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Ideas & Solutions

- Printed Heat– Sink
- Alternative PCB Finishes: Advantages and Disadvantages Comparison
- The Insulated Metal Printed Circuit Board
- Impedance and Multilayer
- The plated through Hole Calculation
- More Then "Just Copper-Foil"
- SMD Assembley Equipment
- Rigid - flexible Multilayer Boards
- Multilayer PCBs with special dielectric constant
- Conduction heat transfer in a printed circuit board
- Metal Circuit Boards
- The effect of etch factor on printed wiring characteristic impedance
- Test coupons for the measurement of wave impedance
- Introduction of an innovative foil material "FRAFLEX®"
- New Base Materials for High-Speed Digital and RF Applications
- PCB specification for press-fit technology
- Hybrid Multilayer PCBs.
- Fungus Resistance – Conformal Coating

 Printed Heat– Sink

As density of electrical components at PCBs is increasing, the problem of conduction off the heat produced by that parts gets more and more essential. The usual solutions are so called
“ Heat sinks” which are cooling elements made of metal (aluminium or copper) which are to be mounted at spots on the PCB from which the heat need to be leaded away. The disadvantages of these Heat Sinks are obvious :
- additional mounting work;
- precious space at PCB is kept away by Heat Sinks;
- as these are electricity conduction metals, a proper insulation from PCB layers is necessary to avoid shorts;
- Heat sinks causing additional weight and increasing the volume of the PCB assembly.

What are the technical alternatives to Heat Sinks ?

a) metal core PCBs (basic material is an aluminium or copper alloy itself – these metals have the higher specific heat conductivity (HC): copper – 400 W / mK, aluminium – 200 W / mK
compared to epoxy glass fibre, the PCB basic material which has only about 0.4 W/mK HC.
Disadvantages : expensive in material and workmanship, only 1and max. 2 Layer PCB possible.

b) thick copper foils as “Thermo – Layers” … these are available at thickness up to 400 micron. They can be used in Multilayer PCB constructions as inner layers or even outer
layers (with inner signal layers and isolated vias through the Thermo – Layer. Disadvantage : complicate construction and higher costs.

c) a new elegant solution is the “Heat Sink Paste” (also called“ Printed Heat Sink”) which is offer by a German producer of PCB– chemicals Lackwerke Peters GmbH + Co KG, Germany. The processing of that Heat Sink Paste is by screen printing directly to PCB through a screen like a solder paste stencil and curing afterwards. The HC of the paste – it consists of
solid particles based in an epoxy resin - is by 2 W/mK – as via hole of the PCB are partly filled, additionally head dissipation is achieved.

The advantages of such Heat Sink Paste are as follows :

- easy application by screen printing and usual drying methods;
- high definition for design of variable structures and layer thickness;
- heat is dissipated from source of its generation, possibility to fill heat vias with the paste;
- economical process compared with alternatives;
- continuous heat resistant to 155° C;
- electrical isolator itself (no insulation necessary);
- no volume shrinking, good mechanical, chemical and solder bath resistance;
- flame class V0 as per UL .


 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).

 The Insulated Metal Printed Circuit Board

The Insulated Metal Printed Circuit Board (IMpcb) can replace Standard FR4 Boards or Ceramic Substrates in Power or Thermal applications. The basic construction IMpcb is a dielectric layer (2), between copper foil tracks (1) and a metal base plate (3).

The primary technology is in the dielectric material, which must provide good thermal conductivity and good dielectric isolation. Power Electronic Products today are being required to provide more performance, in less space, and at lower costs. As a result, the PCB or substrate must provide improved electrical, thermal and mechanical performance. To meet these needs, designers must have the same electrical, thermal and mechanical information, which would be expected with any electrical component. The material manufacturers provide this type of information in Data Sheets, Design Guidelines and Computer Models. The information allows the designer to plan and optimize for performance, reliability, manufacturability and low cost. The IMpcb typically simplifies the system architecture, resulting in performance, size, reliability and cost advantages, which extend beyond the substrate or board. For instance here you can find the Thermal Clad selection guide (10 Mb) of Co. Bergquist.

Also Co. Thermagon has issued some usefull for all manufacturers,  PCB designers and end-users documents:

aa). Design Guidelines for Performance 1.0 Thermal Properties
1.1 Thermal Conductivity of the T-preg
1.2 Thermal Resistance of the IMpcb
1.3 Thermal and Power Management
2.0 Dielectric Isolation
2.1 Hipot Testing
2.2 Dielectric Strength
2.3 Reliability and Operation Life
3.0 Foil Resistivity
4.0 Maximum Copper Foil Current
5.0 Capacitance
6.0 Inductance
7.0 Electrical Vias between Foil Layers
7.1 Maximum Via Current
7.2 Via Resistance
8.0 Thermal Vias Application
8.1 Thermal resistance of Via Pads
8.2 General Thermal Via Considerations
8.3 Thermal Vias in Applications without Base Plates
More details about Design Guidelines you can find here .

bb) Guidelines for Manufacturability with Thermagon IMpcb
1.0 Basic Architecture
1.1 Single Sided T-lam with Base Plate
1.2 Double Sided Layer T-lam
1.3 Multilayer T-lam IMpcb
1.4 Multilayer IMpcb Hybrid with T-preg and FR4
2.0 Base Plate
2.1 Aluminum and Copper Alloys for Stamping, V-Scoring and Routing
2.2 Properties of Aluminum and Copper Base Plates
2.3 Special Base Plate Materials
2.4 Anodized Aluminum Base Plates
2.5 Singulation by Stamping, V-Scoring and Routing
2.6 Substrate Camber and Flatness
2.7 Panelization and Substrate Tolerances
2.8 Substrate Radius, Holes, Bridges and Base Plate Ground Connections
3.0 Dielectric Layer
3.1 General Considerations
3.2 T-preg, Thermal Dielectric
3.3 FR4 and Special Dielectrics
4.0 Copper Foil
4.1 Material Selection and Properties
4.2 Line and Space Considerations for Manufacturability
4.3 Line and Space Considerations for Performance and Safety
4.4 Plating, Solder and Special Coatings
5.0 Electrical & Thermal Vias
5.1 Via Size, Pitch and Plating
5.2 Electrical Connections
5.3 Thermal Enhancement
6.0 Component, Mechanical Hardware and Mounting
6.1 Component Considerations
6.2 Mechanical Hardware and Mounting Issues
6.3 T-lam Board and Substrate Mounting Issues
7.0 Inspection & Test
7.1 Electrical Inspection
7.2 Mechanical Inspection
7.3 Visual Inspection
8.0 Procurement and Ordering
8.1 Part Number System
8.2 Typical Procurement Specification
9.0 Assembly Guidelines
9.1 SMD Assemblies
9.2 Chip & Wire Assemblies
9.3 Mechanical Assemblies
9.4 Coatings, Encapsulations and Potting
10.0 Special Applications

More details about T-guide for Manufacturability with Thermagon IMpcb you can find here .

cc) Single Layered IMpcb Fabrication Guidelines you can find here .

dd) Double sided and Multilayer Mpcb Fabrication Guidelines you can find here .


 Impedance and Multilayer

Developers and designers of high-frequency circuitry know that they must calculate the impedance of printed conductors and allow for it in drafting CAD layouts. What is less well known is that in many standard digital circuits the impedance of printed signal conductors and the power supply increasingly play a role that is functionally decisive. The reason for this lies in the shorter switching times (signal rising edge) of modern modules. Although not always essential to practical use, the clock-pulse rates of signal circuits now approach the 100 MHz range and that of chips the GHz range. The switching times of associated chips have fallen to 1 or 2 nanoseconds or even less. The end result is high-speed circuitry requiring allowance to be made for physical conditions that could previously be disregarded. Elementary design rules must be observed to ensure reliable future module function. This includes overall power planes offering wideband decoupling via capacitive properties. Strict routing strategies avoiding slot antennae (which can cause high-frequency noise interference emission from the planes) and ensuring optimal signal feedback must also be applied. The circuitry design and choice of components must guarantee signal transmission in an homogenous electrical field using impedance-controlled circuitry. A pre-condition in fulfilling this requirement is the application of these technical specifications to the PCB, which hence has a functionally decisive role as carrier of the module. There is no reason to believe the PCB might be unable to meet the specifications set.

If you have interest to find out more about this topic, we suggest you to read the Workshop Lecture "Impedance and Multilayer"  of a leading specialist in this field ? Mr. Arnold Wiemers, company ILFA Feinstleitertechnik GmbH.


 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.

 More Then "Just Copper-Foil"

We want to draw attention of our customers at the possibility of using different copper foil modifications, manufactured by the company GOULD ELECTRONICS INC., which are: ultra thin copper foil TCU (3-9 mkm), laser drillable foil (LD foil), as well as thin film resistor foil (TCR) and planar capacitor foil (TCC). Short description of the above listed copper foils you can find here. For more information, please download in PDF files from here. In case you are interested to purchase this products, you can inquire directly by manufacturer (http://www.gould.com) or (in case of ordering of printed circuit boards) at our company.


 SMD Assembley Equipment

                                       Product pallet of Co. ELLWEST KG include different kind of PCBs (single & double sided, multilayer, flexible, impedance controlled, metal core) also assembling of boards and shipment as per requirement of customer.    Product pallet of Co. ELLWEST KG include different kind of PCBs (single & double sided, multilayer, flexible, impedance controlled, metal core) also assembling of boards and shipment as per requirement of customer.    Product pallet of Co. ELLWEST KG include different kind of PCBs

Have you consided to establish your own through-hole assembly or (and) SMD assembly at your own production ?

We can offer you top level through-hole assembly equipment (e.g. for 25.000 - 30.000 typical (250 x 110 mm) units / month) and SMD assembly equipment (for 8.000 - 9.000 typical (235 x 110 mm) units / month).

Please find the details on our PCB Assembly Equipment list and send us your inquiry.


 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.

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.


 Multilayer PCBs with special dielectric constant

Sometimes you need to have a special dielectric constant of PCB material, but it should be not expensive. For multilayer PCB’s we can offer you in this case following solution :

Click to enlarge

A schematic picture of Teflon and FR4 combination
DK of FR-4 is 4.1-4.8, DK of Teflon –2.1 – 3.0.

Go back


 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.

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


 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).

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)

 METAL/ALLOY

THERMAL CONDUCTIVITY
(W/m-K)

COEFFICIENT OF
THERMAL EXPANSION (ppm/K)

Copper

 400

 17

 Aluminum

 150

 25

304 Stainless Steel

 16

 16.3

Cold Rolled Steel

 50

12.5 

Iron

 80

 11.8

 CIC Copper - Invar - Copper

 20

 5.2

 CMC Copper - Molly - Copper

 200

 6.5

 20% ALSIC/Aluminum

 175

 15

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


 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).
Cross Section of PCB Outerlayer After Etching
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.

 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).

 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


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.


Pic.1.FRAFLEX® cross-cut.


Pic.2. FRAFLEX® surface after etching.

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”).

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.

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  

 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.


 PCB specification for press-fit technology

Production of printed circuit boards that meet the requirements of press-fit technology is extremely important for a good press-fit connection. The material and structure of a printed circuit board plated through hole has at least as high an influence on the quality of a press-fit connection as the press-fit zone itself.
The DIN standard 41 611, part 5, "solder less electric connections - press in connections" covers the specifications for the structure of a printed circuit board.
The IPC standard IPC-D-422 contain the design guide for Press Fit Rigid Printed Board Backplanes.
The decisive characteristic of a high quality press in connection is not only a final diameter of the plated through hole withing the permissible tolerances, but also a correct hole structure.

More about the PCB hole requirements for the press-fit technology (PCB hole HAL tin, PCB hole copper) you can find here (de/eng) and here (eng)

 Hybrid Multilayer PCBs.

Base materials made from PTFE (Teflon) are being used in larger and larger volumes. The reason for it is based on the continuously increasing operating frequencies of electrical appliances. The dielectric properties of FR4 are not adequate any longer to ensure functionality: Dielectric Constant (DK) incl. tolerance, dielectric loss, etc. The correct solution lies in the use of PTFE base material, which is manufactured according to applications, even in Europe.
Multilayers made of PTFE have been virtually unknown until recent time, but total system cost analyses have changed the rules: Multilayers made entirely from thermoplastic materials are being used as well as hybrid multilayers composed of PTFE and FR4 (pls. see lay-up example http://www.ellwest-pcb.com/solutions.php?year=2003#5 ). In all-RF multilayers thermoplastic bonding films made of CTFE (for instance TacBond HT1.5 bonding film from TACONIC) or FEP are used to bond the inner layers instead of prepregs. As with FR4 multilayers, increasing packaging densities of printed circuit boards are the decisive factors. It is also possible to obtain coupler structures, whereby PTFE inner layers of different thicknesses and dielectric constants can be bonded together (pls. see a picture below)

More about this matter you can find find in article of Dipl.-Ing. (FH) Manfred Huschka here.

 Fungus Resistance – Conformal Coating

Some electronics products require because of the environmental condition (high humidity and/or temperature, presence of inorganic salts) applicatioon of such printed circuit boards for theis construction, which are covered with a special mask resistant against different kinds of fungi and mould. General requirements for solder masks for the printed-circuit boards are formulated in IPC-SM-840C "Standart - Permanent Polymer Coating (Solder Mask) for Printed Boards". A corresponding test method is described in standart: IPC TM-650 - chapter: 2.6.1.1.“Fungus Resistance – Conform Coating" - test is to be made for following kinds of Fungus:

Scientific name:                              ATCC code:
Aspergillus niger ....................................9642
Chaetomium globosum ............................6205
Gliocladium virans ..................................9645
Aureobasidium pullulans ..........................9348
Penicillium funiculosum ...........................9644

We can introduce - and apply in our PCB production - fungus resistant PCB solder masks of manufacturers:
- Peters GmbH & Co., KG (Germany)
- GOO CHEMICAL CO., LTD.(Japan)


 

 

 

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