You have an idea. You make a prototype at home on your bench. Crowdfunding is an astonishing success. Now you need thousands of identical widgets. The scale is beyond your home bench and it is time to manufacture on a larger scale. What now?
In the world of contract manufacturing, we rely on automation. Robots, glorious robots, perform most of our tasks. This is accomplished both with physical machines transferring, placing, and soldering parts, as well as with scripts processing data sets. The goal of automation is to reduce the opportunities for error. This begins with the quoting process, which it is important to understand.
For a board containing surface-mount and through-hole parts, the following items are needed to produce an assembly:
- Gerber file
- PCB fab file
- XY placement file
- Bill of material
- Requirements (sometimes on an assembly drawing)
Since all of these items are needed to manufacture, try to provide this information up front at the time of quoting. Missing information will result in delay, production errors, or a change in cost. Due to the nature of development, it is understandable that you might not have all of these items available. If so provide as much information as you can.
The world of contract manufacturing currently has no accepted standard for how these files are packaged; an unfortunate state. However, while everyone does it different, in the end, we all need the same things. If possible, wrap it up in a tidy zip file and forward it via email or through your favorite cloud storage solution.
Now you have the list, let’s dive into more detail and look at how to avoid errors:
This artwork is exported out of your CAM/CAD software and should be in RS274x format. All current popular packages have this export feature.
It’s important to realise that the contract manufacturer should not work directly with the raw design files. There are many options when data is exported and it is ultimately up to the customer to provide a universally accepted data set with all the decisions about options made.
Please provide the Gerber file as a 1-up, not panelized. Your contract manufacturer and PCB Fab house will determine the best way to ensure compatibility with their manufacturing setup.
PCB Fab File
Printed circuit boards can be manufactured with a variety of options, and it is important to understand some basics so that you can both support your design, and avoid unnecessary cost. A PCB fab file basically states the fabrication requirements in plain ol’ black and white for easy understanding. There are many websites that provide detail about this subject. For the moment, we will keep this to a quick and simple crash course. This information should be documented within a simple PDF file. At a minimum, it should contain the following:
- Finished PCB Thickness: A normal, run of the mill circuit board is usually .062 inches thick. High reliability or rugged designs are typically .093 inches thick. Really thin consumer products usually run .031 inches thick. If you are unsure of what you need, stick to the .062 as this will be fine for most applications.
- Finished Copper Thickness: Specify the final thickness of copper for both the inner layers and outer layers. This is typically stated as “1 oz finished copper”. High reliability, high power products may have a greater copper thickness, such as 2 oz finished copper. Consumer products are usually ½ oz finished copper. A printed circuit board fab house will sometimes start with a ½ oz copper sheet, and plate it up to 1 oz, hence the “finished copper thickness”. If you don’t know, just stick with a blanket statement of “1 oz finished copper for all layers” and you should be fine.
- Layer Stackup: You can name files whatever you want to. In the end, though, a designer should label the order of the files being stacked up. By providing this information, you will eliminate an opportunity for error. This is especially important if your design requires controlled impedance or differential pairs.
- Color: Specify the PCB mask color and silkscreen color. Typical colors are green, black, white, yellow, red, and blue. Any color is possible, but you will likely incur additional charges and are less likely to get the exact color you desire. Stick with the basics for greatest success. Note that silk screen colors should be complementary: it is not a good idea to specify black ink on a black board. Typical silkscreen color is white or black.
A few notes are worth making about white PCBs. If you desire a white finish, the surface treatment is very important! Do not specify immersion gold with white as the gold color will bleed through the masking and your board will end up with an undesired pinkish color. Stick with immersion silver or immersion tin. Also, reflow temperatures for RoHS (lead free) can sometimes discolor white finishes, so it should be specified as “high-temperature white masking” or “double-painted white masking” for the greatest chance of success.
- Drill File: Some CAD/CAM software exports the drill file in a separate folder. Make sure you include both the drill file and drill chart in the final data.
- Surface Treatment: All exposed copper where your parts ultimately end up must have a surface treatment as exposed copper all by itself will corrode. If you are unsure of what to specify, stick with immersion silver for good performance and lower cost. The available options are usually:
- LF-HASL – Hot Air Solder Leveling. “LF” stands for Lead Free. Get used to going lead free! LF-HASL is the lowest cost surface treatment, and should only be used for designs that are completely through-hole or surface-mount assemblies which do not have any fine pitch parts. LF-HASL will cause erroneous shorts on fine pitch designs.
- Immersion Tin – This is the lowest cost solution for high volume and fine pitch designs. The electrical performance of immersion tin is not as good as that of silver. Also, if not used immediately, immersion tin will corrode over time. If you intend to have your boards sitting around waiting for customer orders, do not use this finish. It should only be used if you immediately run your production order upon arrival.
- Immersion Silver – Slightly lower cost and performance than immersion gold, typically anything using this finish will perform just fine. Silver suffers from the same corrosion as described above under immersion tin.
- Immersion Gold aka ENIG – this is the gold standard. It has the best electrical performance, looks great, and will not corrode. It is also the most expensive finish. Use this for any assemblies that require lots of fine pitch, ball grid arrays, or other advanced features.
- * Special Features – Controlled impedance or differential pairs should be specifically identified! If your design contains high-speed flash memory and/or USB connected directly to a microcontroller, you probably need this. Controlled impedance is characterized by a trace width and trace separation note. A printed circuit board fab house will typically ask for minor alterations to these widths to hold your desired impedance. Make a note stating it is okay to alter widths to hold the desired impedance.
XY Placement File
An XY file is a text file that locates all of your surface mount parts. This file is translated into machine code for pick-and-place machines to perform automated surface mount placement.
Remember those robots mentioned in the beginning? These are the instructions that the robots use. As with a Gerber, an XY file can be exported out of your CAD/CAM design suite. Without this file, a programmer would need to put your board on the placement machine and manually enter every position. You don’t want that to happen, since generating the XY yourself costs nothing. Some quoting formulas use this file to automate analysis and calculate labor times. It’s a nice thing to have up front.
Bill Of Material
It is beyond expression how valuable a correct bill of material (BOM) from the customer is to a contract manufacturer. Correctly formatted, this data will lower your costs, cut down how long it takes to get your quote, avoid additional charges later, and ensure correct production the first time.
However your CAD/CAM software exports data, it is likely incorrect. Here what you want it to look like: this is your master template forevermore. Free. Here. Now. The best format for a bill of material will look like this:
Oh, one more thing: .XLS format is a must. For all of you Open Source types, I respect your decisions. Just know that in the contract manufacturing world this file format is king. As you are able to export into the .XLS format, save everyone time by providing it up front. Little tangent here: All machines used to make Apple products run on Windows. All of them.
You are of course welcome to add additional information. This is the bare minimum that is needed. Absolutely nothing less. Spend time on your bill of material. Google part numbers, make sure they are easily found. If you are using something not readily available out of a standard vendor or catalog company, include the contact people needed to buy. Make sure the quantity listed actually matches the number of reference designators.
Feel free to refer to the bill of material examples sheet we uploaded to see many examples of problem BOMs and the opportunities for error that are introduced.
Remember the beginning, when we talked about robots and automation? When quoting your project, a BOM is sent to various vendors for pricing. They use scripts to automate matching the listed part numbers to their internal data, which then gives a price per line item. If a number cannot be matched through automation, it is done by a real live human being, which takes more time. Most customers demand that a quote be finished yesterday, so don’t shoot yourself in the foot on this one.
You may have designed a product exclusively using products from a particular popular vendor, and have that vendor’s internal part number on the BOM. This is a bad practice! The same part can be sourced from my set of vendors at a lower price. Take advantage of my lower price by listing the actual manufacturer and actual real-world part number. Consider this: you are Vendor A and need to price a bill of material. All of the part numbers are specific to Vendor B only. This requires someone to Google each part number on Vendor B’s site, and translate it to a real-world part number. Not only does this eat up time, but there is an opportunity for someone other than you to make a parts decision. Keep control over the design by providing real-world part numbers that do not require translation! Historic note – Part numbers ending with “-ND” do not have anything to do with Nu Disco. It used to mean “No Discount”.
Find numbers are useful for everyone. When issues come up, it is very expedient to refer to bill of material items by the find number. Think about it: “I would like to discuss Find #6” versus “Hey, the transistor on the ladder filter”. This helps expedite questions, and thus helps you get a faster and more accurate quote.
Do not put assembly notes on the bill of material (things like, “Place U2 upside down” or “Remove J1 Pin 4”). These should be communicated through a vehicle such as an assembly drawing or other PDF specification. Most companies make use of a database system called MRP (Material Resource Planning) or ERP (Enterprise Resource Planning) software. This is a big fancy database where every relevant number is stored. BOMs are often uploaded and converted to a format that is specific to a particular MRP system. Thus, it is possible that critical information can get stripped out.
Now we are in miscellaneous item territory. Remember contract manufacturers are not a one-person show. There are departments, such as quoting, engineering, manufacturing, and materials. All of these departments talk to each other but sometimes details can be missed. Given this, it’s important that you make certain that your requirements are easily available to everyone that needs to know them.
Assembly drawings are a good vehicle for all the miscellaneous requirements. If you can provide an assembly drawing, please provide, at a minimum, top and bottom side rendering with a full set of reference designators. Through-hole parts don’t always get mounted on the board’s top-side. If any through-hole parts are facing down, it is a good idea to note this. Special features such as manual wire modifications, heat sink assembly and so on should be noted on a drawing.
Examples of common requirements are lead free status, first article, pre-programming of ICs, packaging, shipping address information, etc. A simple PDF document is usually sufficient.
Can you get by with less? Yes, it happens all the time. About as often as a purchase order is cut for rework. Understanding the process from a contract manufacturer’s perspective will certainly help you get manufacturing completed faster, error free, and on budget.
Jason Duerr is director of engineering for Aimtron Corporation, a contract manufacturer in Chicago.