8 causes of bridging
The formation of solder bridges is one of the most common defects - the article presents eight of the most important causes of this phenomenon.
- No solder mask between adjacent pads. Placing a solder mask between adjacent pads can prevent solder bridges. A solder mask is an epoxy layer that is applied to those parts of the PCB where the designer does not want the solder to stick. It is essential that the designer places the solder mask between the adjacent pads of the SMD components. The designer should keep the solder mask at least 2 miles (0.002 inch) wide.
- Inadequate pad spacing or improper pad design. It is very important that the designer follows design standards when determining the size and spacing between adjacent pads. Following the manufacturer's suggestion for pad size needed for a particular IC is usually the best practice, in line with IPC guidelines. Typically designers will place some space around the pads, a 3-8 mil space between the edges of the pads and the solder mask (the width depends mainly on the space available between adjacent pads). Such areas without a solder mask as an additional measure to prevent the formation of solder bridges.
- Using a dirty template. Contamination of the stencil can also cause bridging. The "extra" amount of solder paste on the bottom of the template, leftover from previous printing, can be transferred to the PCB, thus creating a solder bridge. Optimizing the process and wiping the template frequently will usually prevent this problem. Another option is to apply the paste with a jet paste printer.
- Incorrect chemical composition of the solder paste. Poor solder paste chemistry, out-of-date solder paste, or operating temperature/humidity not as recommended by the supplier may also cause bridging problems. For example, higher than recommended operating temperatures may cause excessive spreading of the paste from the pads, which is a direct cause of solder bridges.
- Template incorrectly positioned. Any misalignment of the template apertures with the PCB pads will lead to solder build-up outside the pad. In the case of fine pitch precision circuits, this inevitably leads to solder bridges.
- Incorrect placement / dimensioning of components. Another common cause of solder bridges is the use of incorrectly sized SMD components, component misalignment, or misalignment. The use of an oversized SMD increases the likelihood of a solder bridge forming. A misaligned component may have one of its corners closer to its neighbor, and an item misaligned by a pick-and-place machine may be very close to the next: in both cases, there is a very high probability of solder bridges.
- Too much pressure when placing components. Even when a pick-n-place machine places items correctly and in the intended positions, the pressure it exerts on the item as it is placed is also important. The machine program takes into account four coordinates - X, Y, Z, and Ɵ or rotation. While the X, Y, and Ɵ coordinate position the component on the PCB surface, the Z factor controls the alignment height of the component from the PCB surface as the machine releases it. Standard practice is to release the component approximately 1/3 of the amount of solder paste deposit above the PCB surface, letting it lightly rest on the solder paste. If the operator programs the P&P machine to apply more pressure to the component, there is a risk of the solder paste being squeezed out from under the component and spilling the solder paste onto the adjacent pad, which can lead to a solder bridge.
- Inadequate preheating time. An inadequate flow profile, consisting of the wrong combination of temperature and heating time, can also cause solder bridges. The deposit of the paste requires some time after liquefaction to reach the intended position. This only happens if the solder is able to wet both the pads and leads of the component when all contact areas are at the same temperature. However, component leads may e.g. have a lower thermal mass and tend to heat up faster than PCB pads, which often have a higher thermal mass. In addition, greater airflow around the components helps them heat up faster. As a result, the hotter leads of the components attract molten solder faster than the pads. Therefore, the preheating process requires a longer preheating time for the pads to catch up before the solder can evenly wet the component and pads.