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PCB Design &
Fabrication Tips

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We want you to have the best possible experience when purchasing your printed circuit boards from PCB Train. To help us get your finished PCBs to you as quickly and efficiently as possible, we've put together the following guidelines to assist you when sending your order to us. Please consider these tips when completing your order as they help to minimise delays. If you're unsure on anything or need further technical assistance, feel free to send us an email and we'll be happy to advise.

Ball Grid Arrays (BGA's)


Via holes and tracks within a BGA footprint must be entirely covered with solder resist. Failure to do so will cause solder to wick from the BGA pad during reflow resulting in failure of the solder joint. Similarly, do not put a via hole within a surface mount pad. Do not use the PCB Train Express service for designs containing BGA's because this service does not offer a solder resist. All other PCB Train services include a solder resist as standard.


Blind & buried vias

Avoid using blind or buried vias. 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.

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. So 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

Not much to be said about these. The default drill file output of your CAD design software will be fine because it will be excellon format, which is what we expect.

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 PXCB design, ask us to quote specifically. Consider using a standard PCB Train gold over nickel finish, (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 clipped against the corresponding soldermask 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 soldermask layer, this can cause a blurred affect and lift off easily.

Clipping process:

• We will clip the legend 0.0254mm (1mil) back from the soldermask openings. Please note you need to allow for the soldermask clearance from copper pad of 0.127mm (5mil) annular.
• If there is no soldermask 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 centerline 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. So 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. So 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

Very often solder resist artwork can fit only one way. 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 finishes

Our recommended standard PCB finish is I.S. (Immersion Silver) which is lead free. This finish is very flat and perfect for SMD assembly. The application of this finish imparts little stress on the PCB and the process has low environmental impact. Hot Air Solder Leveled (HASL), lead-free finishes and 0.1 micron of immersion gold over 5 microns electroless nickel (Au/Ni) finishes are available as well on some PCB Train services.

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.