Redesigned for manufacturability
Design for manufacturability (also sometimes known as design for manufacturing or DFM) is the general engineering practice of designing
products in such a way that they are easy to manufacture. The concept
exists in almost all engineering disciplines, but the implementation
differs widely depending on the manufacturing technology. DFM describes
the process of designing or engineering a product in order to facilitate
the manufacturing
process in order to reduce its manufacturing costs. DFM will allow
potential problems to be fixed in the design phase which is the least
expensive place to address them. Other factors may affect the
manufacturability such as the type of raw material, the form of the raw
material, dimensional tolerances, and secondary processing such as
finishing.
Depending on various types of manufacturing processes there are
set guidelines for DFM practices. These DFM guidelines help to precisely
define various tolerances, rules and common manufacturing checks
related to DFM.
While DFM is applicable to the design process, a similar concept called DFSS (Design for Six Sigma) is also practiced in many organizations.
For printed circuit boards (PCB)
In the PCB
design process, DFM leads to a set of design guidelines that attempt to
ensure manufacturability. By doing so, probable production problems may
be addressed during the design stage.
Ideally, DFM guidelines take into account the processes and
capabilities of the manufacturing industry. Therefore, DFM is constantly
evolving.
As manufacturing companies evolve and automate more and more
stages of the processes, these processes tend to become cheaper. DFM is
usually used to reduce these costs.[1] For example, if a process may be done automatically by machines (i.e. SMT component placement and soldering), such process is likely to be cheaper than doing so by hand.
For integrated circuits (IC)
Achieving high-yielding designs, in the state of the art VLSI
technology has become an extremely challenging task due to the
miniaturization as well as the complexity of leading-edge products.
Here, the DFM methodology includes a set of techniques to modify the
design of integrated circuits
(IC) in order to make them more manufacturable, i.e., to improve their
functional yield, parametric yield, or their reliability.
Background
Traditionally,
in the pre-nanometer era, DFM consisted of a set of different
methodologies trying to enforce some soft (recommended) design rules
regarding the shapes and polygons of the physical layout of an integrated circuit.
These DFM methodologies worked primarily at the full chip level.
Additionally, worst-case simulations at different levels of abstraction
were applied to minimize the impact of process variations on performance
and other types of parametric yield loss. All these different types of
worst-case simulations were essentially based on a base set of
worst-case (or corner) SPICE
device parameter files that were intended to represent the variability
of transistor performance over the full range of variation in a
fabrication process.
Taxonomy of yield loss mechanisms
The most important yield loss models (YLMs) for VLSI ICs can be classified into several categories based on their nature.
- Functional yield loss is still the dominant factor and is caused by mechanisms such as misprocessing (e.g., equipment-related problems), systematic effects such as printability or planarization problems, and purely random defects.
- High-performance products may exhibit parametric design marginalities caused by either process fluctuations or environmental factors (such as supply voltage or temperature).
- The test-related yield losses, which are caused by incorrect testing, can also play a significant role.
Techniques
After
understanding the causes of yield loss, the next step is to make the
design as resistant as possible. Techniques used for this include:
- Substituting higher yield cells where permitted by timing, power, and routability.
- Changing the spacing and width of the interconnect wires, where possible
- Optimizing the amount of redundancy in internal memories.
- Substituting fault tolerant (redundant) vias in a design where possible
All of these require a detailed understanding of yield loss
mechanisms, since these changes trade off against one another. For
example, introducing redundant vias
will reduce the chance of via problems, but increase the chance of
unwanted shorts. Whether this is good idea, therefore, depends on the
details of the yield loss models and the characteristics of the
particular design.
For CNC machining
Objective
The
objective is to design for lower cost. The cost is driven by time, so
the design must minimize the time required to not just machine (remove
the material), but also the set-up time of the CNC machine, NC programming, fixturing and many other activities that are dependent on the complexity and size of the part.
Set-Up Time of Operations (Flip of the Part)
Unless
a 4th- &/or 5th-Axis is used, a CNC can only approach the part from
a single direction. One side must be machined at a time (called an
operation or Op). Then the part must be flipped from side to side to
machine all of the features. The geometry of the features dictates
whether the part must be flipped over or not. The more Ops (flip of the
part), the more expensive the part because it incurs substantial
"Set-up" and "Load/Unload" time.
Each operation (flip of the part) has set-up time, machine time,
time to load/unload tools, time to load/unload parts, and time to create
the NC program for each operation. If a part has only 1 operation, then
parts only have to be loaded/unloaded once. If it has 5 operations,
then load/unload time is significant.
The low hanging fruit is minimizing the number of operations
(flip of the part) to create significant savings. For example, it may
take only 2 minutes to machine the face of a small part, but it will
take an hour to set the machine up to do it. Or, if there are 5
operations at 1.5 hours each, but only 30 minutes total machine time,
then 7.5 hours is charged for just 30 minutes of machining.
Lastly, the volume (number of parts to machine) plays a critical
role in amortizing the set-up time, programming time and other
activities into the cost of the part. In the example above, the part in
quantities of 10 could cost 7–10X the cost in quantities of 100.
Typically, the law of diminishing returns presents itself at
volumes of 100–300 because set-up times, custom tooling and fixturing
can be amortized into the noise.
Material type
The most easily machined types of metals include aluminum, brass, and softer metals. As materials get harder, denser and stronger, such as steel, stainless steel, titanium, and exotic alloys, they become much harder to machine and take much longer, thus being less manufacturable. Most types of plastic
are easy to machine, although additions of fiberglass or carbon fiber
can reduce the machinability. Plastics that are particularly soft and
gummy may have machinability problems of their own.
Material form
Metals
come in all forms. In the case of aluminum as an example, bar stock and
plate are the two most common forms from which machined parts are made.
The size and shape of the component may determine which form of
material must be used. It is common for engineering drawings to specify
one form over the other. Bar stock is generally close to 1/2 of the
cost of plate on a per pound basis. So although the material form isn't
directly related to the geometry of the component, cost can be removed
at the design stage by specifying the least expensive form of the
material.
Tolerances
A
significant contributing factor to the cost of a machined component is
the geometric tolerance to which the features must be made. The tighter
the tolerance required, the more expensive the component will be to
machine. When designing, specify the loosest tolerance that will serve
the function of the component. Tolerances must be specified on a
feature by feature basis. There are creative ways to engineer
components with lower tolerances that still perform as well as ones with
higher tolerances.
Design and shape
As
machining is a subtractive process, the time to remove the material is a
major factor in determining the machining cost. The volume and shape
of the material to be removed as well as how fast the tools can be fed
will determine the machining time. When using milling cutters,
the strength and stiffness of the tool which is determined in part by
the length to diameter ratio of the tool will play the largest role in
determining that speed. The shorter the tool is relative to its
diameter the faster it can be fed through the material. A ratio of 3:1
(L:D) or under is optimum. If that ratio cannot be achieved, a solution like this depicted here can be used. For holes, the length to diameter ratio of the tools are less critical, but should still be kept under 10:1.
There are many other types of features which are more or less
expensive to machine. Generally chamfers cost less to machine than
radii on outer horizontal edges. 3D interpolation is used to create
radii on edges that are not on the same plane which incur 10X the cost. Undercuts are more expensive to machine. Features that require smaller tools, regardless of L:D ratio, are more expensive.
Design for Inspection
The concept of Design for Inspection (DFI) should complement and work in collaboration with Design for Manufacturability (DFM) and Design for Assembly
(DFA) to reduce product manufacturing cost and increase manufacturing
practicality. There are instances when this method could cause calendar
delays since it consumes many hours of additional work such as the case
of the need to prepare for design review presentations and documents. To
address this, it is proposed that instead of periodic inspections,
organizations could adopt the framework of empowerment, particularly at
the stage of product development, wherein the senior management empowers
the project leader to evaluate manufacturing processes and outcomes
against expectations on product performance, cost, quality and
development time. Experts, however, cite the necessity for the DFI because it is crucial in performance and quality control, determining key factors such as product reliability, safety, and life cycles. For an aerospace
components company, where inspection is mandatory, there is the
requirement for the suitability of the manufacturing process for
inspection. Here, a mechanism is adopted such as an inspectability
index, which evaluates design proposals.
Another example of DFI is the concept of cumulative count of conforming
chart (CCC chart), which is applied in inspection and maintenance
planning for systems where different types of inspection and maintenance
are available.
Design for additive manufacturing
Additive manufacturing
broadens the ability of a designer to optimize the design of a product
or part (to save materials for example). Designs tailored for additive
manufacturing are sometimes very different from designs tailored for
machining or forming manufacturing operations.
In addition, due to some size constraints of additive
manufacturing machines, sometimes the related bigger designs are split
into smaller sections with self-assembly features or fasteners locators.
