The increasing level of automation in PCB manufacturing has led to the production of extremely fine pitch devices. Surface Mount Technology (SMT) PCB design has been a huge catalyst for this result. Nonetheless, every PCB manufacturing process is bound to come across some limitations in terms of prototyping, mechanics, or manufacturing. For this reason, it is always important to ensure that certain best practices be maintained during the PCB design and manufacturing processes.
SMT Best Practices
The best practices given below are used to ensure that a SMT device can be manufactured.
Avoid Natural Bridges: Natural bridges are straight tracks between leads that create a trace over which the solder can run. This result can be seen as a defect during inspection, and may cause delays in the production process. This can be avoided by designing tracks from the ends of the pads instead of linear connections.
Weight and Thermal Balance: It is important to balance the weight of the PCB and thermal mass of the leads. These problems take place when a double sided board does not have enough support, and when the lead chip carrier creates too much heat within the PCB. Clear edges for stiffeners can help to provide support to the PCB and maintain its weight. Titanium stiffeners are a good choice as they provide strength and stability. For thermal imbalances, solder spheres can be attached between the IC and the PCB.
Vias and BGAs: Vias that occur under Ball Grid Arrays (BGAs) should always be masked to ensure that no solder from the balls leak onto the vias, creating a defect. Also, vias should be avoided in pads as the solder can run down the via. This can lead to difficult reworking and loss of control over the volume of the solder joint.
Plating: Appropriate plating can prevent the occurrence of open circuits and open leads. Plating also helps to maintain a flat finish on the PCB when it comes to materials such as tin finishes and gold plating.
Damming: Use solder mask dams to minimize solder bridging between pads. These are extremely important to fine pitch devices. The reason is that a PCB without solder dams can increase the risk of short circuiting. Generally, the minimum size of the dams should be .003″ between leads.