7 Guidelines for PCB Design For Manufacturing (DFM)
Designing any part of manufacturing is the first and the most crucial stage as it helps to carry out the manufacturing process in a cost-effective manner. Not only conducive to cost control, the purpose of design for manufacturing is to improve the quality of final output also. Once the manufacturing process is clearly defined, the turnaround time of products can remarkably be reduced; also, the use of best components in the application making ensures less number of breakdowns in future. Thus, DFM is believed to be a key to achieving global standards of quality and reliability and also of cost-effectiveness, which help printed circuit board manufacturers increase their profit margin and chances of sustainability in the long run. Designing the manufacturing process also helps identify the loopholes and the common mistakes and provides ways to avoid repeating them.
Here are some of the guidelines that can be used in design for manufacturing (DFM) concept applicable for PCBs:
Layering in PCB
It is essential to find a balance between the layer count and over the board sides. The balanced way of defining layers helps in avoiding warpage which is mainly responsible for interference and poor signal flow.
Specifying surface finishes
If the applications involve tin-lead material, then Horizontal Hot Air Leveling is a preferred finishing style. In some cases such as ultra-fine pitch parts, CSP or QFN, HASL is to be avoided. Pad co-planarity and pad doming need to be tackled by using other alternatives to HASL. While designing RoHS applications, Electroless Nickel Immersion Gold is preferred as a surface finish material. ENIG offers better resistance to oxidation, has commendable surface planarity in addition to being comparatively inexpensive. It also offers better usability on uncoated contact points.
Decision about Fiducials
It is advisable to have 3 fiducials placed on the panel or board corners. These should be 0.050” round dot and should have 0.100” solder mask clearance. From rail edges, these fiducials should be of 0.190” from rail edges so that the obstruction due to clamps can be avoided. While designing larger arrays, the fine pitch components are to be placed nearby local fiducials.
The minimum solder mask clearance required is 0.002” from copper made pad edges. The solder mask dam size preferred between pads for minimizing solder bridging is 0.005”, industry approved minimum is 0.003”
Component spacing guide
Component spacing clearance surrounding BGAs should be 0.060” for easier inspection and seamless reworking. One must refer to IPC-7351 or device component specification chart to decide upon component footprints. The pad sizes should be uniform for QFN (Quad Fine Pitch No Lead) components. These should also be consistent with respect to each other for preventing the tilting that may cause ‘open’ position. For cleaner and effective inspection, the pad size should be extended beyond the shape of the component.
Covered, Plugged, Tented or Flooded are popular via masking options. You must cover vias to prevent solder from getting lost into the vias, and to avoid leaving the joint starved. Vias should not be used within the pattern of land, neither should these be closely connected nor made underneath components. These should also not be connected closely to pads. If at all vias are kept closer to pad, these should be masked or a solder mask dam is to be provided between the via and pad. This helps keeping the solder from getting absorbed into via which may cause the target joint to be devoid of solder.
Some general guidelines
1). Non-polarized parts should be provided with clear preference. Polarity should be placed in one direction with consistency to enable easy inspection and eliminate the possibility of errors.
2). Hole spacing for radial through-hole components should measure on the center side – 0.100”, 0.200” or 0.300”, while axial through-hole components can have center spacing ranging from 0.300” to 0.800”. A 0.125” of Keep Out Area should be maintained between the surface mount technology components and plated through hole lead.
3). The primary side should have all large massy arts while the secondary part should be fixed for lighter components.
4). Bal Grid Array (BGA) should not be mirrored on opposing sides as it makes reworking and inspection a bit tedious.
5). Test pads should be 0.030” – 0.040” wide where centers should measure 0.100”, and these and other components should be placed 0.030” apart.
6). And lastly, Environmental Test Screening requirements should be defined clearly as this screening is essential for ascertaining goodness of quality in the PCB.
So, these are a few guidelines on design for manufacturing PCB. Most of these guidelines are derived from the points laid out by the industry approved standards. PCB manufacturers find great support in standards as these give clear-cut guidance on how to carry out the whole process of making parts of the products so that they can win the trust of the bulk buyers. Working on laid guidelines gives a feeling of comfort and the buyers are able to trust the quality of the product. Moreover, following a DFM helps deliver the orders within the stipulated deadline.
Adopting a clear-cut and predefined ways of producing and creating a design out of these procedures helps a lot when the process is repetitive and the part is to be manufactured in insanely large numbers. A uniform code of fabricating ensures the diminished possibility of errors and the quality standards are met in every unit produced. Also, it helps save a lot of time on designing each product separately.