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Match Your High-Tech PCB Design To Your Suppliers Capabilities: Q&A

Al Wright
Written by Al Wright
Posted on July 30, 2019 at 2:04 PM

At the conclusion of our recent webinar – Match Your High-Tech PCB Design To Your Supplies Capabilities – we had a number of questions for our presenter, PCB Field Applications Engineer Al Wright. We decided to compile these into a readable format on our blog.

Q&A From Our Live PCB Webinar

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Question: Are you capable of metal clad PCB?

Answer: Yes, we offer metal clad PCB, particularly for LED applications. Read more about it in our Metal Core PCB Vs. Standard Circuit Boards blog post.

Question: Can you please quickly go over your cost model? Do you charge per square foot of the panel or the board itself? Do you charge for the unused area, too?

Answer: We charge by area of the PCB, and do not include unused areas of the raw material sheet. The only exception is with our extreme plating product (10-oz copper and above.) Because of the long cycle time required for extreme plating, a printed circuit board which yields very poorly from the sheet of raw material has a large negative impact on throughput in the plating and image departments. For this one category of work, we may make a pricing adjustment to account for the extra time per panel.

Question: How do I ensure working with a particular vendor on design rules, materials, etc.? Doesn't it cause issues when purchasing wants to have multiple PCB manufacturers?

Answer: Agreed. Multiple sources may have different capabilities, depending on their equipment and controls. It can be tricky to manage. A lot of design rules are common around the industry, but as some companies improve their capabilities with processes such as laser direct imaging, there can be significant differences in capability between suppliers at some points in the process.

One strategy we’ve used when working with our own suppliers on design rules is to send the rules we typically use for fabrication to the prospective partner in the form of a survey. We ask them to review our rules and to note any parameters they may have trouble meeting.

If they are generally as capable as our existing sources, but need relief in a couple of areas, we consider whether we may be able to adjust our rules to the wider value without creating significant negative consequences. If we decide that it’s OK to go wider, then it won’t bother the existing source – they can already do better anyway, so a more forgiving value is just a wider processing window for them.

Materials are another issue. I have always favored the use of generic industry material callouts, as opposed to specific material products, whenever possible. Calling out material by the IPC spec designation (“material in accordance with IPC-4101/126”) leaves it to the vendor to select an appropriate product. There is some detail around this topic in one of my 2017 webinars (slides 3 through 6). You can view those on YouTube. The relevant slides occur between 2:00 and 6:00 of the presentation.

If it is critical that all boards from every supplier use exactly the same material products, then you may need to survey a handful of companies on their material preferences as a first step. Weed out the ones who don’t or won’t use what you need. Then, from the ones who do use the right materials, see whose design rules track most closely with your preferences. That should be a good fit.

Question: With RF traces, would you suggest allowing a supplier to adjust traces to meet impedance requirements or redesign the stackup so the traces won't need adjusting?

Answer: I generally agree. I don’t like to change a customer’s metal features on RF work if possible. With FR4, where there are many types of prepreg and many core thicknesses available, it is usually possible to change the dielectrics to something that will work. If you’re using higher frequency materials (Rogers, Taconic, etc.) then most of the time the strategy is to select an available core material at the layout stage and design the trace width according to the distance across that core to the reference plane. It’s common on such designs to have the trace width tolerance after etching called out as a drawing note.

If you’re not doing so already, it is useful to define what adjustments are and are not permitted, within your documentation package. We have a few customers who do this, and we find it useful to have a clear guideline. It also gives you irrefutable recourse if somebody ignores it and makes unauthorized adjustments that result in a board that doesn’t function.

The statement usually goes more or less like this: “Fabricator may adjust dielectric material thickness to meet impedance target. If impedance target cannot be met by dielectric adjustment alone, then trace width may be adjusted by +/-0.001” or +/-10%, whichever is smaller. If this amount of adjustment still does not guarantee correct impedance, then contact (your name here) before proceeding.”

You can, of course, modify the parameters in that statement to fit your needs. You can also add a “not less than ____ thick” to the dielectric adjustment part of the statement, to be sure that the adjusted thickness does not violate your minimum insulation requirements.

Whichever type of adjustment is used, it’s also not a bad idea to ask the fabricator to notify you of any adjustments they make, so that if you ever need to use a second source, you’ll be aware of exactly what’s been done in the past.

Question: I’ve always tried to use material my board house stocks, but as a contract designer this is not always possible. How much extra time does it take to buy material that is not part of the board house’s standard inventory?

Answer: That depends on what the material happens to be. For FR4 products by other manufacturers, it should not be too bad if the board house has a distributor who can get it. For other materials like high frequency products, time can range from days to weeks. For some relatively exotic types, sometimes the manufacturer will not build a batch until there is sufficient demand to justify the build.

Question: I’m starting to get into more complex designs and considering switching to a higher end layout software. Is there a program you recommend that handles blind drilling, or vias-in-pads, particularly well?

Answer: I’m not a PCB designer, so I will tread lightly here. I will say that we’ve seen a significant migration towards Cadence Allegro over the past several years. Anecdotally, most of the higher end work we see that involves stacked vias and HDI is generated in either Cadence Allegro or Altium Designer.

Question: If I’m looking for a quick-turn prototype of a very dense design, and if I use materials known to be stocked, will a denser design necessarily take longer to manufacture than a board that follows standard rules?

Answer: Not necessarily. If your design is just tighter and finer than normal, but you have followed the advanced design rules and have avoided using some features that require extra processing, then all other factors being equal, lead times should be pretty similar. It depends upon factors like: does it require multiple laminations, via-in-pad, back drilling, possible outside processing such as ENEPIG? Those processes do add time. If the board is a little bit finer, as long as the supplier has the right copper weights in stock on their core materials, you should be able to get a reasonably quick turn on a dense board.

Question: I have designed boards following what I thought were my board house’s published design rules, but sometimes have delays anyway. Why would that be?

Answer: Sometimes it comes down to design rule check (DRC) vs. design for manufacturability (DFM). DRC checks are the automated checks that catch most run-of-the-mill errors like spacing violations, missing mask openings, etc. Even when you follow the design rules, other items sometimes won’t be caught without human interaction – someone experienced noticing that something just does not look right. An automated DRC won’t usually catch disagreements between the Gerber files and the supplied documentation. Sometimes generic fabrication notes are flown into a drawing and don’t match the design. Other times drill tables don’t include plating or tolerance information. Older drawings can include callouts for obsolete processes or materials. It is the fabricator’s responsibility to notify the customer whenever such discrepancies come up.

Question: One board house’s site specifically warned not to design a board solely according to the design rules on their Web page. Isn’t that why the rules are listed in the first place?

Answer: You can’t necessarily have everything. Some design rules are interactive. We’ve seen many instances of designers making an honest effort to design to our rules, but maybe overlooking the importance of how copper weight changes the rules. A 0.5mm surface mount component, which is hardly bleeding-edge by today’s standards, will process with one set of rules on H-oz copper, but at 2 or 3 ounces the etching resolution and the solder mask adhesion begin to suffer, so the rules need to be opened up to compensate.

Question: I have looked on several websites, and one thing I don’t always see is their capabilities around micro BGAs. Isn’t that something where there should be listed rules?

Answer: One might think so. Bear in mind that Web page information is not always up to the minute, and as capabilities move ahead, Web information may lag a bit. That is one reason we suggest calling when you get into a high-end design cycle. Another reason is that not every situation is cookie cutter, especially when things get quite dense. It may be that there are multiple ways to skin the cat, and 15 minutes discussing what those are may save hours of misspent time.

Question: Altium Designer offers outstanding support. How do you feel about Altium Designer?

Answer: Because I am not a designer myself, I do not feel qualified to express informed opinions about the features and usability of various PCB layout platforms. I leave that to those, such as yourself, who are “driving” every day. I do, however, feel that Allegro and Altium Designer export files are the best we see in terms of Gerber and other fabrication data when buried and blind via processing is required. Also, I find that about two thirds of the selective copper work we quote or build is done in Altium.

Designers I have talked to who have used both systems have reported that those buried/blind features within the Altium program are relatively easier to master than the similar features in Allegro. In my own experience, dealing with the outputs, which become my inputs for manufacturing, I find some of the Allegro layer naming and drill report text files are not the easiest to read through when setting up a job, whereas Altium reports are more straightforward.

Question: How do I explain to a manufacturer that I need a semi flex PCB?

Answer: I believe what you may be looking for is a board that will bend somewhat in order that it can be installed, but that will not require the ability to bend back and forth many cycles after installation.

The industry term for such a board is “Flex-To-Install.” Unlike thinner, more flexible PCBs, which must be made very thin so the circuitry will not be damaged by high numbers of flex cycles, flex-to-install can be built somewhat thicker and may use slightly heavier copper. If you have ever worked with, for example, the lead sheet material that is used to flash the area between the bottom of a chimney and the shingles of a roof, that is a useful comparison.

Flex-to-install is still a flex circuit, manufactured by a flex circuit specialist manufacturer and from the same materials (typically flexible polyimide) as dynamic flex. There is another type, called “Rigid-Flex.” This is a multilayer structure with a cross section of rigid external layers (for relatively complex assembly and circuit routing) plus flexible polyimide between. It is like a PCB with wires attached but comes all as one unit for convenience.

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Topics: Printed Circuit Boards, Product Design

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