Common PCB Design Decisions that Cause Manufacturing Problems

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Common PCB Design Decisions that Cause Manufacturing Problems

What are the most common design decisions that can have a negative impact on the PCB assembly manufacture and how could we correct them during the design? The Assembly Brothers, Phil Zarrow and Jim Hall, discuss this scenario and share their suggestions.
Board Talk

Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes

Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.

Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript

Phil
And welcome to Board Talk with Jim Hall and Phil Zarrow, The Assembly Brothers, who by day go as ITM Consulting. We are here to answer your questions on assembly processes, methodologies, equipment, materials, and all the good stuff.

Today, we have a question from G.B. What are the most common design decisions that can have a negative impact on the PCB assembly manufacture, and how could we correct them during the design? Jim, I don’t know where to begin. We could do a whole workshop on this. In fact, I believe we have. Since we are limited to a five-minute sound bite here, let’s talk about the top two. How does that sound, Jim?

Jim
And far away is the spacing of the SMD components on the board relative to through-hole joints. You want to have as much clearance space between any surface-mounted part and any through-hole joint to make the through-hole soldering as less problematic as possible. Unfortunately, on some boards, surface mount parts get really close to through-hole joints and it becomes more and more difficult to do the through-hole soldering.

Where you are not just heating the board up like you are with reflow, but you are having to add solder and flux and heat. Now, obviously, if you can do pin-in-paste and you do the whole thing with reflow, then it goes away. There are some issues there too, but most of us don’t. Most of us are going to have to do some separate through-hole soldering step.

They get too close immediately; you have to eliminate wave soldering. Now, you are moving to selective soldering. And even there, you can only make the nozzle so small on a selective soldering machine. As surface mount parts get closer and closer to the through-hole lead, it just gets more and more problematic. Even good hand soldering, with a good skilled operator, can be difficult if the location of the surface mount parts is too close to the through-holes. So that is my number one Phil.

Phil
I definitely concur with you. I think that is definitely one of the big nasties. I would say second to that, the next thing that comes up is making sure the design uses the correct pad size and shape. As we all know, if you deviate from those, there are a lot of other things that could go wrong. The design probably has a major contribution to the tombstone defect that we see.

There are a lot of other things. By misdoing that, we could have components shifting, which you might make electrical contact, but you may not have optimum physical contact with a component. The same thing with when using solder mask-defined pads versus non-solder mask-defined pads with regard to the pitch of the components you are using.

There is a lot. They should really be doing their due diligence. Unfortunately, you have to keep on them with regard to the actual pad size and shapes. Again, like we said, there are a whole lot of other things that designers do to screw us up. As we like to say, forgive them, Father they know not what they are doing.

They know, but the fact is that most designers are not exposed to the assembly area, what we are trying to do every day. Things used to be a lot more tolerant back in the through-hole days, but with surface mount, all bets are off. I am happy to see that from what we have seen, most of the CAD programs are getting better and better in bridging the gap, but it still isn’t there.

In future episodes, we will talk about some of the other problems. Things we like to see and do, like please pick the lowest MSD level of components. There are a whole number of things. Watching the component count. Most designers are not familiar with how many feeders you can have on a given pass. I think that is it, Jim. Anything you want to add to that?

Jim
No, I think you have covered it all, Phil.

Phil
Well good, you have been listening to Jim Hall and Phil Zarrow, The Assembly Brothers, who by day go as ITM Consulting. Whatever that designer has done to you, please don’t solder that board like my brother does.

Jim
Don’t solder like my brother, either.

Comments

Your summary is only a good beginning point. There are literally thousands more examples. I do not have enough time to list them all, but it would be great if everybody could chime in and respond. Here is one example: Radial PTH relays with glass seals on the leads.

The companies I work for all have design rules that specify this type of component must be purchased with lead lengths that do not require trimming, or an exemption is placed on the drawing to allow them to protrude more than the J-STD-001 maximum length of .060" provided there is no fit issue. This is because the shock of trimming these leads causes cracked glass seals even if trimmed after soldering and the kinetic shock also is known to damage the delicate and very tiny "teeter-totter" copper reed contacts that are just on the other side of the lead entrance inside the relays.

Cracked glass seals are very difficult to detect using visual inspection after trimming at the component level, can easily propagate, cannot be inspected after assembly of the relay to the board, and can cause latent relay failures due to moisture ingression which will oxidize the bare copper reeds or their fulcrums. The latent failures can be catastrophic if used in high-rel applications such as avionics, etc.

Quite often a design "newbie" will specify a relay with leads long enough to stick through the PWB, and will depend on the manufacturing engineer to come up with a trimming process to ensure the leads do not violate the protrusion limit, whether it be the .060" standard or something less as specified on the assembly drawings. The Manufacturing engineer may also be ignorant of this issue and be trimming away, even with no subsequent inspection of the glass seals. This, like many, many other issues is not captured in the J-STD-001 but the cracked glass seals ARE defined and there are acceptance criteria for them in MIL-STD-883 Cracked Glass Seal Criteria, along with excellent failure analysis examples.

Along with damage to the reeds, contacts, and glass seals, there is a very real possibility of cracked solder joints, especially in lead-free solder which is much more brittle. There are many known catastrophic failures that have occurred from the phenomena of the kinetic energy imparted by the shock of trimming, even when using angled shear plates, special hand trimmers with an "overbite" design, etc.

Not to mention the added cost of the trimming itself and the cost of inspection after trimming, all of which can be avoided by doing a good job of selecting components with standard lead lengths that do not require trimming in the first place.

Richard Stadem, General Dynamics Mission Systems