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How to avoid the PCB tombstone in PCBA process?

The PCB tombstone , also known as the Manhattan or drawbridge effect, is a chip (passive) component assembly defect situation. The cause of the PCB tombstone is that the melting time of the solder paste at both ends of the part is inconsistent, resulting in uneven force at both ends of the chip component, which is relatively light in mass and will cause one side to roll up under the action of stress.

Introduction of PCB tombstone

Introduction of PCB tombstone
Introduction of PCB tombstone

Before reflow soldering or before the solder paste melts, due to the gel component in the solder paste, the two ends of the component are fixed on the PCB pad by the adhesion force of the solder paste and the gravity itself, and the components will not move when the PCB is on the track.

In the reflow soldering phase, the adhesion of the paste disappears as the solder paste melts. Before wetting begins, the electronic component is actually suspended on the surface of the liquid solder due to the surface tension of the liquid, subject to an upward buoyancy force.

If the solder of the two pads of the component does not wet the two solder terminals at the same time, that is, the rate difference between the wetting of the two ends is large, or the wetting force gap between the two ends is large, the phenomenon of PCB tombstone or offset will occur.

In fact, most of the locations where the PCB tombstone occurs are chip capacitors or inductors, while resistance rarely occurs. This can actually be seen from the shape and structure of the component, the chip capacitor or inductor end face is usually square, the height value is large, and the resistance is basically a rectangular shape with a small height, so the probability of the resistor PCB tombstone will be relatively small.

How did the PCB tombstone come about

1. The PCB stencil is too thick and the amount of solder paste is too much;
2. The spacing between the stencil and the PCB is too large;
3. The pressure of the scraper of the printing machine is too small;
4. Multiple printing;
5. Solder paste collapse (low viscosity);
6. Solder paste printing offset;
7. Components placement offset;
8. The preheating temperature of the reflow soldering furnace is too high;
9. Use a nitrogen stove;
10. The peak temperature is too high/the secondary temperature ramp rate is too steep;
11. The width of termination electrode of the component is too small.

How to solve the PCB tombstone effect

1. Change the stencil thinner;
2. Adjust the parameters of the solder paste printing machine;
3. Adjust the scraper pressure of the solder paste printing machine;
4. Print solder paste only once;
5. Shorten the storage time of SMT solder paste after printing (adjust the viscosity of solder paste);
6. Adjust the parameters of the solder paste printing machine;
7. Adjust the placement machine data files;
8. Adjust the reflow oven settings.

How to avoid the failure of the PCB tombstone caused by SMT

How to avoid the failure of the PCB tombstone caused by SMT
How to avoid the failure of the PCB tombstone caused by SMT

In fact, it is to avoid the uneven force at both ends of the component during the welding process.

If the plating solderability of the two soldering terminals of the component is poor, the solderability between the two terminals is different, and the wetting ability is not the same, poor wetting, offset or PCB tombstone problems may occur.

The temperature of the reflow soldering furnace is uneven, resulting in inconsistent temperature at both ends of the chip, while the temperature difference causes the melting degree of solder paste to be inconsistent, and the stress difference at both ends of the component causes PCB tombstone.

Solder paste printing offset. If the paste print is offset and not completely deposited on the pad, this can result in component terminals not making effective contact with the paste, which is highly likely to create a PCB tombstone or offset problem.

Insufficient placement accuracy can also lead to monuments. This is the same as the offset of solder paste printing, where the paste cannot be in sufficient contact with the two terminals of the component, resulting in a wetting difference at both ends, and a monument or offset may occur.

The reflow temperature curve is too steep, and the temperature rises suddenly, which is easy to cause the temperature of the solder pads at both ends of the component to be uneven, and the temperature rise slope of less than 2 °C per second can prevent the PCB tombstone problem.

The quality of the solder paste is not good. Solder paste is an essential material for the reflow soldering process and is a paste solder made by homogeneous mixing of alloy powder (granules) with a paste flux (rosin, thinner, stabilizer, etc.) carrier.

Among them, alloy particles are the main component of forming solder joints; Flux removes the oxide layer on the solder surface and improves wettability, which is a key material to ensure the quality of solder paste. In terms of weight, solder paste is generally 80%~90% metal alloy, and in terms of volume, metal and flux each account for 50%.

To ensure the quality of solder paste is mainly reflected in the storage and use of two aspects, solder paste is generally stored between 0 °C ~ 10 °C (or stored according to the manufacturer’s requirements).

In terms of use, the use environment of solder paste generally requires the temperature of SMT patch processing workshop to be 25±3 °C, humidity: 50±10% (related to the characteristics of solder paste), the use should be in accordance with the “first in, first out” principle, and make a good record of take-up to ensure that the temperature recovery time is greater than four hours.

Sufficient stirring is required before use to make it viscous with excellent printability and mold release. After adding the solder paste, the inner and outer lids of the solder paste can should be immediately closed, and the reflow soldering should be completed within 2 hours after printing.

The above are all problems that PCBA manufacturers need to guarantee, but sometimes PCB tombstone may also occur when layout, so how to avoid PCB tombstone during PCB Layout design?

How to avoid PCB tombstone when layout

How to avoid PCB tombstone when layout
How to avoid PCB tombstone when layout

First of all, it is necessary to pay attention to a reasonable pad design.

The pad design should strictly maintain symmetry, that is, the shape and size of the pads on both sides should be exactly the same to keep the stress on both sides of the component the same when the solder paste melts, and avoid the phenomenon of PCB tombstone.

SMT pad design is a very important part of PCB design, which determines the soldering position of components on the PCB. With a reasonable pad design, a small amount of placement offset during SMT can be automatically corrected by the surface tension of the molten solder paste during reflow soldering.

If the PCB pad design is unreasonable, the molten solder paste causes uneven forces on both sides of the component, which will cause component offset and soldering defects such as PCB tombstone.

If the length of the pad of the chip component is too short, it will cause defects such as shifting, open circuit, and inability to solder, and conversely, the width of the pad is too wide, which will also lead to component displacement, PCB tombstone and other defects.

The pad connection copper plate needs to be made to make a thermal relief connection to avoid a large area of copper connection caused by pad heat dissipation vias, resulting in uneven heating at both ends of the component.

The wire should exit symmetrically from the pad, and the width of the trace into the pad should be symmetrical.

Also note that when PCB design , the two components should not share common pads.

Conclusion

The above is all the content related to why the PCB tombstone occurs in the PCBA SMT patch, in fact, the PCB tombstone can also be prevented by a series of methods, such as the correct design and layout pad, the appropriate increase in the temperature of the insulation zone of the preset stage, the adjustment of the patch accuracy of the component, etc. can reduce the occurrence of the PCB tombstone, and can improve the processing quality of the product.

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