Going from a PCB design house to a non-associated PCB assembly house often poses a problem for the OEM customer. Simply put, design-for-manufacturing or DFM is completely neglected. A number of PCB design mis-steps can occur at layout, especially when you have inexperienced designers who have good intentions, but fail to understand the nuances of implementing that design through manufacturing.
Poorly executed PCB layout can cause fabrication and assembly issues. Areas of concern include pad definition, component footprint, layer stack-up, material selection, fan out, trace width, trace clearance, and others. For example, let’s take a fabrication issue. It occurs due to acute angle traces where a chemical is trapped in “acid traps” at those acute angles shown in Fig. 1. When the chemical isn’t cleaned away, it can eat away at the traces even after assembly and cause intermittent connections in the field, whereby the product might have a higher fail rate in the field.
For a more comprehensive study about neglecting DFM, read our EE Times article.
In the meantime, here are some tips and hints to get you on the road to DFM.
- Poor pad definition at layout can cause opens and shorts at assembly.
- Misplacement of a bypass capacitor can create noise in the circuitry.
- An inaccurate component footprint can cause non-manufacturability if the component library isn’t correct.
- Poor layout also results in registration issues. When there are many PCB layers, fine lines and spacing may cause mis-registration of holes and pads, leading to adverse effects on pads and vias during fabrication.
- Solder mask slivers may result due to poor layout. That’s when there isn’t enough solder mask between pads and vias.
- Also, tombstoning results due to non-uniform pad thickness or incorrect registration for the assembly line.
The PCB layout design engineer has to move gingerly through various stages to avoid these fabrication and assembly problems. It’s important to note that 80 percent of PCB layout errors happen due to incorrect part geometry or creation, bad hole definition, inadequate spacing between through holes and surface mount components, and lack of rework ability around critical components.