PCB Fabrication Technical Information

We recommend that your order is placed online. This ensures all the information we require is presented to us in one document.

We need PCB layer files in GERBER format plus standard CNC drill files (usually designated "excellon"). The default plot files from your CAD system are invariably suitable. ZIP them up and upload through our price calculator or just email them your order to us here.

Circuits or Panels ?

You have a choice. Most customers want PCBs supplied as single circuits. However, you can order them as step & repeated panels. Either way, you send us a set of GERBER data with a single image and we do the panelisation.

PCBS supplied as single circuits

Most customers will want their PCBs supplied as single circuits and prices can be obtained from the price calculator and orders may be placed online.

Step & repeated panels of the same circuit attached by tabs with PCB train standard handling frame

Simply provide us with single image PCB GERBER data. We will place your circuit in a "standard" panel in a format which we know to be acceptable for most automated assembly. Our standard panel design with handling frame has 2.00mm gaps between circuits with a 10.00mm handling frame on each of 4 sides. 3.00mm tooling holes and fiducial markers will be placed in each corner. Individual circuits will be fixed by 2.0mm tabs within the panel. During the "Quote and Order" process, select "Yes" to supply in panels. Enter the required number of images in the "X" axis and "Y" axes. Prices are still quoted as per circuit, but your order will be supplied in the specified panel arrangement. Note that the maximum overall step & repeated panel size should be no more than 200mm x 300mm, about the size of an A4 sheet of paper.

Step & repeated panels of the same circuit attached by tabs with customer designed handling frame (Additional charge £ 100.00)

Customer designed handling frames present us with difficulties because of the wide variation in complexity. We recommend that you consider using the PCB Train standard handling frame for which there is no charge. However, if this is unsuitable, then PCB pricing will be based on the overall dimensions of your panel including the handling frame, plus an additional £100 charge for processing panel GERBER data or for creating the panel data from single image GERBER and your panel design. You cannot at present place this type of order online. Calculate the base order value from the website, add £100, and email your order details to us.

Solder paste files  (No extra charge)

Solder paste data for step & repeated panels will be supplied upon request at no charge subject to solder paste layers having been provided within the original PCB data. email your request to us.

Multi circuit panels containing a mixture of different circuirts (Additional charge - see below)

Pricing will be based on the overall dimensions of the panel. A multi-circuit panel is defined as a PCB which contains two or more different circuits on the same sheet. You design the panel and send us a single set of GERBER data. Multi-circuit panels can be extremely complicated. These types of configuration present problems for us during tooling and final profiling. There are extra charges if the lot price is less than our minimum lot prices for multi-circuit panels. We will manufacture multi-circuit panels at our quoted PCB Train prices but subject to minimum lot value of

Layers	Price
2 layers	£200
4 layers	£300
6 layers	£400

If your lot price is less than this, we will charge an additional lot premium of £10 for each circuit contained in one panel of your multi-circuit panel design. Remember to space the circuits at least 2.00 mm apart to allow for our standard 2.00mm router tool. You cannot at present place this type of order online. Calculate the order value from the website and email your order details to us

Circuit board technical capability

The design features of circuits must be within the PCB Train technical capability. We do not warrant to identify non-conformities in design files. Any impairment of functionality as a result of this is your responsibility. If your circuit falls outside the PCB Train technical capability, ask us for a special quote. We manufacture in a wide variety of materials and specifications. E-mail your requirement to PCB Train

FR4 (150 deg C) middle Tg laminate Ventec VT-481TC/Laminate datasheet

This laminate is suitable for all single sided, double sided, & multi-layer PCBs & lead-free assembly operations with a middle range heat tolerance typical for FR4.

• Mid-Tg FR4.0
• Phenolic Cure System & Lead Free Compatible
• Excellent Thermal Reliability
• Low Z-CTE
• CAF Resistance
• UV Blocking

Applications for this grade of laminate include: Computers, Communication Equipment, Instrumentation, Electronic Gaming Machines, Automotive, etc

 

FR4 (180 deg C) high Tg laminate Ventec VT-47 Laminate datasheet.

This laminate is suitable for all single sided, double sided, & multi-layer PCBs & lead-free assembly operations with a higher range heat tolerance typical for FR4.

• High Tg FR4.0
• Phenolic Cure System & Lead Free Compatible
• Excellent Thermal Reliability
• CAF Resistance
• UV Blocking
• Low Z-CTE

Applications for this grade of laminate include: Computers, Communication Equipment, Instrumentation, Precision Apparatus & Instruments, Servers, Routers, Automotive, Medical, etc

 

Polyimide (250 deg C) high Tg laminate Ventec VT-90H Laminate datasheet.

This laminate is suitable for all single sided, double sided, & multi-layer PCBs & lead-free assembly operations with a very high range heat tolerance.

• Tg 250 deg C
• High Td
• Improved Fracture Toughness
• Low Z-axis CTE
• Non-Brominated Chemistry

Applications for this grade of laminate include: Chip Manufacturers, Engine/Flight Controls, Power Supply/Backplanes, Military and Burn-in Boards

 

Aluminium base copper clad laminate specification

Aluminium base copper clad laminate is commonly a thin sheet of copper clad FR4 bonded to a thicker sheet of aluminium which acts as a heat-sink. The bulk of aluminium clad PCBs are single sided SMD only circuits. The presence of a conduction layer of aluminium prevents the use of through hole components. The superior heat dissipation of aluminium clad laminate allows the design of smaller lower cost PCBs with thinner tracking, and denser component population than would otherwise be possible with standard FR 4 laminate.

This data provides information about: Aluminium thickness, FR4 dielectric thickness, Copper thickness, Overall thickness, Thickness tolerance, Thermal conductivity, Flammability rating, & UL file number. This data is frequently required by circuit designers.

Aluminium clad VT-4A2 laminate Link to specification and datasheet

• 1.5 mm thick aluminium,
• 100 um thick ceramic filled dielectric
• 35 um thickness of copper, single sided
• Thermal conductivity of 2.2 W/m*K.

Applications for this grade of laminate include: Power conversion, PDP, LED, Regulator for televisions, Monitor drives, Rectifiers, Power supply, etc

With high-speed digital signals, it can be crucial to predict and match the impedance of the transmission line (relevant tracks and ground plane) with the input and output impedance to minimise losses and reflections. Impedance factors are increasingly important as frequencies increase.

Manufacturing custom-made controlled impedance PCBs can be expensive. PCB Train's circuit boards are fabricated as consistently as possible, and we use our technical knowledge to advise what feature widths (track widths and gaps) to use to achieve commonly required impedances. Because of this, our clients can receive the benefits of controlled impedance circuits without suffering the additional costs.

The number of possible transmission line structures is infinitely vast. Variables influencing the impedance are track width, insulation gaps, both in the “x, y” axis and “z” axis, the geometry of the transmission line structure, and the dielectric constant of the insulation material. We use intelligent software to design a test coupon of the same structure and impedance as that required within the final circuit. A time domain reflectometer (TDR) is used to measure the impedance of the test structure which is manufactured on the same sheet as the PCB. A digital microscope is used to measure the geometry of the structure. If the impedance of the test coupon structure agrees with the predicted value, then the reasonable assumption is that the similar structure in the body of the PCB will also have the required impedance.

This impedance information is published to assist users of the PCB Train service who are looking for a track impedance calculator to predict track impedance.

These guidelines relate only to PCBs fabricated on the PCB Train 4 Layer M/L 1.6 mm FR4 1 oz copper service. View the multi-layer build here.

Customers use this information at their own risk.

Layer descriptions are defined as follows:

• Layer (1) = top copper
• Layer (2) = inner-layer top
• Layer (3) = inner-layer bottom
• Layer (4) = bottom copper

Our calculations and measurements have established the following:

• A layer (1) single free-standing copper track (a micro-strip) of 0.40 mm width will have an impedance of 50 ohms with respect to a solid copper ground plane on layer (2)
• A layer (1) single free-standing copper track (a micro-strip) of 0.18 mm width will have an impedance of 75 ohms with respect to a solid copper ground plane on layer (2)
• A layer (2) single free-standing copper track (a micro-strip) of 1.64 mm width will have an impedance of 50 ohms with respect to a solid copper ground plane on layer (3)
• A layer (2) single free-standing copper track (a micro-strip) of 0.68 mm width will have an impedance of 75 ohms with respect to a solid copper ground plane on layer (3)
• A layer (2) single free-standing copper track (a micro-strip) of 0.26 mm width will have an impedance of 100 ohms with respect to a solid copper ground plane on layer (3)

These guidelines relate only to PCBs fabricated on PCB Train 4 Layer M/L 0.8 mm FR4 1 oz. copper service. View the multi-layer build here.

• A layer (1) single free-standing copper track (a micro-strip) of 0.40 mm width will have an impedance of 50 ohms with respect to a solid copper ground plane on layer (2)
• A layer (1) single free-standing copper track (a micro-strip) of 0.18 mm width will have an impedance of 75 ohms with respect to a solid copper ground plane on layer (2)

These guidelines relate only to PCBs fabricated on the PCB Train 6 Layer M/L 1.6 mm FR4 1 oz. copper service. View the multi-layer build here.

• Layer (1) = top copper
• Layer (2) = inner-layer top
• Layer (3) = inner-layer 2
• Layer (4) = inner-layer 3
• Layer (5) = inner-layer bottom
• Layer (6) = bottom copper
• A layer (1) single free-standing copper track (a micro-strip) of 0.40 mm width will have an impedance of 50 ohms with respect to a solid copper ground plane on layer (2)
• A layer (1) single free-standing copper track (a micro-strip) of 0.18 mm width will have an impedance of 75 ohms with respect to a solid copper ground plane on layer (2)

All tolerances + / - 10%
If you require controlled impedance PCBs complete with measured impedance test coupons, please ask us to quote.

• It is a "drop in" HASL replacement, requiring no change to assembly processes.
• It has excellent wettability for soldering.
• It is environmentally friendly.
• The solder joint strength is superior to that of AU/Ni PCB finishes.
• The resistance to environmental ageing is superior to that of organic solderability preservative (OSP).
• Flatness, electrical properties and solder joint strength are superior to HASL.
• For touchpad contacts, it is better than carbon ink and as good as Au/Ni finish, see the Motorola report into the use of immersion silver for keypad / touchpad contacts.
• It has similar performance Au/Ni for ultrasonic aluminium wire bonding.
• It can withstand multiple reflow.
• The process is supplied and supported by Macdermid PLC

Handling and storage of immersion silver finished PCBS

• Avoid finger contact with the Immersion Silver finish. It is recommended that gloves be used for handling panels/boards. Salts/acids from fingerprints may tarnish the finish, and impair solderability.
• The generally accepted shelf storage life for Immersion Silver is 6 to 12 months in dry conditions, the same as that for HASL.
• The bright appearance of Immersion Silver finished PCBs should not suffer from any yellow or black discolouration which will indicate contamination. If contamination is suspected, please contact us for advice.
• If PCBs have been in storage for some time, it is wise to perform a solderability test prior to assembly. For SMD applications this might consist of solder paste and reflow of a bare PCB. 100% solder wetting would indicate a satisfactory pass.
• When board cleaning or washing, surfactants and/or acid cleaners should not be used for bare boards with Immersion Silver finish. Only water washes should be used, with a pH > 6.0.
• Immersion Silver finished boards should be packaged to prevent exposure to air circulation. Storage could be in sealed bags or similar containers to exclude direct contact with air. Storage conditions should be < 30 deg. C and < 50 % RH. Use sulphur free paper to interleaf parts.
• If Immersion Silver finished boards must be stress relieved for warp & twist problems, the boards should be tightly wrapped in aluminium foil to prevent oxidation of the silver which will affect solderability. The bake conditions are: Temperature 150deg C for 2 hours. If it becomes necessary to perform a bake to remove moisture, the time and temperature should be reduced to a minimum. Do not exceed 2 hours at 122 deg. C.

Blind & buried vias

Avoid using blind or buried vias. The extra cost is only justified on specialist applications. A blind via is a connection that only goes part way through a PCB. A buried via is a connection that is completely internal in the multi-layer PCB. These types of vias are often suggested in text books as a way of routing more tracks in a given space, because they do not use board area on the layers that they do not penetrate. In practice, these techniques are rarely used for commercial application PCBs because of the high additional cost. An 8 layer PCB will often be cheaper than a 4 layer PCB with blind or buried vias. Blind & buried via PCBs are not a standard service on PCB Train but are available to special quote.

Copper layers

It is a golden PCB design rule to maintain uniform copper density across a track side and between track sides. There are two reasons for this. Firstly, copper imbalance will cause the undesirable bi-metallic strip effect of bow and twist. Secondly, it imbalances the thickness of copper plating in the electrolytic plating tanks because of differences in current density. This is another way of saying that the PCB will be under and over copper plated. The solution is to add copper planes to areas where there are few tracks. We recommend simple solid copper planes rather than hatching. Hatching by its nature, has lots of corners, and copper fragments can become detached at the hatch corners they have the potential to cause short circuits.

Data files

Standard default extended GERBER RS274X output will be fine for the tracking, solder resist, and legend layers. If possible, use "flashed pads" rather than "painted pads". This reduces the file sizes, and assists with identifying net ends for testing Output all pads as "filled", because that is almost certainly what you want, i.e. a complete solid copper pad, and not a thin copper outline for a pad. The standard convention is that all data is output looking down through the PCB from the component or top side. If this is not what you are sending us, please say so. This view is the same as you will see when you design the PCB. There is no need for you to mirror anything. A drawing showing how the layers build up in cross-section can be helpful. By convention, we treat one data-file as one PCB type designated by one PCB reference or name. If you have incorporated several different PCBs in one data-file, we will treat it as one (albeit complicated) PCB type. PCB data-files may be supplied step & repeated if you wish. If you have design notes, ZIP them up with the GERBER files in a README file PCB data-files must be compressed using PKzip before you upload them to us. The PCB Train website upload function requires data-files to be in .zip format.

Drill files

he default drill file output of your CAD design software will be fine.

Data file layer naming conventions

Your CAD design software will automatically name the layers of your PCB design output data-file. We are familiar with these outputs and you can just send us the default output.

Electrical testing

We do not require electrical test data. Our test data is derived from the GERBER data of your design. When we receive your data, one of the first processes is to take a reference snapshot of the XY coordinates of the nets and their ends. After we have finished producing the PCB production tooling, we compare the production data "net ends" with the reference data "net ends". They must be identical.

If possible, create component pads (= net ends) from flashed pads rather than painted lines. If painted pads are present, we have to convert them to flashed pads as part of the production tooling process. This is not just to generate electrical test coordinates, but also to reduce the data size. GERBER data is used throughout the production process, and smaller file sizes assist the speed of processing.

Gold connectors

If you require electrolytic plating of gold edge connectors on your PCB design, this is not available on PCB Train. We can supply this type of gold plating to special quote. Ask us to quote specifically. Consider using a standard PCB Train gold over nickel finish, (ENIG or Au/Ni finish, 0.1 microns of immersion gold over 5 microns of electroless nickel). This finish is good for a limited number of insertions in the connector.

Layer identification

We are not talking about filenames here. What we are referring to is the benefit of physical marking the application of each printed layer in the PCB build-up. The purpose of this is to make it easy to quality check that your PCB has been built correctly at all stages of the production process. For example, the topside of the PCB should be marked as "trackside 1" for example. Or the bottom solder resist could be marked as "bottom solder resist" say. Similarly, inner tracking layers of multi-layer PCBs should be marked in sequence, for example, "inner layer 1"," inner layer 2", etc. Convention is that the 1st inner layer starts from the topside or component side. Although the CAD layout screen view will be through the PCB viewed from the topside, the layer description text should be mirrored in this view appropriately so the text is "right reading" when printed and viewed on the finished PCB component (think of the individual printing of inner layer pairs 1 + 2, 3 + 4, etc., and top and bottom sides).

Legends or ident layers

It is bad practice to print legend ink on component pads. As part of our design rule checks, legends which may encroach on pads will be shaved back, and this may affect legibility.

The technical details are as follows:

• Minimum line width 0.10mm (4mil)
• Minimum text height for good readability: 1.00mm (39.5mil).
• The legend print will be clipped against the corresponding solder resist layer.
• All legend text must be right reading. The text on the top layer of your board should be right reading, the text on the bottom layer should be mirrored
• It is helpful to include the board outline on your legend print layer using a 0.25mm (10mil) line width. We will take the centre of this for the profile of the board. This line will be removed from your legend print layer prior to production.
• We would recommend there should be no legend print added on the copper layer side of a PCB without a solder resist layer, this can cause a blurred affect and lift off easily.
• We will clip the legend 0.0254mm (1mil) back from the solder resist openings. Please note you need to allow for the solder resist clearance from copper pad of 0.127mm (5mil) annular.
• If there is no solder resist layer, the legend print will be clipped against the corresponding copper layer, otherwise we will clip against the drill layer.
• If you do not want your legend to be clipped, then you need to allow for this on your design. Otherwise please add a note when ordering to advise us that you do not want us to clip your legend.

Rout files

We do not need a specific CNC rout file programme. What we do need is a graphic drawing showing the cutting line of the PCB profile. It is preferable to draw this on your screen with a zero-width line. If you use a thicker line, our working assumption is that the cut edge will be the centreline of the drawn line. Do not try to compensate for the router cutter width or try to show the cutter path. This is not necessary. Keep it simple and just show the actual cut line. Cut-outs and slots can be missed unless shown clearly in your drawing. It is good practice to annotate your drawing with text to indicate which areas are cut-outs and slots to avoid errors.

Routed internal corners: minimum radii

Our standard router is 2.00 mm diameter. The minimum radius of an internal corner will be 1.00 mm. The narrowest slot will be 2.00 mm. If smaller radii or narrower slots are required, draw our attention to this and ask us to quote specifically.

Solder resist

All solder resists are of the photo-imageable type, printed by direct mirror imaging without the use of photography. The use of automatic camera based fiducial registration gives superb line up of solder resist to copper pad.

Very often solder resist artwork will only physically line up one way with the appropriate tracking layer. However, it is very good practice to identify solder resist sides with appropriate text printed right reading when viewing the finished PCB, e.g. solder resist side 1, solder resist side 2 for example. We will check the solder resist clearances around component pads as part of our design rule checks, and increase them to our minimum if necessary.

Surface finish

Our recommended standard PCB finish is Immersion Silver (I.S.) This finish is flat and perfect for SMD assembly. It consists of an organic anti-tarnish layer over a thin layer of silver over exposed copper. The application of this finish imparts little stress on the PCB and the process has low environmental impact.

An optional alternative surface finish is electroless nickel immersion gold (ENIG). This consists of 0.1 micron immersion gold over 5 microns electroless nickel (also known as Au/Ni surface finish). ENIG is a more expensive process with a greater environmental impact than I.S.

The above finishes are lead-free (RoHS compliant), have excellent solderability and are ultrasonic wire bondable. They will withstand multiple reflow operations. They are both suitable for touch pad contacts.

Avoid finger marking any of these finishes as this may jeopardise solderability. All PCBs should be stored in a dry environment and kept wrapped up in order to extend their shelf life which will be a minimum of 6 months.

Symmetrical PCBS

Be very careful with the design of PCBs which have some degree of symmetry. We strongly advise that some features are designed in on each layer which breaks up any symmetry. This will avoid any possibility of mirroring or miss-rotation of layers during fabrication.