prevent cracking in flexible PCBs
As with any type of product, if flexible PCBs are handled or bent beyond their mechanical capabilities, they may crack and break. Fortunately, cracking in flex circuits can be prevented by careful design and planning. Here are the three main causes of cracked flex PCBs:
The most common reason for a flexible pcb to crack is due to excessive bending or twisting. This can cause the copper layer to weaken or break off from the substrate. The best way to prevent this is by designing the PCB with a minimum number of layers and by following proper bend guidelines.
It is also important to route traces perpendicular to the neutral bend axis. This will eliminate stress points and help ensure that traces don’t break during flexing. It is also important to avoid abrupt changes from wide to narrow traces. This can also cause stress and a weak spot in the traces. Instead, try to use fillets or a gradual transition from the thin to the thick trace.
How do you prevent cracking in flexible PCBs?
Another common reason for a flex circuit to crack is the thermal cycling that occurs during soldering. This can be caused by high temperatures and uneven heat distribution. The best way to prevent this is by using a solder paste with low melting point and by avoiding using solders with a high melting point. It is also important to use a FR-4 or Kapton stiffener at critical locations on the flex circuit. This will help reduce the stress on the components and connectors attached to the flex.
Via construction can also play an important role in preventing cracking in flex PCBs. Through-hole vias are not recommended for flex circuits, but surface mount (SMT) vias can be used. These are smaller than through-hole vias and can better dissipate the heat from underneath them during soldering processes. Through-hole vias can also be prone to warping when the flex circuit is heated or bent, which can lead to pad lifting during assembly.
When using plated through-holes on a flex PCB, it is recommended to use annular rings rather than regular round pins. This will allow the plated through-hole to be positioned closer to the copper trace and will eliminate the stress concentration spots that are commonly found in pins with smooth, circular edges.
Finally, it is important to properly design the flex circuit and follow manufacturer guidelines when assembling it. This includes ensuring that the thickness of the prepreg used is greater than the coverlay by at least 2 – 3 mils. It is also important to consider the environmental factors that the flex circuit will be exposed to when designing it. For example, the flex circuit should be designed to withstand a temperature range of -20oC to 120oC. The flex circuit should also be designed to be able to resist repeated flexing up to 20 times. Finally, the flex circuit should be designed with a rigid section and a flex section to protect components and connectors.
