When it comes to reducing the manufacturing cost of a PCB, it is important to remember that a significant part of the cost of a PCB assembly is already baked in at the design concept stage. The product concept defines the PCB size and shape, while the performance of the system dictates the microprocessor, memory and other functional elements of the circuit.
However, some of the assembly cost can still be affected by design choices made by the PCB designer. A PCB that is designed for manufacturing (DFM) can reduce both the fabrication cost of the PCB and the costs of component assembly and soldering. A DFM approach to design can also reduce the likelihood of the hidden costs of poor yield in production.
The best way to achieve the lowest cost, most manufacturable design is to work closely with the prospective manufacturer, since the way to maximize DFM cost savings is to design for a manufacturer’s specific equipment and technology, rather than to rely on general rules.
The first step to reducing costs in PCB fabrication to reduce the number of operations performed by the manufacturer. The second step is to optimize the PCB design to leverage a manufacturer’s particular fabrication technology.
One way to reduce costs is to eliminate or minimize the amount of machining required around the board edge or within the PCB itself. Rectilinear PCB outlines without internal slots will minimize machining, and the rectangular shape allows the PCB to be grouped in larger panels that are separated after assembly. These large panels streamline assembly by allowing a several PCB to go through component assembly and reflow at the same time, improving throughput. For example, PNC can process a maximum panel size of 18” X 24.” To ensure that the components on the PCB are not damaged during the scoring and separation from the panel, components should be kept 200 mil from the board edge.
The generally accepted rule that reducing the number of PCB layers in a stack-up will reduce cost has become more complicated with the advent of HDI technology. The reason is that the cost of an additional Printed Circuit Board layer is not linear, so a cost calculation needs to be made for each jump in the number of layers. Is it cheaper to use finer trace widths and buried vias to reduce the layer count from six to four? Only the manufacturer is going to know. However, as a board gets past eight layers costs increase non-linearly with each additional layer. The aspect ratio of the through hole vias begin to become a factor, as well as the sheer number of vias that need to be drilled and plated to connect all those layers. At an eight layer stack up or above, the additional cost of HDI technology begins to make economic sense if it is used to reduce the number of stack-up layers required.
Respect the design rules for hole sizes and hole to copper clearance. If the real estate on the PCB allows it, selecting hole sizes clearances and annular ring sizes larger than the absolute minimum will improve fabrication yield. Here are the through hole design rules for PNC:
Non-Plated Through Hole (NPTH)
● Finished hole size (minimum)= 0.006″
● Edge to edge clearance (from any other surface element) (minimum)= 0.005″
Plated Through Hole (PTH)
● Finished hole size (Minimum) = 0.004″
● Annular ring size (Minimum)= 0.004”
● Edge to edge clearance (from any other surface element) (minimum) = 0.009″
To reduce assembly costs the objective is the same as reducing PCB fabrication costs; reduce the number of operations, and optimize the PCB design to leverage a manufacturer’s particular fabrication technology.
One easy way to reduce assembly costs is to stay away from the smallest passive packages. 0603 passives are easier to place than 0402 or the almost invisible 0201. If possible, chose active parts that have leads rather than ball grids, because they are easier to place, they can be visually inspected instead of x-rayed, and they are easier to rework.
Manual operations are always expensive, and the designer should do everything they can to avoid the need for them.
Component manufacturers have recognized this and now offer through-hole components (typically connectors) that can be reflow soldered. This technology called “Through-Hole Reflow” allows through-hole components to be soldered in the same reflow process as the SMD components, eliminating a pass through the wave soldering machine or manual soldering.
Finally, if possible, avoid putting components on both sides of the board. The cost of a higher density PCB with components on one side may be cheaper than a lower density PCB with components on both sides.
The best time to talk with the PC Board design experts at PNC is early in the layout process. They can tell you when to use HDI to reduce costs and can advise on how to optimize panel size. The experts in the assembly department can also work with you to select components that will reduce assembly costs and increase yield.
Give PNC a call today.