If you’re getting ready to put a new PCB design into motion, chances are this is the umpteenth “X Common Design Mistakes” or “Here’s the Right Way to…” article you’ve skimmed through. We understand.
What many of those articles don’t account for, however, are the issues that can show up only after the production process has begun. So, we’re addressing some lesser-mentioned mistakes that we see from a contract manufacturer’s point of view.
Implementing these layout design tips will help you get the most out of manufacturing your PCB.
Ok, this one is mentioned so often we’re not actually counting it on the list. That being said, it cannot be overstated.
By letting your contract manufacturer in on the design ASAP, they can point out issues that may affect the manufacturability of your product. All it takes is one slip-up to push your timetable way out or cost you a detrimental amount of dough in redesigns and supplies.
Manufacturers also know their capabilities and limits. Your blueprint should line up with their equipment in order to reach peak efficiency.
Collaborate early and you’ll have saved yourself and everyone you work with from risking a legendary headache of a situation.
Large, heavy ground planes act as heat sinks, sucking away the heat that should be flowing into your solder joint. The bigger the plane, the more heat is needed to achieve the melting point.
What makes this a big deal? Imagine a pan of cookies in the oven, but one side of the oven heats up faster than the other. You’re either going to end up with under-baked goo on one side, or hockey pucks on the other.
It’s the same with solder joints.
You may get perfect reflow on some joints while others can end up under-melted, leaving a bumpy finish and an unreliable circuit. Turn too far the other way, and you get extreme reactions that raise or fling components off the board, or you end up with exhausted flux that leaves potentially cold solder joints.
The way to negate this risk is to add thermal relief traces – or “spokes” (pictured above) – between the pads and the ground planes. This way, you keep the electrical connection intact while slowing the heat transfer from the solder joint to the ground plane.
Modern machinery is relying less and less on tooling pins for processing, but that doesn’t mean the need has been eliminated. Many machines currently in use still rely on tooling holes to help clamp a PCB panel in place.
Check with your manufacturer to see if adding tooling holes is a requirement for them. They can specify where the optimal hole placement will be, and what size to make them.
Most of today’s PCB designs incorporate surface mount components. In order for SMT pick-and-place machines to get the job done, they need fiducials.
Fiducials are vital in lining up the PCB with the software’s placement data. If a PCB has poorly positioned or missing fiducials, placement accuracy goes out the window. Some machines won’t even bother to try.
On the flip side, get the fiducials in the right spot and you’ll help your manufacturer nail down even the smallest of component positioning.
Add fiducials on both the PCB and the panel. This allows your manufacturer’s SMT team to optimize the project for the best balance of accuracy and speed.
If you want to get those fiducials just right (of course you do!), we’ll show you how right here.
A descriptive silkscreen legend is essential for getting things done right the first time.
First, make sure you get your reference designators in place. Get them as close to the corresponding components as you can.
We get it though. Sometimes you don’t have enough clear space near the component for the silkscreen. If you can, get the designator in a logical place where someone looking at the board for the first time could still figure out what it goes to.
Of course, if you’re so short on space you can’t fit any reference information at all, well, like we said. We get it. In that case, offer detailed close-up drawings that a production associate can easily follow. One way or another, make sure they can see what goes where.
Second, use the silkscreen to mark polarities. This one you should always have space for.
Most of the more complex component designs have a polarity indicated on the component itself. In production, the associate needs to match this mark to the mark on the board. When there’s no mark on the board, this launches an often-extensive research mission of digging through documents, then double-checking e-mails, then ultimately contacting you, the designer, to find out which way the part goes on.
That’s a lot of wasted time trying to find a dot.
Yep, a dot is all you need to add. Or a line. Or any shape that points out Pin 1 of the component.
You don’t need to be an artist. Just show the manufacturer where stuff goes. That’s the whole point of the silkscreen.
Surface mount components can shrink down to near-microscopic levels and still accomplish the same tasks, allowing for heavy population in small spaces. So why not design like the future is here? Good question.
Incredibly small components aren’t exactly easy to work with. True, SMT placement equipment technology is progressing, but not every manufacturer has the same capabilities.
More pressing, though, is the matter of rework, inspection, properly reflowing solder joints… All the work that happens after pick-and-place.
Smaller components are difficult to handle and require more fine-tuning of equipment, which in turn means more time and supplies go into getting the process right. If the manufacturer has to spend extra time, you’ll have to spend extra money.
Here’s how to keep your design cost-effective:
First, if you have unused real estate left on your board, consider spreading out clumps of components. This will allow for more even reflow temperatures across the assembly which means better, more uniform solder joints. It also means easier access for any handwork (rework, hand soldering and such).
Second, only use components as small as you need, not as small as you can. Larger components simplify machine placement accuracy, are easier to work with by hand, and can ultimately speed up the assembly process as a whole.
There are few moments as disappointing as sinking days, weeks, months into a design only to discover a vital piece is suddenly unavailable.
Don’t let this happen to you.
Make sure the components you choose are, if at all possible, manufactured by more than one company.
Be careful not to confuse manufacturers (who create the parts) with suppliers (who sell the parts.) Suppliers often buy from the same manufacturer, so what happens when that one manufacturer stops making the part you need?
A lot of activity happens near the edge of the PCB during assembly. Boards are handled by the edge during transport. The removal process of panel rails puts extra stress around the edge. Machinery grips boards at – you guess it – the edge. It can be a rough place for parts.
Any component that connects near the edge is more susceptible to stress and other dangerous forces. Something as general as being placed in a storage rack could fracture the component or tear it off of the board, requiring serious rework.
Small components such as chips are especially vulnerable to stress fracturing. If possible, try to keep them at least 2mm from the edge for safety.
Now, obviously this concept has exceptions. Some parts such as connectors are designed specifically to rest at the edge. While this isn’t a problem per se, make sure the part does not extend beyond the edge if it doesn’t need to. Otherwise, it can interfere with adjacent boards in a panel or get in the way of board separation equipment.
Oftentimes a design will call for a large surface mount pad to contain vias for thermal or electrical transfer. However, leaving those vias open can have a dangerous side-effect: Insufficient solder.
When a solder pad has open vias, the vias actually suck away some of the solder that was meant for the component. The intended solder joint thus becomes insufficient or remains unsoldered entirely.
The simple fix for this is to have the vias filled and plated. They’ll retain their function while keeping all the solder where it’s supposed to be.
As a side note, some designers place open vias on pads (as shown above) to intentionally reduce the solder level. When used correctly, this can actually aid the manufacturing process. Just be sure the vias are the right size for the job, or you may still end up with an empty solder joint.
Or, more to the point: Make sure your components are going to fit. For instance, make sure you’re buying 0603 chips for 0603 pads, not 0402 pads. Components have very specific footprints, and if even one ends up not matching, the delay in purchasing the right type could cost you a massive amount in assembly line shutdowns.
You’ll also want to make sure the pads and through-holes are not only spaced out correctly but sized correctly. The size of the connection point determines how much solder the component will have on it. Too large and the component might float around on the solder’s surface. Too little and the solder joints won’t form to acceptable standards.
This might be a more controversial argument as BGAs offer excellent functional and space-saving options. But from a manufacturing standpoint, BGAs can become a risky business.
Unlike most components, BGAs carry the assembly requirement of being pasted, placed and reflowed perfectly in one shot. These complex parts can have up to several hundred connections, and if even one fails, expensive and often lengthy rework follows.
BGAs are also shrinking in size as technology progresses. And as we talked earlier about the increased difficulty with smaller components, compounding the small component risk with the BGA risk means a greater challenge for the assembly team. This could come at a price.
In short: Use BGAs where needed, but minimize their use when you can.
Some design pitfalls are obvious at a glance, but others might not be clear until your design hits production. By adding these 10 checks to your design process and communicating with your manufacturer right away, you can clear these hurdles up front and be well on your way to a faster, cheaper and smoother production run.
Have you encountered PCB layout factors not listed here that tripped up the manufacturing process? Or maybe you’ve found new solutions to the issues mentioned here? Help others avoid a costly situation by sharing your discoveries in the comments below!