We all hope that the SMT process is perfect, but the reality is cruel. The following is some knowledge about the possible problems of SMT products and their countermeasures.
Next, we describe these issues in detail.
1. Tombstone phenomenon
Tombstoning, as shown, is a problem in which sheet components rise on one side. This defect can occur if the surface tension on both sides of the part is not balanced.
To prevent this from happening, we can:
- Increased time in the active zone;
- Optimize pad design;
- Prevent oxidation or contamination of component ends;
- Calibrate the parameters of solder paste printers and placement machines;
- Improve template design.
2. Solder bridge
When solder paste forms an abnormal connection between pins or components, it is called a solder bridge.
Countermeasures include:
- Calibrate the printer to control the print shape;
- Use a solder paste with the correct viscosity;
- Optimizing the aperture on the template;
- Optimize pick and place machines to adjust component position and apply pressure.
3. Damaged parts
Components may have cracks if they are damaged as a raw material or during placement and reflow
To prevent this problem:
- Inspect and discard damaged material;
- Avoid false contact between components and machines during SMT processing;
- Control the cooling rate below 4°C per second.
4. damage
If the pins are damaged, they will lift off the pads and the part may not solder to the pads.
To avoid this, we should:
- Check the material to discard parts with bad pins;
- Inspect manually placed parts before sending them to the reflow process.
5. Wrong position or orientation of parts
This problem includes several situations such as misalignment or wrong orientation/polarity where parts are welded in opposite directions.
Countermeasures:
- Correction of the parameters of the placement machine;
- Check manually placed parts;
- Avoid contact errors before entering the reflow process;
- Adjust the airflow during reflow, which may blow the part out of its correct position.
6. Solder paste problem
The picture shows three situations related to solder paste volume:
(1) Excess solder
(2) Insufficient solder
(3) No solder.
There are mainly 3 factors causing the problem.
1) First, the template holes may be blocked or incorrect.
2) Second, the viscosity of the solder paste may not be correct.
3) Third, poor solderability of components or pads may result in insufficient or no solder.
Countermeasures:
- clean template;
- Ensure standard alignment of templates;
- Precise control of solder paste volume;
- Discard components or pads with low solderability.
7. Abnormal solder joints
If some soldering steps go wrong, the solder joints will form different and unexpected shapes.
Imprecise stencil holes may result in (1) solder balls.
Oxidation of pads or components, insufficient time in the soak phase and rapid rise in reflow temperature can cause solder balls and (2) solder holes, low soldering temperature and short soldering time can cause (3) solder icicles.
Countermeasures are as follows:
- clean template;
- Baking PCBs before SMT processing to avoid oxidation;
- Precisely adjust the temperature during the welding process.
The above are the common quality problems and solutions proposed by the reflow soldering manufacturer Chengyuan Industry in the SMT process. I hope it will be helpful to you.
Post time: May-17-2023