What are the common causes of tin beads
Tin bead is common welding defect during reflow soldering, which occurs during the rapid heating welding process; or suddenly enters the welding zone while too low temperature in the preheating zone , which is easy to produce tin beads. The common causes of tin beads are concluded as following.
(1) The reflow temperature curve set improperly.
First, insufficient preheating and the temperature or time requirements are not met, so that the flux is not only less active, but also has less amount volatilization, not only cannot remove the oxide film on the surface of the pad and the solder particles, but also the solder paste powder can’t rises to the solder surface . It is impossible to improve the wettability of the liquid, and it is easy to produce tin beads.
Solution: Prolong the preheating time at 120 to 150 °C. Secondly, if the temperature of the preheating zone rises too fast, the time to reach the flat top temperature is too short, leading to the moisture and solvent inside the solder paste be not completely volatilized, the water and the solvent may be boiled,spilled tin beads when reaching the reflow soldering temperature zone. Therefore, attention should be paid to the heating rate, and the wettability of the preheated solder is affected, leading to generate tin beads easily. As the temperature increases, the wettability of the liquid solder will be significantly improved, thereby reducing the generation of tin beads. However, too high reflow temperature will damage the components, printed boards and pads. Therefore, the proper welding temperature should be selected to make the solder have better wettability.
(2) The flux not working.
The function of the flux is to remove the oxide film on the surface of the pad and solder particles, thereby improving the wettability between the liquid solder and the pads and component leads (weld ends). If the soldering paste is coated for a long time and the flux is easily volatilized, the deoxidation of the flux is lost, the wettability of the liquid solder is deteriorated, and tin beads are inevitably generated during reflow soldering.
Solution: selecting solder paste with working life of over 4h, or to minimize the placement time as short as possible.
(3) Too large opening or severely deformed of stencil .
If tin beads always appear in the same position, it’s necessary to check the design structure of the metal stencil. The dimensional accuracy of the stencil opening is not up to standard. The rather large pad and the softer surface material (such as copper stencil),causing the outline of the solder paste will be unclear and bridging interconnected. Which mostly occur in the pad printing of the pitch device, a large number of tin balls between the pins are inevitably generated after reflow soldering.
Solution: The appropriate stencil material and manufacturing crafts should be selected according to the different shapes and center distance of stencil, strictly control the stencil manufacturing craft, or use the method of laser cutting and electropolishing to make the stencil.
Excessive pressure placed on the chip components
Excessive placement pressure can squeeze the solder paste out of the pad. If the solder paste is coated too thick, the excessive placement pressure makes it easier to squeeze the solder paste out of the pad. Tin beads are inevitable to generated.
Solution : controlling the thickness of the solder paste while reducing the placement pressure of the chip components head at the same time.
(5) The solder paste contains moisture.
If the solder paste is taken out from the refrigerator and directly opened for usage, the water vapor will be condensed due to the large temperature difference, which easily causing boiling and splashing of water to form tin beads during reflow soldering.
Solution: after the solder paste is taken out from the refrigerator, it should be placed at room temperature for over 4 hours. Until the temperature of the pad in the sealed cylinder reaches the environment temperature, then open the cover.
(6) The printed board is not cleaned completely, so that the solder paste remains on the surface of the printed board and through holes.
Solution: strengthening the responsibility of operators and craft personnel in the production process, strictly follow the craft requirements and operating procedures for production, and strengthen the quality control of the craft process.
(7) Using non-contact printing or excessive printing pressure .
There is a certain gap between the stencil and the PCB in the non-contact printing. If the pressure of the squeegee is not controlled well, it is easy to make the solder paste under the stencil to the non-pad area of the PCB surface, and then the tin balls will inevitably be generated after the reflow soldering.
Solution: using contact printing or reduce printing pressure if there are no special requirements.
(8) Flux failure.
If the time from SMT to reflow is too long, the solder particles will deteriorate due to oxidation of solder particles in the solder paste, and the activity will be reduced, which will cause the solder paste to no longer flow and the tin balls will be generated.
Solution: using longer life solder paste (at least 4h).
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