RWKN.COM
Acronym Definition
RWK Rework.
RWKN: Rework Network.
Rework Network RWKN.com
Rework is a term used in several contexts. For example:
* Rework is the step in Fagan Inspections in which the defects found during the
inspection meeting are resolved by the author, designer or programmer. On the
basis of the list of defects the Low-level document is corrected until the
requirements in the High-level document are met.
* Rework is a process of removing an IC component off a printed circuit board
(PCB) and re-soldering it back to the PCB.
Fagan inspection refers to a structured process of trying to find defects in
development documents such as programming code, specifications, designs and
others during various phases of the software development process. It is named
after Michael Fagan who is credited with being the inventor of formal software
inspections.
Definition
Fagan Inspection is a group review method used to evaluate output of a given
process.
Fagan Inspection defines a process as a certain activity with a pre-specified
entry and exit criteria. In every activity or operation for which entry and exit
criteria are specified Fagan Inspections can be used to validate if the output
of the process complies with the exit criteria specified for the process.
Examples of activities for which Fagan Inspection can be used are:
* Requirement specification
* Software/Information System architecture (for example DYA)
* Programming (for example for in iterations in XP or DSDM)
* Software testing (for example when creating test scripts)
Usage
The software development process is a typical application of Fagan Inspection;
software development process is a series of operations which will deliver a
certain end product and consists of operations like requirements definition,
design, coding up to testing and maintenance. As the costs to remedy a defect
are up to 10-100 times less in the early operations compared to fixing a defect
in the maintenance phase it is essential to find defects as close to the point
of insertion as possible. This is done by inspecting the output of each
operation and comparing that to the output requirements, or exit-criteria of
that operation.
Criteria
Entry criteria are the criteria or requirements which must be met to enter a
specific process [Fagan, 1986]. For example for Fagan inspections the high- and
low-level documents must comply with specific entry-criteria before they can be
used for a formal inspection process.
Exit criteria are the criteria or requirements which must be met to complete a
specific process. For example for Fagan inspections the low-level document must
comply with specific exit-criteria (as specified in the high-level document)
before the development process can be taken to the next phase.
The exit-criteria are specified in a high-level document, which is then used as
the standard to compare the operation result (low-level document) to during the
inspections. Deviations of the low-level document from the requirements
specified in the high-level document are called defects and can be categorized
in Major Defects and Minor Defects.
Defects
“A defect is an instance in which a requirement is not satisfied.” [Fagan, 1986]
In the process of software inspection the defects which are found are
categorized in two categories: major and minor defects (often many more
categories are used). The defects which are incorrect or even missing
functionality or specifications can be classified as major defects: the software
will not function correctly when these defects are not being solved.
In contrast to major defects, minor defects do not threaten the correct
functioning of the software, but are mostly small errors like spelling mistakes
in documents or optical issues like incorrect positioning of controls in a
program interface.
Typical operations
In a typical Fagan inspection the inspection process consists of the following
operations [Fagan, 1986; Eickelmann, 2003]:
* Planning
* Preparation of materials
* Arranging of participants
* Arranging of meeting place
* Overview
* Group education of participants in the to be inspected materials
* Assignment of roles
* Preparation
* The participants prepare their roles
* Inspection meeting
* Actual finding of defect
* Rework
* Rework is the step in software inspection in which the defects found during
the inspection meeting are resolved by the author, designer or programmer. On
the basis of the list of defects the low-level document is corrected until the
requirements in the high-level document are met.
* Follow-up
* In the follow-up phase of software inspections all defect found in the
inspection meeting should be corrected (as they have been fixed in the rework
phase). The moderator is responsible for verifying that this is indeed the case.
He should verify if all defects are fixed and no new defects are inserted while
trying to fix the initial defects. It is trivial that all defects are corrected
as the costs of fixing them in a later phase of the project will be 10 to 100
times higher compared to the current costs.
An more comprehensive flow is available at .
A meta data model is available at .
Follow-up
In the follow-up phase of Fagan Inspection all defects fixed(by the author) in
the rework phase should be verified. The moderator, usually, is responsible for
verifying it. Sometimes, fixed work can be accepted without verifying(when the
defect was trivial), or full reinspection process is performed by the inspection
team(not only the moderator) when the defect was complicated or critical.
If verification fails, go back to rework process; the author tries to fix the
defect again.
Roles
The participants of the inspection process are normally just members of the team
that is performing the project. The participants fulfill different roles within
the inspection process [Fagan, 1976]:
* Author/Designer/Coder: the person who wrote the low-level document
* Reader: paraphrases the document
* Tester: reviews the document from a testing standpoint
* Moderator: responsible for the inspection session, functions as a coach
Benefits and results
By using inspections the amount of errors in the final product can significantly
decrease, creating a higher quality product. In the future the team will even be
able to avoid errors as the inspection sessions give them insight in the most
frequently made errors in both design and coding providing avoidance of error at
the root of their occurrence. By continuously improving the inspection process
these insights can even further be used [Fagan, 1986].
Together with the qualitative benefits mentioned above major 'cost improvements'
can be reached as the avoidance and earlier detection of errors will reduce the
amount of resources needed for debugging in later phases of the project.
In practice very positive results have been reported by large corporations like
IBM indicating that 80-90% of defects can be found and savings in resources up
to 25% can be reached [Fagan, 1986]...
Improvements
Although the Fagan Inspection method has proved to be very effective,
improvements have been suggested by multiple researchers. Genuchten for example
has been researching the usage of an Electronic Meeting System (EMS) to improve
the productivity of the meetings with positive results [Genuchten, 1997].
Other researchers propose the usage of software that keeps a database of
detected errors and automatically scans program code for these common errors
[Doolan,1992]. This again should result in improved productivity.
Example
In the diagram a very simple example is given of an inspection process in which
a two-line piece of code is inspected on the basis on a high-level document with
a single requirement.
As can be seen in the high-level document for this project is specified that in
all software code produced variables should be declared ‘strong typed’. On the
basis of this requirement the low-level document is checked for defects.
Unfortunately a defect is found on line 1, as a variable is not declared ‘strong
typed’. The defect found is then reported in the list of defects founds and
categorized according to the categorizations specified in the low-level
document.
In electronics, printed circuit boards, or PCBs, are used to mechanically
support and electrically connect electronic components using conductive
pathways, or traces, etched from copper sheets laminated onto a non-conductive
substrate. Alternative names are printed wiring board (PWB),and etched wiring
board. Populating the board with electronic components forms a printed circuit
assembly (PCA), also known as a printed circuit board assembly (PCBA).
PCBs are rugged, inexpensive, and can be highly reliable. They require much more
layout effort and higher initial cost than either wire-wrapped or point-to-point
constructed circuits, but are much cheaper, faster, and consistent in high
volume production. Much of the electronics industry's PCB design, assembly, and
quality control needs are set by standards that are published by the IPC
organization.
Manufacturing
Enlarge picture
A PCB (left) as a design on a computer and (right) realised as a board and
populated with components. The board is double sided, with through-hole plating,
green solder resist, and white silkscreen printing. Both surface mount and
through-hole components have been used.
Patterning (etching)
The vast majority of printed circuit boards are made by adhering a layer of
copper over the entire substrate, sometimes on both sides, (creating a "blank
PCB") then removing unwanted copper after applying a temporary mask (eg. by
etching), leaving only the desired copper traces. A few PCBs are made by adding
traces to the bare substrate (or a substrate with a very thin layer of copper)
usually by a complex process of multiple electroplating steps.
There are three common "subtractive" methods (methods that remove copper) used
for the production of printed circuit boards:
1. Silk screen printing uses etch-resistant inks to protect the copper foil.
Subsequent etching removes the unwanted copper. Alternatively, the ink may be
conductive, printed on a blank (non-conductive) board. The latter technique is
also used in the manufacture of hybrid circuits.
2. Photoengraving uses a photomask and chemical etching to remove the copper
foil from the substrate. The photomask is usually prepared with a photoplotter
from data produced by a technician using CAM, or computer-aided manufacturing
software. Laser-printed transparencies are typically employed for phototools;
however, direct laser imaging techniques are being employed to replace
phototools for high-resolution requirements.
3. PCB milling uses a two or three-axis mechanical milling system to mill away
the copper foil from the substrate. A PCB milling machine (referred to as a 'PCB
Prototyper') operates in a similar way to a plotter, receiving commands from the
host software that control the position of the milling head in the x, y, and (if
relevant) z axis. Data to drive the Prototyper is extracted from files generated
in PCB design software and stored in HPGL or Gerber file format.
"Additive" processes also exist. The most common is the "semi-additive" process.
In this version, the unpatterned board has a thin layer of copper already on it.
A reverse mask is then applied. (Unlike a subtractive process mask, this mask
exposes those parts of the substrate that will eventually become the traces.)
Additional copper is then plated onto the board in the unmasked areas; copper
may be plated to any desired weight. Tin-lead or other surface platings are then
applied. The mask is stripped away and a brief etching step removes the
now-exposed original copper laminate from the board, isolating the individual
traces.
The additive process is commonly used for multi-layer boards as it facilitates
the plating-through of the holes (vias) in the circuit board.
Lamination
Some PCBs have trace layers inside the PCB and are called multi-layer PCBs.
These are formed by bonding together separately etched thin boards.
Drilling
Holes, or vias, through a PCB are typically drilled with tiny drill bits made of
solid tungsten carbide. The drilling is performed by automated drilling machines
with placement controlled by a drill tape or drill file. These
computer-generated files are also called numerically controlled drill (NCD)
files or "Excellon files". The drill file describes the location and size of
each drilled hole.
When very small vias are required, drilling with mechanical bits is costly
because of high rates of wear and breakage. In this case, the vias may be
evaporated by lasers. Laser-drilled vias typically have an inferior surface
finish inside the hole. These holes are called micro vias.
It is also possible with controlled-depth drilling, laser drilling, or by
pre-drilling the individual sheets of the PCB before lamination, to produce
holes that connect only some of the copper layers, rather than passing through
the entire board. These holes are called blind vias when they connect an
internal copper layer to an outer layer, or buried vias when they connect two or
more internal copper layers and no outer layers.
The walls of the holes, for boards with 2 or more layers, are plated with copper
to form plated-through holes that electrically connect the conducting layers of
the PCB. For multilayer boards, those with 4 layers or more, drilling typically
produces a smear comprised of the bonding agent in the laminate system. Before
the holes can be plated through, this smear must be removed by a chemical
de-smear process, or by plasma-etch.
Exposed conductor plating and coating
The pads and lands to which components will be mounted are typically plated,
because bare copper oxidizes quickly, and therefore is not readily solderable.
Traditionally, any exposed copper was plated with solder. This solder was a
tin-lead alloy, however new solder compounds are now used to achieve compliance
with the RoHS directive in the EU, which restricts the use of lead. Other
platings used are OSP (organic surface protectant), immersion silver,
electroless nickel with immersion gold coating (ENIG), and direct gold. Edge
connectors, placed along one edge of some boards, are often gold plated.
Electrochemical migration (ECM) is the growth of conductive metal filaments on
or in a printed circuit board (PCB) under the influence of a DC voltage
bias.[1][2]
Solder resist
Areas that should not be soldered to may be covered with a polymer solder resist
(solder mask) coating. The solder resist prevents solder from bridging between
conductors and thereby creating short circuits. Solder resist also provides some
protection from the environment.
Screen printing
Line art and text may be printed onto the outer surfaces of a PCB by screen
printing. When space permits, the screen print text can indicate component
designators, switch setting requirements, test points, and other features
helpful in assembling, testing, and servicing the circuit board.
Screen print is also known as the silk screen, or, in one sided PCBs, the red
print.
Lately some digital printing solutions have been developed to substitute the
traditional screen printing process. This technology allows printing variable
data onto the PCB, including serialization and barcode information for
traceability purposes.
Test
Unpopulated boards may be subjected to a bare-board test where each circuit
connection (as defined in a netlist) is verified as correct on the finished
board. For high-volume production, a Bed of nails tester, a fixture or a Rigid
needle adapter is used to make contact with copper lands or holes on one or both
sides of the board to facilitate testing. A computer will instruct the
electrical test unit to send a small amount of current through each contact
point on the bed-of-nails as required, and verify that such current can be seen
on the other appropriate contact points. A "short" on a board would be a solid
connection where there should be no connection. An "open" is between two points
that should be connected and are not. For small- or medium-volume boards,
flying-probe testers use moving test heads to make contact with the copper lands
or holes to verify the electrical connectivity of the board under test.
Populating
After the PCB is completed, electronic components must be attached to form a
functional printed circuit assembly, or PCA(sometimes called a "printed circuit
board assembly" PCBA). In through-hole construction, component leads are
inserted in holes. In surface-mount construction, the components are placed on
pads or lands on the outer surfaces of the PCB. In both kinds of construction,
component leads are electrically and mechanically fixed to the board with a
molten metal solder.
There are a variety of soldering techniques used to attach components to a PCB.
High volume production is usually done with machine placement and bulk wave
soldering or reflow ovens, but skilled technicians are able to solder very tiny
parts (for instance 0201 packages which are 0.02" by 0.01") by hand under a
microscope tweezers and a fine tip soldering iron for small volume prototypes.
Some parts are impossible to solder by hand, such as Ball Grid Array (BGA)
packages.
Often, through-hole and surface-mount construction must be combined in a single
PCA because some required components are available only in surface-mount
packages, while others are available only in through-hole packages. Another
reason to use both methods is that through-hole mounting can provide needed
strength for components likely to endure physical stress, while components that
are expected to go untouched will take up less space using surface-mount
techniques.
JEDEC guidelines for PCB component placement, soldering, and inspection are
commonly used to maintain quality control in this stage of PCB manufacturing.
After the board is populated, the populated board may be tested with an
in-circuit test system. To facilitate this test, PCBs may be designed with extra
pads to make temporary connections. Sometimes these pads must be isolated with
resistors. The in-circuit test may also exercise boundary scan test features of
some components. In-circuit test systems may also be used to program nonvolatile
memory components on the board.
In boundary scan testing, test circuits integrated into various ICs on the board
form temporary connections between the PCB traces to test that the ICs are
mounted correctly. Boundary scan testing requires that all the ICs to be tested
use a standard test configuration procedure, the most common one being the Joint
Test Action Group (JTAG) standard.
When boards fail the test, technicians may desolder and replace failed
components.
Protection and packaging
PCBs intended for extreme environments often have a conformal coat, which is
applied by dipping or spraying after the components have been soldered. The coat
prevents corrosion and leakage currents or shorting due to condensation. The
earliest conformal coats were wax. Modern conformal coats are usually dips of
dilute solutions of silicone rubber, polyurethane, acrylic, or epoxy. Some are
engineering plastics sputtered onto the PCB in a vacuum chamber.
Many assembled PCBs are static sensitive, and therefore must be placed in
antistatic bags during transport. When handling these boards, the user must be
earthed; failure to do this might transmit an accumulated static charge through
the board, damaging or destroying it. Even bare boards are sometimes static
sensitive. Traces have gotten so fine that it's quite possible to blow an etch
off the board (or change its characteristics) with a static charge. This is
especially true on non-traditional PCBs such as MCMs and microwave PCBs.
Safety Certification (US)
Safety Standard UL 796 covers component safety requirements for printed wiring
boards for use as components in devices or appliances. Testing analyzes
characteristics such as flammability, maximum operating temperature, electrical
tracking, heat deflection, and direct support of live electrical parts.
The boards may use organic or inorganic base materials in a single or
multilayer, rigid or flexible form. Circuitry construction may include etched,
die stamped, precut, flush press, additive, and plated conductor techniques.
Printed-component parts may be used.
The suitability of the pattern parameters, temperature and maximum solder limits
shall be determined in accordance with the applicable end-product construction
and requirements.
"Cordwood" construction
Enlarge picture
A cordwood module.
Cordwood construction can give large space-saving advantages and was often used
with wire-ended components in applications where space was at a premium (such as
missile guidance and telemetry systems). In 'cordwood' construction, two leaded
components are mounted axially between two parallel planes. Instead of soldering
the components, they were connected to other components by thin nickel tapes
welded at right angles onto the component leads. To avoid shorting together of
different interconnection layers, thin insulating cards were placed between
them. Perforations or holes in the cards would allow component leads to project
through to the next interconnection layer. One disadvantage of this system was
that special nickel leaded components had to be used to allow the
interconnecting welds to be made. Some versions of cordwood construction used
single sided PCBs as the interconnection method (as pictured). This meant that
normal leaded components could be used.
Before the advent of integrated circuits, this method allowed the highest
possible component packing density; because of this, it was used by a number of
computer vendors including Control Data Corporation. The cordwood method of
construction now appears to have fallen into disuse, probably because high
packing densities can be more easily achieved using surface mount techniques and
integrated circuits.
Multiwire boards
Multiwire is a patented technique of interconnection which uses machine-routed
insulated wires embedded in a non-conducting matrix (often plastic resin). It
was used during the 1980s and 1990s. (Augat Inc., U.S. Patent 4,648,180)
Since it was quite easy to stack interconnections (wires) inside the embedding
matrix, the approach allowed to forget completely about the routing of wires
(usually a time-consuming operation of PCB design): Anywhere the designer needs
a connection, the machine will draw a wire in straight line from one
location/pin to another. This led to very short design times (no complex
algorithms to use even for high density designs), reduced cross talk (an
electrical phenomenon appearing where a current in one wire generates another
current in another conductor, that is highly amplified when wires are parallel -
this nearly never happens in Multiwire), but the cost is too high to compete
with cheaper PCB technologies when large quantities are needed.
Surface-mount technology
surface-mount technology
Surface mount components, including resistors, transistors and an integrated
circuit.
Surface-mount technology was developed in the 1960s, gained momentum in Japan in
the early 1980s and became widely used globally by the mid 1990s. Components
were mechanically redesigned to have small metal tabs or end caps that could be
directly soldered to the surface of the PCB. Components became much smaller and
component placement on both sides of the board became far more common with
surface-mounting than through-hole mounting, allowing much higher circuit
densities. Surface mounting lends itself well to a high degree of automation,
reducing labour cost and greatly increasing production and quality rates. SMDs
can be one-quarter to one-tenth the size and weight, and passive components can
be one-half to one-quarter the cost of through-hole parts. Integrated circuits
(where the chip itself is the most expensive part) are often priced the same
regardless of package type however. As of 2006, some wire-ended components, such
as small signal switch diodes (philips 1N4148 for instance), are actually
significantly cheaper than corresponding SMD versions
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