-
Types of PCBs
A printed circuit board, or PCB, is used to mechanically support
and electrically connect electronic components using conductive
pathways, tracks or traces etched from copper sheets laminated onto
a non-conductive substrate. It is also referred to as printed
wiring board (PWB) or etched wiring board. A PCB populated with
electronic components is a printed circuit assembly (PCA), also
known as a printed circuit board assembly (PCBA).
PCBs are 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 and faster for 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.
The board is typically coated with a solder mask that is green
in color. Other colors that are normally available are blue and
red. There are quite a few different dielectrics that can be chosen
to provide different insulating values depending on the
requirements of the circuit. Some of these dielectrics are
polytetrafluoroethylene (Teflon), FR-4, FR-1, CEM-1 or CEM-3. Well
known prepreg materials used in the PCB industry are FR-2 (Phenolic
cotton paper), FR-3 (Cotton paper and epoxy), FR-4 (Woven glass and
epoxy), FR-5 (Woven glass and epoxy), FR-6 (Matte glass and
polyester), G-10 (Woven glass and epoxy), CEM-1 (Cotton paper and
epoxy), CEM-2 (Cotton paper and epoxy), CEM-3 (Woven glass and
epoxy), CEM-4 (Woven glass and epoxy), CEM-5 (Woven glass and
polyester). Thermal expansion is an important consideration
especially with BGA and naked die technologies, and glass fiber
offers the best dimensional stability.
The thickness of the PCB can be 1.0mm, 1.2mm or 1.6mm. They can
be single sided or double sided with cooper clad of 1 oz or 2
oz.
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
[email protected]
-
the Prototyper is extracted from files generated in PCB design
software and stored in HPGL or Gerber file format.
What is Wave soldering??
Wave soldering is a large-scale soldering process by which
electronic components are soldered to a printed circuit board (PCB)
to form an electronic assembly. The name is derived from the use of
waves of molten solder to attach metal components to the PCB. The
process uses a tank to hold a quantity of molten solder; the
components are inserted into or placed on the PCB and the loaded
PCB is passed across a pumped wave or waterfall of solder. The
solder wets the exposed metallic areas of the board (those not
protected with solder mask, a protective coating that prevents the
solder from bridging between connections), creating a reliable
mechanical and electrical connection. The process is much faster
and can create a higher quality product than manual soldering of
components.
Wave soldering is used for both through-hole printed circuit
assemblies, and surface mount Terminal (SMT). In the latter case,
the components are glued by the placement equipment onto the
printed circuit board surface before being run through the molten
solder wave.
As through-hole components have been largely replaced by surface
mount components, wave soldering has been supplanted by reflow
soldering methods in many large-scale electronics applications.
However, there is still significant wave soldering where SMT is not
suitable (e.g. large power devices and high pin count connectors),
or where simple through-hole technology prevails (certain major
appliances).
Wave soldering Process...
There are many types of wave solder machines, however the basic
components and principles of these machines are the same. A
standard wave solder machine consists of three zones: the
preheating zone, the fluxing zone, and the soldering zone. An
additional fourth zone, cleaning, is used depending on the type of
flux applied. The basic equipment used during the process is a
conveyor that moves the PCB through the different zones, a
[email protected]
-
pan of solder used in the soldering process, a pump that
produces the actual wave, the sprayer for the flux and the
preheating pad. The solder is usually a mixture of metals. A
typical solder has the chemical makeup of 50% tin, 49.5% lead, and
0.5% antimony.
It is important that the PCBs be allowed to cool at a reasonable
rate. If they are cooled too fast, then the PCB can become warped
and the solder can be compromised. On the other hand if the PCB is
allowed to cool too slowly, then the PCB can become brittle and
some components may be damaged by heat. The PCB should be cooled by
either a fine water spray or air cooled to decrease the amount of
damage to the board.
Some types of flux, called "no-clean" fluxes, do not require
cleaning; their residues are benign after the soldering process.
Others, however, require a cleaning stage, in which the PCB is
washed with solvents and/or deionized water to remove flux
residue.
The following are characteristics of the wave soldering
process:
* The solder connection is very reliable and also a clean
connection * The process is automated * The process reuses the flux
and solder that is left over * It does require inspection, some
touch ups, and also testing * The productivity and efficiency is
increased
What is Solder??
Solder is a fusible metal alloy with a melting point or melting
range of 90 to 450 C (200 to 840 F), used in a process called
soldering where it is melted to join metallic surfaces. It is
especially useful in electronics and plumbing. Alloys that melt
between 180 and 190 C are the most commonly used.
What is Lead solder??
Tin/lead solders, also called soft solders, are commercially
available with tin concentrations between 5% and 70% by weight. The
greater the tin concentration, the greater the solders tensile and
shear strengths. At the retail level, the two most common alloys
are 60/40 Sn/Pb which melts at 370 F or 188 C and 63/37 Sn/Pb used
principally in electrical work. The 63/37 ratio is notable in that
it is a eutectic mixture, which means:
1. It has the lowest melting point (183 C or 361.4 F) of all the
tin/lead alloys; and 2. The melting point is truly a point not a
range.
At a eutectic composition, the liquid solder solidifies at a
single temperature. Tin/lead solder solidifies to fine grains of
nearly pure lead and nearly pure tin phases, there are no tin/lead
intermetallics and no solubility of tin in lead or lead in tin, as
can be seen from a tin/lead equilibrium diagram.
[email protected]
-
In plumbing, a higher proportion of lead was used, commonly
50/50. This had the advantage of making the alloy solidify more
slowly, so that it could be wiped over the joint to ensure
watertightness, the pipes being physically fitted together before
soldering. Although lead water pipes were displaced by copper when
the significance of lead poisoning began to be fully appreciated,
lead solder was still used until the 1980s because it was thought
that the amount of lead that could leach into water from the solder
was negligible from a properly soldered joint. The electrochemical
couple of copper and lead promotes corrosion of the lead and tin,
however tin is protected by insoluble oxide. Since even small
amounts of lead have been found detrimental to health, Lead in
plumbing solder was replaced by silver (food grade applications) or
antimony, with copper often added, and the proportion of tin was
increased.
Most lead-free replacements for conventional Sn60/Pb40 and
Sn63/Pb37 solder have melting points from 520 C higher, though
solders with much lower melting points are available.
In electronics, the traditional use of solder was to fortify
mechanically made electrical contacts, e.g. two solid copper wires
twisted together. This was in part due to the higher electrical
resistance of solder versus copper. Printed circuit boards use
solder joints to mount components and create a circuit, also
replacing the use of solid solder with solder paste.
What is Flux??
Flux is a reducing agent designed to help reduce (return
oxidized metals to their metallic state) at the points of contact
to improve the electrical connection and mechanical strength. The
two principal types of flux are acid flux, used for metal mending
and plumbing, and rosin flux, used in electronics, where the
corrosiveness of acid flux and vapours released when solder is
heated would risk damaging delicate circuitry.
Due to concerns over atmospheric pollution and hazardous waste
disposal, the electronics industry has been gradually shifting from
rosin flux to water-soluble flux, which can be removed with
deionised water and detergent, instead of hydrocarbon solvents.
In contrast to using traditional bars or coiled wires of
all-metal solder and manually applying flux to the parts being
joined, some light hand soldering since the mid-20th century has
used flux-core solder. This is manufactured as a coiled wire of
solder, with one or more continuous bodies of non-acid flux
embedded lengthwise inside it. As the solder melts onto the joint,
it frees the flux and releases that on it as well.
[email protected]
