How to Generate Assembly Files and Why They are Important

In an ideal world, you can just give the bill of materials to an assembly house and they will have enough information to populate your PCBs correctly. In reality, it’s rarely that simple.

From our experience, even the most standard of standards, whether as interpreted by customers or provided directly by manufacturers can be flawed. Datasheets missing data or being unnecessarily cryptic, manufacturers deciding to go against widely accepted standards, designers failing to indicate orientations and polarity markings or just plain silly mistakes, there are a plethora of potential pitfalls designers can fall victim to. These issues usually rear their ugly head at the last minute, at which point they can be a production nightmare to both designer and assembler. The amount of time and resources that could be lost should not be underestimated.

The other day, someone rather bluntly asked one of our engineers, “Well, what can we do about this?”. For the here and now, there isn’t really an easy solution, except just to be vigilant. Check, check again and check some more. There is ample talk of idealistic ways of tackling the issues from utilizing smart software to panning together to make standard footprint libraries, but none can ever guarantee flawless assembly.

Some examples of the problems we commonly face as an assembler:

• Incorrect footprint: Part R1 is size 0603, but the land pattern on the PCB is 0402.
• Quantity mismatch: There are 6 different designators in the BOM for this part, but the quantity indicated in the BOM is less than that – is one missing?
• Missing designator/footprint: The BOM has part L4, but we can’t find this part on the board. Where should it be placed?
• Lack of polarity markers: Part D2 and D3 do not have polarity markings, please provide us with the correct orientation.
• Lack of “pin 1” markers: We don’t know how to mount IC U4, there are no indicators on the board.
• Lack of orientation indicators for asymmetric parts: Which direction should the connector be facing? Should the header be facing inwards or outwards?

While we can’t check you’ve used the correct footprint for all your BOM parts or make sure you haven’t left out a part (that’s another thing entirely), what we can do is let you know what information we need and how best to present it. In this article, we share some of our experience with problems encountered during the PCB assembly stage and how best to prevent them.

There are three common issues we encounter that can easily be remedied by ensuring that the information is marked clearly and it boils down to 3 basic elements: 1. Designator, 2. Polarity markings, and 3. Orientation indicators.

All this information can be communicated in the assembly files

Why are they important?

• They are the main means of communicating a whole wealth of precise information to your fab house and are even more paramount if you are handing the job of assembly to a shop where the language of choice may not be your own.
• When it comes to populating your design, it only takes a single mistake to ruin a production run and waste valuable time and resources on costly repairs.
• X-Y or pick and place files can help with orientation and placement, but humans don’t read in X-Y and they don’t cover through-hole components which are arguably even easier to mount incorrectly.
• So, assembly files are all the more valuable when hand soldering a couple of boards for prototyping since everything is done manually.
• If the silkscreen is unclear or if, like some of our clients, you don’t want the board surface to be scribbled with silkscreen text, then the assembly files can serve as a complete reference to populating the boards.
• For ICs, it’s pretty obvious that they have special orientation requirements but for simpler components like diodes, the lack of polarity markings can result in the component being placed willy nilly, and you usually discover this when it’s too late.

How do I generate assembly files?

Every piece of PCB design software has different ways of generating assembly data (some even in Gerber format) but the general idea is the same: to have a clear and quick reference of the component locations, with positions and orientations clearly visible. That may mean having component outlines and or courtyard information, something that is not always printed in its entirety on the silkscreen. Keep things simple; things like component values and MPNs don’t really help at the assembly stage and can contribute to clutter in densely populated designs.

While we cannot speak for all PCB design software, here’s some brief pointers on how to export this information from some of the more popular packages:

• In Eagle, the useful layers are tPlace, tNames and tDocu (and likewise for the bottom side). The additional courtyard layer is tDocu.

• In KiCad, the Courtyard and Fab layers typically contain the essential data.

• For Altium Designer and CircuitMaker users, the software has the option to export a PDF of the assembly drawings directly.

Check out our official FAQ for more step by step instructions. These are not industry standards but they usually contain more than enough information for the assembler. Bear in mind, for this to work, the footprints you are using, whether drawn by yourself or by others. should follow design package specific footprint standards and contain all the relevant information in these layers.

If you are looking for X-Y pick and place files, we have it covered.

Ideally, technicians don’t even need to look up the datasheet to know how a part is supposed to be mounted. That is what the little dot is for right? But it still amazes us how many “conventions” are intertwined and made unnecessarily complicated. It’s all in the datasheet they say. It may very well be, but no technician wants to waste ten minutes for each possibly-polar component trying to figure out the orientation from a poorly written datasheet.

Here are some examples of the odd situations we’ve found ourselves in:

ICs: I’ve received the PCBs and they are completely dead! Checking the parts I noticed the IC is soldered upside down.

Ok, this one was our bad. Don’t get these two mixed up. On the left, the lines partially show the component outline including the ‘notch’. The right is the convention where the longer line indicates the ‘notch’ or dot indicator. Easily overlooked if you’re not paying attention.

Not to mention, datasheets and footprints often don’t come in the same package, especially if drawn up by third parties that claim to be standard. There’s no one library of recommended conventions – even if there were it seems someone is always trying to invent a new one. Some make sense, some don’t.

The best thing to do is imagine if you were to place this part yourself without datasheet assistance. Look at the recommended land pattern to get a feel of what the package looks like, check what indicators are on the package, be it a dot or notch or whatever, then make sure these are visible on your PCB design and/or the fab drawing. Is the silkscreen dot overlapping a pad or some other text? Are there other indicators that may cause confusion? If so, tweak the layout so it is obvious and easily visible. Along with a clear component courtyard or outline in the assembly files, there should be little room for error.

For this component, by a renowned components manufacturer, the datasheet shows us how the dot should be aligned with the footprint. Good – if only the component body itself had a dot too. The component marker is actually the notch on the bottom of the package that is completely hidden once mounted.

LEDs: I’ve received the PCBs and some of the LEDs don’t light up. The orientation is clearly marked in the silkscreen, what went wrong?

This is, by all means, a real case. Two LEDs, a green and red, sat next to each other with the same silkscreen markings (a rectangular outline of the chip with one end opened), made by the same manufacturer, in the same series, differentiated only by the designator markings D1 and D2, and a single letter in the part number indicating a different color. Nothing can go wrong here, right? But, lo and behold, D2 is actually supposed to be mounted in reverse, opposite to the common convention – what is that about?

Turns out, there is a little asterisk in the datasheet that says the anode and cathode are reversed for the red LED, and only the red LED. Gee, thanks a lot.

In an ideal world, all conventions are just that: conventions. But there always seems to be a manufacturer who wants to be complicated. (If you have an inkling of why they reversed the polarity, do enlighten us). Please don’t expect the technicians to check every little detail in the datasheet. You can make life easier for both parties by recognizing key points early on and marking the silkscreen suitably. As the designer, you should be the most familiar with the components on the board – which are polar, which are orientation specific or have specific mounting requirements. Check the datasheet for physical indicators on the package and match it on the silkscreen or assembly files. It only takes one small mistake to ruin a batch, so leaving a little extra time for file review can easily save you an arm and a leg.

Another odd example, the WS2812 series of RGB LEDs.

As a general rule of thumb, we’re usually told that the notch or dot is the indicator for pin 1. WS decided to throw that out the roof for some LEDs and go for pin 3, just because. Not so much of a problem for assembly as long as you are using THE footprint and schematic symbol for WS2812B and not something else, and that you are not manually routing the PCB for some inexplicable reason.

We’ve made our share of mistakes but the only thing we can do is learn and move on. Got your own assembly woes to share? Any must know tips and tricks? Enlighten us in the comments section below.

Already preparing to hand over the task of assembly to the professionals? We hope this article helped give you some peace of mind when it comes to communicating your intentions with your crew. If not, we hope it has highlighted the importance of having clear and concise instructions and warned you of the perils of failing to do so.

Seeed Fusion online manufacturing services have provided Turnkey PCB Assembly services for over ten years. Pioneering the instant quoting platform, Seeed Fusion’s online system can read formatted BOM files and extract component cost and availability directly to get you a quick quote in seconds. Sourcing from partners like DigiKey and Mouser, local Chinese manufacturers and more, the Seeed Fusion PCB Assembly service is easily one of the most convenient and accessible all-in-one services out there.

About Author