|Ask the Experts
February 20, 2023 - Updated
December 1, 2021 - Originally Posted
Design Considerations and Impact on Assembly
What are the most common design decisions that can have a negative impact on the PCB assembly and manufacture and how can we correct them during the design?
|Expert Panel Responses
An oft forgotten (until testing starts) design consideration is the spacings required by regulatory agencies (UL, VDE, etc) between “mains” and low voltage elements. This not only applies to spacings between traces on the board, but the spacings between and selection of components. Consider these design issues early on in the process, before you start board layout, if you are developing assemblies that must have regulatory agency approvals.
Also, consider thermal loading over the surface of the board(s). Do your best to spread evenly the (thermal) mass of the components over the area of the board. Concentrating heavy components in one area and lighter components in others can cause uneven heating during solder (reflow) which can really mess with the thermal profile required to solder properly, forcing the use special jigs or pallets to compensate. This may mean separating massive components (typically high power/large components) from lighter/smaller components onto different boards; don’t try to cram everything onto one board.
Senior Project Engineer
Electronic Controls Design Inc
These questions have a wide and long answers but i will try to provide some recommendations that helps to you to minimize errors or omissions during the design stage. In my experience, each variable or decision that we take on design stage, have a consequence on manufacturing.
I recommend first that fully know the class and usage of your product to determinate the design rules, the components and kind of the materials to use. this because combined with the design rules, will determinate the reliability and life of your product. The series of J-STD-001 to 006 can help to you to understand and select the right alloy or flux for your product and design according mechanical and chemical properties and characteristics of the solder and flux.
Once have an idea of the class and type of product you need design and manufactured, I fully recommend the series 2220 of IPC. This series is built around IPC-2221, Generic Standard on Printed Board Design, the base document that covers all generic requirements for printed board design, regardless of materials. From there, the designer chooses the appropriate sectional standard for a specific technology. All five sectional standards are included with the series: IPC-2222, Sectional Design Standard for Rigid Organic Printed Boards; IPC-2223, Sectional Design Standard for Flexible Printed Boards; IPC-2224, Sectional Standard for Design of PWBs for PC Cards; IPC-2225, Sectional Design Standard for Organic Multichip Modules (MCM-L) and MCM-L Assemblies; and IPC-2226, Sectional Design Standard for High Density Interconnect (HDI) Printed Boards.
Once you have all this and have a design, produce some samples. build some samples to validate the design and talks with the engineers of manufacturing to make a feasibility report. My finally recommendation is that.. If you will design a product, please create a multidisciplinary team (quality, engineering, manufacturing, purchasing, maintenance etc..) take all his recommendations and apply to you design if is possible.
In my experience, works separately and do not make a team, is the worst and most common error.
Engineering Director / Master IPC Trainer (MIT)
A few of the most common issues include:
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group
One item that I have experienced is to go straight into production without running a design for manufacturing or DFM. First, one thing to see your product on paper but that does not mean it will be easy to build. If this is happening to you I recommend inviting the design group to the production floor. Show them the capabilities and limitations of the manufacturing equipment including the capabilities and limitations of the assemblers. Second, sit down with the designer and discuss the project. I believe that the more you and your design team interact the easier the build.
Senior Manufacturing Engineer
Generally speaking, you should always strive to produce a board that is as simple as possible to manufacture and assemble. Always think about how you can relax your requirements, reduce the layer count to only what's necessary, and make a generous tolerance allowance for manufacturing errors.
The factors that affect PCB design include drill holes, annular ring, trace widths, spacing between the traces, power planes on the edge of the board, and many more.
For drill holes two significant aspects should be considered:
Annular ring width: There should be enough annular ring width to establish a solid connection
Annular ring width = (Diameter of the pad – Diameter of the finished hole) / 2
For instance, if your pad diameter equals 22 mils and the hole diameter equals 10 mils, then the width is calculated in this manner: (22 – 10) / 2 = 6 mils.
Two important elements to consider:
To get an insight into the common design issues that adversely impact the board manufacture and assembly see 6 DFM issues designers should check before PCB manufacturing.
Director of Sales and Marketing
Given below are seven cases:
Vikram Sarabhai Space Centre
An oft-overlooked consideration is testing. This states the obvious, but failure to incorporate testing access into board designs has a major impact on yield, and therefore productive throughput. Bonepile accumulations during functional test can be significantly reduced, if not avoided altogether, if mechanical test access, or the lack of it, is taken into account from the beginning of the design phase. Once a board is committed to CAD and resulting gerber files, it’s too late for test-based alterations until the next major revision spin.
A good starting point for basic test-related design rules is the SMTA TP-101E 2014 Testability Guidelines, available from the SMTA bookstore at smta.org. The TP101E Guidelines address topics such as probing/fixturing rules; flying probe guidelines; vectorless testing; IEEE 1149.1 and 1149.6 JTAG guidelines; Xray inspection rules; In-system programming guidelines; analog and mixed signal problems; built-in self test rules; AOI rules; and general physical layout considerations for all major test platforms such as ICT, Flying Probe, AXI, AOI, JTAG/Boundary Scan, and DFT rules of thumb.
Start with the theory advanced in the TP-101E Guidelines. Then talk to your test department or test engineering services provider for additional rules based on battle scars (real world experience). You won’t regret the effort.
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