With the development of society and technology, electronic products are becoming more and more personalized. Portable, full-featured devices are loved by everyone. So many designers also consider this factor when designing circuit boards. In order to achieve a smaller and more comprehensive purpose, designers will choose to use components with smaller package. This not only satisfies the function, but also makes the product look more refined.
Components with small package have certain difficulties in soldering, the technical requirements in design and production are very high. We must take care in all aspects, otherwise soldering defect may occur.
What is Tombstone?
In early SMT manufacturing, the Manhattan phenomenon was a problem related to vapor phase reflow (VPR) and infrared reflow (IR) welding. In vapor phase reflow soldering, the temperature rises too fast, which causes the solder paste at the two ends of components with different heat capacities not to melt at the same time and caused the Manhattan phenomenon. In infrared reflow soldering, due to the different colors of heat absorption at the two ends of the component, the paste does not melt at the same time and occurred Manhattan phenomenon, and we also call this phenomenon as Tombstone.
Generally speaking, the SMT Tombstone is that after reflow soldering, one end of the component is soldered to the PCB pad, and the other end is not soldered for some reason. And it is obliquely or upright at one end of the pad. Visually, it stands on the PCB like a tombstone, so we use the word “Tombstone” to describe this defect phenomenon.
What kind of components are prone to such defect after reflow soldering?
Firstly, it only occurs in SMD components. As mentioned above, this phenomenon is more likely to occur in chip components with small packages. For example, the capacitance and resistance of 0201, 0402 and 0603. Big packaged resistors and capacitors, such as 2010, are not easy to stand up due to the weight of the components.
The root cause of the tombstone phenomenon is the unbalance soldering tension on the two sides of the component, which causes the wetting of the two ends of the component to be uneven. Then, the following situations will cause an unbalanced wetting force at both ends of the component during reflow soldering during SMT chip processing, resulting in the Tombstone Phenomenon.
1. PCB design problems
A. If the pads are too far apart, the wetting force will cause the solder wetting element to move away from the solder paste.
B. If the size of the two sides of pads is different, the solder paste be subjected to different stresses, which will cause a soldering state during the soldering process.
C. Unreasonable components placement design. Excessive temperature difference in PCB leads to uneven heat absorption on both sides of the components. For example, heterogeneous components, QFP, BGA and so on, with large heat absorption will have uneven temperature on both sides of the solder pads of small packaged components around, so the solder paste dissolves differently, resulting in uneven soldering tension.
2. Material Problems
The non-uniform thermal conductivity of PCB, the poor weldability of components, the oxidation of components will also cause the phenomenon of Tombstone.
3. Soldering Paste Problems
A. If the holes opening of the stencil are not correct, or if the stencil is too thick, these reasons will make the amount of solder paste unsuitable and cause the Tombstone problem.
B. If the parameters of the solder paste printing machine are set incorrectly, the blade pressure of the printing machine is too small, or the printing is repeated for many times, these factors will affect the quality of SMT.
C.The activity of the solder paste is also a factor.
4. Progressing problems
A. Poor accuracy of SMT machine, caused SMD placement deviation. If the component is misaligned, the component offset generated during placement will be automatically corrected during the reflow process due to the surface tension when the solder paste melts. However, if the shift of component’s placement is serious, the tension will cause the component to stand up and cause the Tombstone.
B. When the preheating temperature is set low and the preheating time is short, the probability that the solder paste at both ends of the component does not melt at the same time will increase greatly, resulting in an imbalance in tension at both ends and causing Tombstone.
C.The furnace temperature curve is unreasonable. If the time in the reflow oven is too short and the temperature zone is too small, it will lead to uneven heating of the PCB, excessive temperature differences on the PCB, and resulting an uneven wetting force.
How to avoid the Tombstone phenomenon?
First of all, when designing chip resistors and capacitor pads, it is necessary to strictly maintain their overall symmetry, that is, the shape and size of the pad should be exactly the same to ensure that the total force acting on the solder joints on the component is zero, when the solder paste melts. Designing is the first step in the manufacturing process, and improper pad design may be the main reason for erecting the component.
In addition, to choose the right PCB material. Components must be carefully inspected to ensure no oxidation.
However, choose the solder paste which has good activity and check the solder paste quality regularly.
What’s more, controlling the amount of solder paste. When the thickness of the solder paste becomes thinner, the possibility of the Tombstone will greatly reduce. Thinner the solder paste, the probability of the solder paste on both pads melting at the same time is greatly increased. Therefore, the stencil should be made according to the design characteristics, and the appropriate thickness should be selected to avoid excessive solder paste. The parameter settings and scraper strength of the printing machine also need to be adjusted in time to ensure that the printed solder paste is uniform.
Besides, if component placement deviation is found, you should immediately check whether the accuracy of SMT machine and whether pick and place data is correct.
Lastly, the preheat temperature and time of the reflow furnace must be sufficient. And to set a big furnace temperature curve according to the solder paste, and then optimize the furnace temperature curve according to the structure and layout of different PCBs to optimize the furnace temperature curve for each project. The reflow furnace must be tested and find the suitable temperature curve before mass soldering.
The Tombstone phenomenon is very obvious, and it is easier to find than other soldering defects. This defect can be found immediately after the reflow is completed. There are many causes of tombstones, but as long as to more careful at each aspect, it can be avoided.
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