Understanding PCB Materials and Specifications
When you give a PCB for
fabrication, you need to specify details on PCB materials, impedance controls,
finish, copper thickness and so on. Most of the details are so common that they
are always assumed. However still it is better to know them.
Below is a screenshot of
quotation submitted by a fabricator for a PCB design
Let us understand each term one
by one and things related to them
1) 4 layer specifies NUMBER OF
LAYERS - This means the number of conducting layers. The layers may be either
routing layer or plane layer. The commonly used no. of layers are 1, 2, 4, 6, 8
and other even numbered layers. Odd number layer boards may be formed but they
are not standard boards and doesn’t offer any savings. The PCB materials used
for 3 or 4 layers would almost be same.
Routing layers contains tracks
while plane layer are generally used to connect power or ground. Plane layers
have plane of copper from which connections are drawn up or down wherever
needed. Islands can be formed on plane layers and each island may have
different signal. Suppose some sets of chips on one board use 3.3 V while other
use 5 V then the power plane may be partitioned into two islands of 3.3 V and 5
V.
2) FR4 specifies PCB MATERIAL TYPE
– FR4 is other name for “Glass reinforced epoxy laminated sheets”. It is a
commonly used PCB material. The reasons for its ubiquity as a PCB material are
its high dielectric strength, high mechanical strength, light weight,
resistance to moisture and other properties conducive for PCB manufacturing. FR
in FR4 stands for “fire retardant”, another feather in this PCB material’s cap.
3) 1.6 mm thick specifies
FINISHED BOARD THICKNESS – In multilayer PCB it is thickness after all layers
have been pressed together and all the processes completed. 1.6mm is default
thickness for 4 layer board. Go for higher thickness if you have heavy
components on board to get better strength. Keep an eye on thickness even when
you have high voltages on board.
4) 35 microns copper finish –
The thickness of copper put on board. Another popular way to express is in
grams or Oz. (Ounces per square foot) of copper per unit area. X Oz would mean
that X Ounces were spread on the Board material and it was distributed
uniformly over an area of 1 square foot.
Go for higher copper thickness
if the current density on board is greater than in average digital circuits. In
normal boards 35 microns is good. Up till a current of 1A and copper thickness
of 1 Oz, track width of 10 mils is safe.
5) No impedance Control – When
signal transition time is too small or frequency is too high then tracks may
behave as transmission lines. If unchecked, such tracks may cause significant
power loss. For maximum power to be transferred to other end of track, length
and geometry of the track should be carefully laid taking care of transmission
line equations.
As a rule of thumb, if the
board is of FR4 PCB material with copper tracks and signal rise time is 1
nanosecond then 2 inches is a safe distance. If the track length exceeds this
and signal integrity is important then consider the transmission line effects
also. There are lot of free programs and excel sheet available on net that
calculate impedances for prevalent geometries. If impedance control has been mentioned
to the fabricator then an extra test is done on the completed PCB to check the
required impedances.
6) No Via Filling – Normally
vias are not filled and you clearly see other side of the board through them.
However, they can be filled for various reasons. One need arises when vias are
to be protected from solutions, dust etc. Vias can also be filled with
conducting materials to increase the current carrying capacity of via. One more
reason is to level the board. Via filling is needed many a times on boards with
BGA (Ball Grid array) components. If BGA pins make contact with one of the
inner layer then the via can’t be placed just underneath the pin because then
the solder may flow in the hole and get connected to some other layer. So Via
filling is done so that solder doesn’t flow in the via and contacts are
properly connected.
7) No gold tabs – At any place
on the board when something makes and breaks contact repeatedly then that area
wears out fast. Example is a rubber pushbutton which has contacts printed on
the board. An example is the kind of buttons used in calculator. This wearing
out also happens when daughter card slot are provided on main board. The
contacts on this slot are rubbed when card is pushed in or out. In such cases
it is wise to put gold layer on the surface which makes contact. This enhances
life of the contact. A separate step is added in the PCB fabrication process if
gold tabs are specified. Also remember gold is costly.
8) Green gloss (LPISM)
soldermask - The first image that would come to your mind when you think of PCB
is a green colour board with some shining metal contacts and some holes. The
green colour is because of this soldermask. Now why is soldermask used?? Solder
mask prevents spilling of liquid solder. This spilling has become a serious
problem in recent times. As the pitch of the IC pins has decreased, due to this
spilling the pins get shorted. To feel the miracle of soldermask, try to solder
a board which doesn’t have solder mask (mostly a homemade PCB won’t have) and
the one which has solder mask. You will feel a huge difference. This experience
made me enjoy soldering which I hated earlier.
LPISM stands for Liquid Photo
imageable solder mask. It is a type of soldermask. There are soldermasks
available in other colours as well. Prevalent among them are Red and blue.
9) Leaded HASL finish – HASL
stands for Hot Air Solder Leveling. After soldermask has been put on PCB. The
PCB is passed through molten solder. All the copper which is exposed, like pads
etc. get covered with solder. This is useful in actual soldering of the
components. During soldering of SMD chips on board it is only needed to heat
the board till solder becomes molten again and place the component at the
respective position. The components get soldered as the molten solder again
solidifies. No need to hold soldering guns, wires and flux. So Simple J
Leaded HASL means that lead is
also present in the solder compound put on the board. Nowadays HASL compounds
without lead are also available.
10) Line Width / Spacing : 6/6
mils – This means the minimum track width would be 6 mils and minimum spading
between tracks, tracks and pads and all other spacings should be greater than
or equal to 6 mils. In case you don’t know mils, It is the prevalent unit of
distance used in PCB world. 1 mil = 1/1000th of an inch = .0254mm.
Take care of width and spacing
when higher currents and voltages are expected on board.
11) Electrical test, routed –
In multilayer PCB, It is not possible to do visual inspection of all the tracks
for their correctness. So this test is done. Test probes are put at the end of
tracks or on pads such that signals on it can be accessible. A small voltage is
applied on one such probe. If that voltage is detected at the other end, it
confirms that the tracks are proper. This step may not be necessary in 1 or 2
layer board but if you don’t want to compromise on quality, get this test done.
12) PCB Material-Isola
370HR-High Tg : Isola is a famous company for PCB materials and 370HR is one of
their product’s name. This is an FR4 PCB material. This PCB material in
addition to FR4 properties has additional advantages like superior thermal
performance which helps in dissipating heat on board. The mechanical, chemical
and moisture resistance properties all equal or exceed the performance of
traditional FR-4 materials. “High Tg” stands for “high glass transition
temperature”. This means that the PCB made of this material can sustain high
temperatures. During HASL or soldering, PCBs have to sustain high temperatures
and that’s where this parameter becomes important.
13) No Blind/Buried Vias –
Blind Vias are those vias which join one of the outer layers (TOP or BOTTOM)
with one of the inner layers. Since this via can be seen from only one side, it
is called a blind via (This may not seem logical but just remember itJ). Buried
vias are vias which connect one of the inner layers to another inner layer.
This vias cannot be seen from any of the outer layer and hence is called buried
vias. (Aha!! This is logical) Via filling as we studied above is used many a
times when blind/buried vias are used. It helps level the surface just above
via so that pads/tracks can be placed over the vias without the solder
penetrating in.
Blind via, buried via, via
filling, gold tabs put additional cost on PCB manufacturing. Also, if you go
down 6 mils in line/width spacing, the cost goes up. This is because the
fabrication processes change a little and also the rejection rate is high.
There are PCB fabricators which can go down till 3 mils. Now some may have gone
below that also, but don’t try to touch those limits until you don’t have other
options.
The parameters defined above
are just some of the values to define a PCB.
Thanks for reading my article,
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