This paper introduces that solder bead is a defect that may cause short circuit. It can be improved by reducing the amount of solder paste deposited on the printed circuit board (PCB).

When discussing tin beads, we first need to accurately define smt defects. The tin bead is found on the board that has been reflowed. You can see at a glance that it is a large tin ball, embedded in a pool of flux close to discrete components. These components have very low ground clearance, such as chip resistance and capacitance, thin small outline package (TSOP), small outline transistor (SOT), D-PAK transistor, and resistance assembly (Figure 1). Tin beads are often called “satellites” because of their location in relation to these elements. For obvious reasons, tin beads are sometimes called “extruded balls in the middle of a sheet”, or something similar. Compared with tin balls, tin balls are characterized by some tiny balls agglomerating along the periphery of the flux residue, or these balls are stuck around closely spaced pads and solder resistances. When asked about the prevention and treatment of tin balls or tin beads, the first question should be “What is it like and where does it occur?”

Why is it a problem

Basically, tin beads may form a tin “bridge” from one component terminal to another, thus resulting in no electrical connection in design. This may cause the risk of short circuit. If the vibration causes the solder bead to loosen and move, the short circuit may occur in the place where the solder bead was originally formed or anywhere on the assembly. Although the short circuit may not occur even if the above situation occurs in the tin bead, the tin bead is still a defect that should be minimized or eliminated.

How did it happen

Before discussing the actual causes of tin beads, it is helpful to show their dynamics step by step through images:

—Solder paste is printed on the bonding pad on the circuit board through SMT laser steel screen.

—During component mounting, some solder is squeezed under the component and separated from the solder on the pad.

—During reflow, the solder trapped under the component does not flow back to the pad. In turn, its cohesive property (surface tension) makes it form a large tin ball (bead).

—The surface tension of the cooling solder pulls the element closer to the pad. When the element is pulled down, the tin bead extrudes from the edge and stays there.

The solder paste under the SMT patch steel mesh leaks due to excessive scraper pressure or improper sealing between the SMT steel mesh and PCB,

Tin beads may also occur. These pastes are transferred to PCB beyond the hole size. During reflow, it may be left on the PCB adjacent to the component opening in the form of tin beads.

Why does it happen

In short, tin beads are usually related to excessive solder paste precipitation. Due to its lack of “body”, they are squeezed under discrete elements and then formed as described above. As a common welding defect, the increase of tin beads can be attributed to the increase in the use of wash free solder paste. In the early 1990s, when rosin based RMA solder paste was still popular, solder beads were an uncommon defect. The rosin based solder paste is less likely to be squeezed under the component body than the wash free solder paste; The wash free solder paste has less protective materials than RMA solder paste, and does not have the rigidity or texture of these thicker solder pastes. Therefore, when the chip elements are mounted into the wash free solder paste, the solder paste is easier to squeeze under the components. When too much solder paste is deposited, extrusion is more likely to occur.

Just as there are many reasons for tin balls, there are also some factors that cause or promote the formation of tin balls:

—The misalignment between the SMT mesh opening and the pad may cause the solder paste to be printed on the solder mask layer of the board, forming solder beads.

—The operator is not aware that the solder paste may be sent to the solder mask when attempting to straighten the biased components. It is usually unwise to attempt to move the element before reflow. Twist the mounting position of the element, usually with tweezers, which may cause short circuit or cavity. On the contrary, solder paste and reflow furnace should be used to float the components to the bonding pad, so that the components can be positioned correctly.

—Solder beads and tiny solder balls may also be caused by worn equipment, SMT patch steel mesh and scraper, as well as bent circuit boards or steel meshes that are not rubbed enough.

Experimental methods

Although solder beads are usually application related, the solder paste in use may affect them. Therefore, it is important to develop a set of test methods to determine whether a particular solder paste is more likely to have solder beads than others. Basically, this test should get the maximum amount of solder paste. Some paste manufacturers use test methods to develop pastes that greatly reduce the formation of solder beads. When testing the propensity of a solder paste to solder beads, 1:1 ratio of holes in the pad should be used. The test plate of 6mil laser steel mesh is assembled with misplaced components and correctly placed components. Most commonly, 1206 capacitors or resistors are used to mount with enough pressure, resulting in a backlog on the solder mask. In the reflow process, using a linear reflow temperature curve, reducing long-term insulation may reduce the role of tin beads. These changes help determine which paste formulations tend to form solder beads. Then, it is a simple process to evaluate each plate and corresponding solder paste by the number and size of solder beads.

conclusion

Tin bead is a phenomenon possibly caused by several factors. However, in almost every case, tin beads can be reduced or eliminated by reducing the amount of solder paste deposited on the PCB. In addition, the position of solder paste on the pad is also important. A feasible way to prevent solder beads is to let the SMT stencil manufacturer modify the design of the SMT stencil, that is, reduce the thickness of the SMT stencil, the area where the downward step is used, and reduce the opening in size and shape.