Final revision date: 27/07/2022
Capacitors may become cracked when boards bend during handling in processes after the capacitors are soldered onto the boards. An effective method for preventing this is to orient capacitors relative to the board bending direction so that they are not easily affected by stress.
This lesson introduces part layouts that help prevent board warping and bending from applying stress to parts.
1) Board stress direction and part mounting orientation
Figure 1 shows examples of parts mounted vertically and horizontally relative to the board stress direction. The effects of stress from the board can be reduced by orienting parts horizontally relative to the direction in which the stress acts.
Figure 2 shows an image of the results of evaluating cases 1 and 2 above by board bending-resistance tests. This shows that orienting parts as shown in case 2 increases the resistance to board bending and helps prevent stress from being applied to parts.
2) Capacitor layout near board breaks
Stress from the board is most easily generated in the board break and board cut processes, so care must be taken for these processes. For example, when parts are positioned near board breaks as shown in Figure 3, the parts are more easily affected by stress in ascending order of B, D < C < A.
Now then, let's examine how boards deform depending on the presence of a slit.
Figures 4 and 5 show FEM analysis results of the difference in board strain depending on the presence of a slit.
A part is assumed to be mounted at the location indicated in the model figure. (The boards are 1.6 mm-thick FR4.)
Figure 4 shows the case without a slit. Here the board stress is large, high tensile stress (red to yellow) acts on the location where the part is mounted, and there is the risk of capacitor cracking.
In contrast, Figure 5 shows the case with a slit. Here the location where the part is mounted is green, indicating that there is little or no board strain in this location. This indicates that the stress applied to the part can be greatly suppressed, so this is an effective means of preventing capacitor cracking.
To summarize, orienting parts parallel to the break line (D in Figure 3) is the most effective means of reducing stress due to board breaks. In addition, when the part orientation cannot be changed, making a slit so that the part is not easily affected by board deformation is effective (B in Figure 3).