In the surface mount technology (SMT) process, the "tombstoning" phenomenon (also known as the Manhattan phenomenon, tombstoning) is a common but headache problem. It not only affects the welding quality, but also directly affects the reliability and yield of the product. Especially in mass production, if the tombstoning phenomenon occurs frequently, it will bring huge rework costs and production delays.
Based on actual production experience, this article will analyze the main causes of the PCB tombstoning phenomenon and provide a series of practical and effective solutions.
The so-called "tombstoning" refers to the process of PCB reflow soldering, in which one end of the chip component is molten to complete the soldering, while the other end is not soldered in time, causing the component to stand up like a "tombstone". This phenomenon is particularly common in small components such as chip resistors and capacitors (such as 0402, 0201), affecting the quality of solder joints and even causing circuit breakage.
1. Uneven solder paste printing or inconsistent thickness
If there is a large difference in the amount of solder paste printed on both ends of the component, one end will melt first during reflow heating to form a welding tension, and the other end will be pulled up because it did not melt in time.
2. Asymmetric pad design
Asymmetric pad size or solder mask window differences will cause uneven distribution of solder paste and inconsistent heating at both ends.
3. Improper reflow temperature curve setting
Too fast heating rate or uneven heating will cause one side of the component to reach the welding temperature first, causing unbalanced force.
4. Extremely small components or thin materials
For example, micro devices such as 0201 and 01005 are more easily pulled up by tin liquid when the temperature is uneven due to their small mass and fast heating.
5. PCB board warping or poor flatness
PCB board deformation will cause the soldering points at both ends of the component to be at different heights, thus affecting the solder paste heating and soldering synchronization.
6. Component mounting offset
The mounting position is not centered, which will also cause the solder paste to heat asynchronously, increasing the risk of tombstones.
1. Optimize pad design
Ensure that the pad is symmetrical and appropriately enlarge the pad window area; avoid too large a difference in the design of the pads at both ends to improve the consistency of solder paste distribution.
2. Accurately control the quality of solder paste printing
Use high-quality steel mesh, reasonably design the opening size and shape, ensure uniform solder paste thickness and accurate printing position.
3. Reasonably set the reflow soldering temperature curve
Use the heating slope and peak temperature suitable for the device and board to avoid excessive local temperature difference. The recommended heating rate is controlled at 1\~3℃/second.
4. Use appropriate mounting pressure and center positioning
The placement machine needs to calibrate the nozzle pressure and placement position to avoid thermal imbalance caused by offset.
5. Choose high-quality components
Components with stable quality and standard size can effectively reduce the problem of tombstones caused by uneven heating.
6. Control the warpage of PCB boards
Use PCB boards with consistent thickness and low warpage, and perform flatness detection; if necessary, add a pallet to assist in the process.
Although the tombstone phenomenon is a common process defect, as long as meticulousness is achieved in multiple links such as component selection, pad design, mounting process and reflow control, its occurrence rate can be significantly reduced. For every electronic manufacturing company that focuses on quality, continuous process optimization and experience accumulation are the key to improving product reliability and building a high-quality reputation.
