent cleaning agent to clean the flux residue;
(2) Use mechanical agitation when cleaning (use ultrasonic agitation or use higher spray pressure for cleaning);
(3) Adopt proper cleaning temperature (experiments have been made, black spots can be seen when manually washing with water at 77 ℃, no black spots can be seen when cleaning the components with room temperature water)

Solder cracks: There are visible or invisible fine cracks on the surface or inside of the solder at the solder joint.

the reason:
1. The top of the machine is broken;
2. Turnover storage.
solution:
1. Draw the position map of the thimble when connecting to prevent the component from being damaged;
2. The correct turnover storage method.