The Assembly House Blues: How to Head off a Common Assembly Issue

November 9, 2020 John Burkhert

It’s a hard road when you want to enable something cool. Keeping those assembly machines humming literally involves a lot of moving parts. When we look at a Bill of Materials (BOM) the parts are divided up into three categories; A-parts, B-parts and C-parts. If you sorted the BOM by cost, the items at the top are the A-parts. The dime-a-dozen bulk components are the C-parts and the B-parts are somewhere in between.

“Rework is lost money while shortages are lost time.”

Image Credit: Author. Odd shapes and unusual angles await the factory at every turn. The A-parts are pretty easy to spot here.

If any of those parts are not there, the show is over. Shortages are one of many ongoing challenges on the assembly line. Shortages are lost time. Let’s take a dive into the tactical issue - saving time and making money each and every quarter. 

The Life of a Component

Individual parts go into a shipping box and end up in the customers’ hands. The customer (you) will have a receiving doc that logs the shipment into the system and passes the parcel on to the Receiving Inspection department. This is where items are removed from the box and given a going over to guarantee that they meet all of the requirements set forth in the data sheet for the indicated dash option and revision.

 

assembly

Image Credit: Author. What’s going on around here?

If it is legit in terms of matching the paperwork, then it goes into the stock room. If it is not, the rejected item goes before the Material Review Board (MRB) where it will await further disposition by someone from Quality Engineering. The QE will give a final decision with the usual options: Use As Is (UAI), Return To Vendor, Rework or some other fate. Putting an A-Part into MRB should be done with discretion. Pretty much all PCBs will be on the A-list.

A Winnowing Process of Continuous Improvement

Some of the good parts, including boards, will be skimmed off for destructive testing. The Reliability Lab will be making cross sections and the like using all manner of destructive tools. What a fun job! The remaining good parts are counted into the inventory to become part of an assembly kit. Kitting involves getting the right number of each kind of part into a special anti-static tub so that it can be given to the assembly line.

Before getting around to kitting, the shortage report needs to be shortened. All of the day’s new parts will be automatically checked against any kits that are short on those parts. The chart-toppers on the list are hand carried to the line by the diligent stock clerk. Maintaining cycle counts on all of that inventory is another chance to handle the goods.

Eventually, a complete kit of verified components finds its way to the assembly floor. In real life, they may have to proceed with machine-assisted assembly with one component short. This is exactly what happened with some alarm boards that I needed. The main power diode was all the assembly line required to finish the boards.

3D PCB design

Image Credit: Author. A virtual birdseye view prior to construction.

This was a common component in the telecom business and elsewhere. We were building these seven foot tall racks that held shelves full of boards. The edge connectors on the cards would plug into massive 640-pin connectors that were press fit into extra thick backplanes. All of this circuitry and more were used to convert 24 analog calls (T1) into 458 digital calls.

My part of all of this product sat at the very top of the rack. It was a two rack-unit fuse/alarm panel. So it’s a big metal box with rack ears. The front panel had its row of fuses above a little panel with lights that indicated the state of operation. Different color LEDs displayed Minor, Major and Critical alarms. It’s the cherry on top.

“ We’re going to be shipping 350 racks this quarter”

All of this was wired together with a 140-point wiring harness. Of those 70 wires, there are eight that have to be soldered to the turret terminals of that board. There are two kits on the floor that are built except for the little boards that are sitting over in board assembly. We’re going to be shipping 350 racks this quarter. Pausing is not an option. Each mechanical assembler is expected to build two fuse panels per work-day.

Image Credit: Author. A different iteration of the same design.

So I have eight assemblers on the clock. After questioning the supervisor of the board stuffing group, I found out about the missing diodes. The Material Resource Planning (MRP) software shows that the 60-channel team has hundreds of them; enough to last them three quarters! So I went over to the planner for that group and asked her nicely if she could spare 80 big diodes. Presto. She wrote up a Material Transfer Ticket (MTT) to release them from her stock room and issue them to mine.

The Shoe Leather Network

I walked the parts over to our stock-room and asked if they could bring them into our inventory and fill the shortages on Carina’s assembly line. The guy looked at me like I was nuts. I’m doing something that an expediter is supposed to do. The thing I found is that the chemistry between the two planners wasn’t all that lovely. To me, all of that was beside the point. There’s a bigger picture.

I wasn’t going away so I got my way. The clerk recounted the parts and did his paperwork and data entry. The diodes were hand soldered into the alarm boards. The boards then went through a Quality Control (QC) inspection and were stamped off with zero defects. By ten in the morning, I had them in my cage for my team to install. We had the two lots of fuse panels that went into QC the following day. The carts full of fuse panels then went to Quality Assurance (QA) for electrical testing of a random sample. The size of the sample is based in part on the track-record for the sub-assembly and also the number of units submitted.

work from home office

Image Credit: Author. PCB Design can be a respectable career.

Then it was the phone company’s turn. The AT&T source inspector also has to buy off on the lot before it can go over to the full rack assembly team. We used four productive days to get to this point from the time the shortage was filled. If this big lot of goods passed the source inspection, we would be eligible for reduced levels of inspection. My Manager told me that we were the only ones who were not on tightened inspection. I had mixed feelings about that tidbit.

“The music of a factory has to be heard”

To make matters worse, my planner had to come along with the sad trombone. She’s informing me, with a rather icy tone, that we have a whole reel of 500 diodes in Receiving Inspection. Normal channels will get them to the stock-room in about three days. Inspection was a bottleneck in the supply chain but it was necessary.

Even then, the flow of material would have taken a full week before the completed alarm boards made their way to my team. Equipment racks standing around with nothing in the top rack space was not a good look. The music of a factory has to be heard. I have no time to worry about whose job it is or who gets the credit for doing it. Keep those machines humming!

Compliance with industry standards is a full-time job. Getting all of the material into the factory on schedule is a critical requirement. Hard stop. Keep the amount of inventory logged as Work in Progress (WIP) to a minimum. Maybe someday, I can tell you the story of how two of the fuse panels came up missing one morning. The teaching moment would be the value of the assembly traveler. Until then, just remember this: DFM really stands for Design For Money. Manufacturability and money are entirely interchangeable.

As you create a PCB design, it pays to understand the tactical elements of the supply chain. If we can combine two nearly equivalent line items such as a connector or a resistor, we streamline the whole process. A resistor with 1% tolerance can replace one with a 5% tolerance. A cap rated for 50V can replace a 35V cap. Streamlining the BOM streamlines the whole process. After all, there are only so many reels of components that can be loaded into a pick and place machine.

By bringing you a day in the life on an assembly line, the hope here is to make you aware of the high stakes involved in electronics assembly. In this case, a shortage from an up-stream business unit stands in for a vendor. Fill their shortage in order to fill your shortage. Facilitating your vendor so that they can better serve you is just one way that strong communication leads to a strong project development. The goal is to work within the system to achieve the desired outcome.

 

About the Author

John Burkhert

John Burkhert Jr is a career PCB Designer experienced in Military, Telecom, Consumer Hardware and lately, the Automotive industry. Originally, an RF specialist -- compelled to flip the bit now and then to fill the need for high-speed digital design. John enjoys playing bass and racing bikes when he's not writing about or performing PCB layout. You can find John on LinkedIn.

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