How is Component Tombstoning Prevented During Flexible Printed Circuit Boars?

During Flexible Printed Circuit Boars

Tombstoneing is a common issue that many PCB fabricators and assemblers have to deal with. While most people tend to blame this problem on a misaligned process or the quality of a fab and assembler, there is also a design element that can lead to tombstoning. This article from Rush PCB Inc will discuss how to prevent it in your next flexible printed circuit board.

There are several factors that can cause PCB tombstoning during the reflow soldering process. One of the most common causes is uneven wetting force between two ends of a component. The reflow process causes the solder on one end to melt faster, causing it to rise up and overpower the other side, creating the tombstone effect. Another common cause is a lack of uniform thermal conductivity of the PCB. This can be due to improper temperature control or the printing of the solder paste. The solution to this is to use a stencil, which helps ensure that the correct amount of solder paste is applied and that the thickness is consistent.

Another cause of tombstoning is the incorrect placement of components. It is important to place all components in the same position on the board, and ensure that they are properly aligned. This will help avoid problems like reversed polarity and unplugged leads. It is also a good idea to review the datasheets and documentation for polarized components, such as diodes and electrolytic capacitors, and mark them correctly in their footprints. This will help to ensure that the right orientation is used in the wetting process and minimize the likelihood of tombstoning.

How is Component Tombstoning Prevented During Flexible Printed Circuit Boars?

As a designer, it is important to be aware of the issues that can arise with flex circuits. For example, the choice of copper foil is a critical decision for a flex circuit that will be bending or rolling often. Using low-grade bare copper may cause work hardening or fatigue cracking, which can result in a reduced life of the circuit. Instead, you should consider using higher-grade rolled annealed (RA) copper. This is more springy in the Z-deflection direction, which is the direction that the circuit will be bending or rolling.

To help avoid the problem of tombstoning, you should ensure that your traces and pads are of equal width and length. You should also try to keep the distance between adjacent tracks at a minimum of 1 mm. This will prevent the track with the thicker wire from drawing away heat from the pad, which will lead to an imbalance in wetting forces and cause tombstoning.

In addition, you should ensure that the stencils you use are properly designed and that they have a consistent thickness. You should also check the accuracy of your SMT machine and the pick and placement data to ensure that it is accurate. This will help to avoid issues like tombstoning, and can make your flex circuit production much more efficient.