| | 29.09.2004 | Alternative PCB Finishes: Advantages and Disadvantages Comparison In order to maintain the solderability of PCBs over a period of storage time,
it is necessary to protect the copper surface mount pads with solderable surface finish. Up to now the most common
finish is eutectic tin-lead alloy by hot air solder levelling (HASL) method because it has most desirable properties
of ideal PCB surface. Unfortunately this coating does not meet requirement of soldering pads planarity - fundamental
factor for fine - pitch, very large scale integration surface mount technology. There are over the dozen alternative
PCB finishes on the market. All are useful for some applications, but none really fit as "universal surface finish".
Mike Barbetta has highlighted each of the common surface finishes ( HASL, OSP, ENIG, immersion silver, immersion tin,
electrolytic nickel-gold, electroless nickel/palladium/immersion gold, reflowed tin-lead, selective finishes) with
regard to advantages and shortcomings. More about this you can find
here).
| | 27.08.2004 | The plated through Hole Calculation A clear understanding of the anatomy of a plated through
hole (please see the picture below) is necessary in order to satisfy
the dual needs of assembly and PCB fabrication. The PCB assembler
works with finished hole size. The fabrication process is based on
the drilled hole size.
Along with all of this, it is necessary to size holes in such a way
that the power planes of the PCB are not degraded by placing
holes so close together that they cause slots to be created in
the planes by overlapping clearance holes. It is not possible
to specify generously large hole sizes and insulation spacing
to make manufacturing or fabrication easy without risking degradation
of the environment needed by the high speed signals travelling
across those planes through the PCB. More details how correctly
to calculate the sizes of pads and keep out areas needed to insure
the PCB you can find in article of Lee W. Ritchey here.
| | 30.06.2004 | Rigid - flexible Multilayer Boards Rigid-flex multilayer boards are known since more than
25 years, yet have not been able to penetrate the commercial market and
have always been considered as exotic and expensive. This because rigid-flex
PWBs need special design experience, special manufacturing know how and
special precautions during assembly and soldering.
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In the last few years,
however, rigid-flex constructions have become increasingly popular,
simply because this technology allows an optimal interconnect approach
on the system level. The process is accelerated by the rapidly increasing
demand for portable electronic devices which have to be packed cost
effectively in very small volumes. In addition, the cost for flex materials
have come down as a result of the growing volume and number of supplier
base, both factors which will further promote the use of this fascinating
technology.
Even so, the complexity of some rigid-flex MLBs, in particular in areas,
like avionics and high-end MIL applications, sometimes reaches extreme
levels. Therefore, the aspect of manufacturability has to be designed
into the board, in order to avoid drawbacks, like high costs, long
lead time and poor reliability. At least in cases, where the designer
has low experience in constructing rigid-flex MLBs or when the complexity
is above standard, the board supplier should be consulted and asked
for inputs and advice. In some exceptional cases, the board designs
are so supplier - specific that other suppliers may not be able to
process the boards without modifications in scaling factors, build-up
or materials. Very well experience in rigid-flex Boards design and
manufacturing has company DYCONEX AG (Swiss). More details about Rigid
- flexible Multilayer Boards technology you can find here.
| | 26.05.2004 | Conduction heat transfer in a printed circuit board Most problems in the electronics industry
today are a result of the heat generated by components mounted on a
printed circuit board (PCB). Bruce M. Guenin has examined the heat
flow in a PCB, which typically is a layered composite consisting of
copper foil and a glass-reinforced polymer (FR-4). Calculated value
of heat flow is presented in the graph below. In this calculation,
it is assumed that the total PCB thickness is about 1.6 mm and that
the layers consist only of copper and FR-4, with thermal conductivities
390 and 0.25 W/mK, respectively. |
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Kin plane - heat flow within the plane.
Kthrough - heat flow through the thickness of the plane.
More about conduction heat transfer in a printed circuit
board you can find here.
You can also use the CALCULATOR to calculate the conduction heat transfer
in your PCB.
Useful Software concerning Printed Circuit Board Thermal
Analysis you can find here:
http://www.harvardthermal.com
http://www.thermalman.com
Below you can see the sample of Thermal Analysis in multilayer
PCB using the program TASPCB [5].
References:
[1] J.E. Graebner, “Thermal Conductivity of Printing Wiring Boards,” Technical
Brief, Electronics Cooling Magazine, Vol. 1, No. 2, October, 1995, p. 27.
[2] K. Azar and J.E. Graebner, “Experimental Determination of Thermal Conductivity
of Printed Wiring Boards,” Proceedings, SEMI-THERM XII Conference, March,
1996, pp. 169-182.
[3] Bruce M. Guenin, Conduction Heat Transfer in a Printed Circuit Board
[4] http://www.harvardthermal.com/products/TraceHeating/TraceHeating.htm
[5] http://www.coolingzone.com/Guest/News/NL_APR_2001/Tutorial/pcb.html
| | 28.04.2004 | Metal Circuit Boards Electrically insulated metal substrates offer a low cost alternative
to ceramic boards. Metal substrate circuitry consists of a metal base plate
onto which a copper conductor layer is attached with a thermally conductive
epoxy dielectric (see picture below, 2). |
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In addition to aluminium, metal substrates such as copper,
copper-clad Invar and copper-clad molybdenum are suitable as substrates
(1). An aluminium
alloy is usually chosen for the base metal for its excellent heat dissipation
ability (see table below), mechanical integrity, low cost and lightweight
construction.
Thermal Conductivity and Thermal Expansion
of different PCB Metal base plates (2)
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METAL/ALLOY
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THERMAL CONDUCTIVITY
(W/m-K)
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COEFFICIENT OF
THERMAL EXPANSION (ppm/K)
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Copper
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400
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17
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Aluminum
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150
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25
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304 Stainless Steel
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16
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16.3
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Cold Rolled Steel
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50
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12.5
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Iron
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80
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11.8
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CIC Copper - Invar -
Copper
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20
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5.2
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CMC Copper - Molly -
Copper
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200
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6.5
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20% ALSIC/Aluminum
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175
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15
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Design Considerations When Selecting the
Base Metal Layer
- Coefficient Of Thermal Expansion And
Heat Spreading
- Coefficient Of Thermal Expansion And
Solder Joints
- Strength, Rigidity And Weight
- Electrical Connections To / Through
the Base Layer
- Surface Finish
- Costs
More details about this you can find here.
References
1. Multilayer circuitry on metal
substrates.
Goran Matijasevic, Ormet Corporation,
Carlsbad, CA.
2.Thermal Substrates: base. The Bergquist
Company.
http://www.bergquistcompany.com/thermal_substrates.cfm
| | 31.03.2004 | The effect of etch factor on printed wiring characteristic impedance As logic switching speeds continue to increase, signal
integrity becomes increasingly
important. The signal paths have to be treated as transmission lines
to accurately
predict signal integrity. Many software tools exist to calculate transmission
line
characteristic impedance of printed-wiring traces as part of the overall
signal analysis.
As the logic speeds get even faster, the effect of transmission line
mismatch becomes
more serious. Many of the software tools assume a rectangular cross-section
for the
traces. In actuality, the trace cross-sections more closely approximate
a trapezoid due
to the etching process (see picture).
Steve Monroe and Otto Buhler from Storage Technology Corporation have used
a field modeling program to determine the characteristic impedance of a variety
of traces including buried microstrip, symmetrical stripline, edge-coupled
pair, and broadside-coupled pair. These impedance determinations were performed
with both rectangular and trapezoidal cross sections. In some cases, the
difference in characteristic impedance between rectangular and trapezoidal
cross-sections exceeded six percent.
More about this you can find here.
| | 26.02.2004 | Test coupons for the measurement of wave impedance As it is known, the measurement of wave impedance of printed circuit
boards (PCB) can be executed either directly on the printed-circuit board
(if the parameters of the PCB make it possible to carry out such measurement),
or on a test coupon, which must be specially prepared for such purpose.
The Test coupon can be located directly on the printed-circuit board
(if size of PCB allows this arrangement), or it is carry out at an separate
board, if it cannot be placed at the PCB. In the second case, the test
coupon is placed, as a rule, on the same PCB panel, where main printed
circuit board is located. To achieve a good quality of the measurement
of wave impedance the PCB designer and manufacturer shall follow the
requirements for test coupon design (more details you can see here).
| | 24.01.2004 | Introduction of an innovative foil material "FRAFLEX®"
A polyimide foil which is laminated on one or both sides with copper.
It is applicable as base material for flexible printed circuit boards
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Applications:
- Semiconductor packaging: BGA,
µBGA, TAB, CSP and COF (small circuit feature low profile - high
temperature - compatible with high volume processing);
- Medical (small circuit feature - low profile);
- Space (low outgassing - light weight - high temperature);
- Automotive (high temperature - chemical resistance);.
- Portable device - cell phone, PDA, laptop (small circuit feature
- light weight - low profile);
- Electronic component - embedded devices, resistors, fuses (low
profile - no tiecoat - standard flex circuit processing technologies).
Overview:
FRAFLEX® laminates are adhesiveless,
tiecoat-free, copper-clad polyimide films that are manufactured by
state-of-the-art vacuum metallization and electroplating processes.
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Pic.1.FRAFLEX® cross-cut.
Pic.2. FRAFLEX® surface after etching.
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Due to an innovative technology, FRAFLEX® micromechanical bonding laminates exhibit exceptional peel strength without the use of a tiecoat
between the polyimide film and the copper conductive layer (pict. 1,2).
FRAFLEX® laminates also offer excellent mechanical and thermal characteristics.
The exceptional property of the FRAFLEX® is that the peel strength of
the copper is up to 2,5 N/mm
FRAFLEX® laminates are manufactured in a roll-to-roll process
with four basic steps. In the first step - ion tracking - polyimide film
is irradiated by heavy, high-energy ions. Copper and polyimide are mechanically
interlocking connected, all disadvantages of the material, which are
caused by the presence of intermediate layers, are completely avoided.
Altogether, it is a material which can be fully recycled (“Green PCP”).
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The thickness of the polyimide foil itself
is between 12 and 125 microns. Standard thicknesses of the copper layer
are normally 9, 12, 18, 35, or 70 microns.
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Features:
- Outstanding peel strength / adhesion;
- Excellent flexural endurance;
- Excellent dimensional stability;
- Thin copper for fine line requirements;
- Excellent heat / thermal resistance;
- Compatible with lead free solder;
- Excellent chemical performance;
- Excellent electrical performance;
- Low moisture absorption;
- Completely recyclable;
- Simple etching (no tiecoat);
If you want to learn more about FRAFLEX® or receive
a PCB quotation based on this material please send us your e-mail
here or (and) your inquiry here.
Explanations:
| BGA- Ball Grid Array. |
µBGA -Micro Ball Grid Array |
| COF- Chip on Flex |
TAB-Tape Automated Bonding |
| CSP- Chip Scale Packaging |
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| | 22.01.2004 | New Base Materials for High-Speed Digital and RF Applications A low dielectric constant enables the use of (ultra)
thin base materials for fast clocking speeds and increased packaging
densities at current impedance levels. It is the low dielectric loss
however, which turns high-speed digital into a success story: Signal
traces can become longer, and power input can be reduced - both at
improved signal integrity. Teflon™/glass fabric base materials can
be laminated together with FR4 prepregs and FR4 inner layers for mixed
dielectric multilayers. High-speed digital printed circuit boards are
typically large-sized high-layer count multilayers. Until recently
the unavailability of ultra-thin laminates and prepregs prevented single-component
constructions. However RF-35P is a proven technological advancement
which enables the manufacture of thinnest inner layers down to 50 micron
(0.05 mm) thickness in production quantities. Based on this technology
TacPrepreg TP-32 prepregs have been developed.
For more information, please download the PDF file from
here.
The spec. details for RF-35P material you can find here. In
case you are interested to send us an inquiry for printed circuit boards, based
on RF35P laminates or (and) TacPrepreg TP-32 prepregs
please find the inquiry
form here.
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