We all hope for a flawless SMT process, but it’s the ideal situation. Here are some knowledge about the problems that may occur in the SMT products and their countermeasures.
In the last post, we’ve mentioned that there’re mainly four steps in SMT process, in which the stencil is prepared for solder paste printing.
Now let’s see the distribution of the flaws that occur in different process.
Next, we’ll go into the details of these problems.
As the picture shows on the left, tombstoning is a situation that the sheet components raise up on one side.
If the surface tension of the two sides of the component is unbalanced, this defect will occur.
To prevent this from happening, we can:
- increase time on active zone;
- optimize the design of solder pad;
- prevent oxidation or pollution on the end of component;
- calibrate parameter of solder paste printer and pick-and-place machine;
- improve the design of stencil.
2. Solder bridge
When solder paste forms abnormal connection between pins or components, it’s called solder bridge.
- calibrating the printer to control the printing shape;
- using solder paste of correct viscosity;
- optimizing the apertures on the stencil;
- optimizing the pick-and-place machine to adjust component locations and placing pressure.
3. Parts damaged
Components may have nicks, cracks or fractures if they are damaged as raw material or during the placement and reflow process.
To prevent the problem:
- Check and discard broken materials;
- Avoid wrong contact between components and the machines during SMT;
- Control the cooling rate to below 4℃ per second.
4. Pin damaged
If pins are damaged, they will be lifted from the pad so the parts may fail to solder to the pad.
To avoid this, we should:
- check the materials to discard parts with bad pins;
- review manually placed parts before sending them to reflow process.
5. Wrong location or orientation of parts
This problem includes several situations, such as misalignment (as shown on the left), or wrong orientation/polarity, in which the parts are soldered in the opposite direction.
Here are the countermeasures:
- correct the parameters of the placing machine;
- check the parts that are manually placed;
- for misalignment, avoid wrong touching before going into reflow process;
- adjust the airflow during the reflow process, which may blow the parts out of the correct place.
6. Solder paste problems
Shown on the left are three situations related to the amount of the solder paste:
(1) excess solder
(2) insufficient solder
(3) no solder.
There are mainly 3 factors causing the problem.
1) First, stencil apertures may be blocked or incorrect, so that wrong amount of solder paste are printed.
2) Second, the solder paste may be of wrong viscosity.
3) Third, bad solderability of the parts or the pad may cause insufficient or no solder.
- clean the stencil;
- ensure the standard alignment of the stencil;
- precisely control the solder paste amount;
- discard parts or pads with low solderability.
7. Welding point abnormality
The welding point will form different unexpected shapes (shown on the left) if some soldering steps are going wrong.
Inaccurate stencil apertures may cause (1) solder balls.
Oxidation of the pad or parts, not enough time in soak phase and rapid raising of reflow temperature may cause solder balls and (2) solder holes.
Low soldering temperature and short soldering time may cause (3) solder icicles.
Countermeasures are as below:
- clean the stencil;
- bake PCBs before SMT to avoid oxidation;
- Precisely adjust the temperature during the soldering process.
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