Seven Things Your Assembly House Needs to Know

The PCB assembly drawing is more than a diagram of where things go. Its main reason for being is as an inspection document. As the board outline shrinks while the functionality and connectivity increase, the silkscreen gets crowded out of the picture. This makes the assembly drawing more relevant to people doing troubleshooting and/or rework.

The initial assembly process has become more of a machine operation. An X-Y coordinate file drives location and orientation. A Bill of Materials (BOM) gives the specific quantities and descriptions for the hardware and electrical components. A “pick and place” robot follows the CAD data to place the components. An Automatic Optical Inspection (AOI) machine looks for common defects. Functional testing, boundary scan, JTAG, In-Circuit-Test (ICT) or a combination of them decide if the device is good to go. None of those require a picture. What, then, is the use of the archaic assembly drawing?

The answer is in the assembly notes. Last week, we looked at the fab notes so this is an extension of that effort. Probably not the first time I’ve mentioned that the Assembly Drawing are there to describe “what is” rather than”how to”.

Go down that rabbit hole; you and Elon will both be in “Production Hell.”

Well meaning Manufacturing Engineers would have us detailing the type of solder tip to use, the IR oven profile, the minimum spacing or maximum height or any number of process parameters. They would like to see a single document that covers the assembly steps from end to end. The assembler has everything they need right there on one “B” sized document. I get that.

Going down this road will mean that the assembly drawing will be forever under construction in order to keep up with the latest instructions. We have IPC Standards, Workmanship Manuals, Outline Drawings, Interface Control Drawings and a load of process docs tailored to each function. And, if we don’t, we need to consider the whole package in those terms. The part played by the Assembly drawing is to compare it against the actual assembly in order to determine if the part is accepted or rejected. That’s it.

So, let’s get to some sample NOTES UNLESS OTHERWISE SPECIFIED.

1. INTERPRET DRAWING IN ACCORDANCE WITH ASME Y14.5M 1994.
2. WORKMANSHIP SHALL BE IN ACCORDANCE WITH THE CURRENT REVISION OF J-STD-100 AND IPC-HDBK-001
3. INSPECTION SHALL BE IN ACCORDANCE WITH THE CURRENT REVISION OF IPC-A-610, CLASS 2.
4. THE CONTROLLING DOCUMENT FOR COMPONENT PLACEMENT SHALL BE XY-nnn-nnnn-nn.
5. MARK DASH AND REVISION LEVEL IN PERMANENT BLACK INK AS SHOWN.
6. AFFIX BARCODE, DATE CODE AND SERIAL NUMBER LABELS ON SECONDARY SIDE AS SHOWN. BAG AND TAG ACCEPTABLE FOR ASSEMBLIES THAT ARE TOO SMALL FOR LABELS.
7. YourCo REFERENCE DOCUMENTS AS FOLLOWS:

• BILL OF MATERIAL nnn-nnnn-nn
• PLACEMENT FILE nn-nnnn-nn
• PASTE STENCIL nnn-nnnn-nn
• SCHEMATIC nnn-nnnn-nn
• FPGA PROGRAMMING nnn-nnnn-nn
• TEST PROCEDURE nnn-nnnn-nn
• Whatever else is relevant and not documented elsewhere.

So, what’s missing? I just wrote them on the spot so they’ve never been on a real drawing or in front of vendors. Whether you call out the specific revision of each referenced doc can make things clearer at the risk of getting stale. If you use them, you’ll eventually find out what else your shop or shops want. Who develops the stencil thickness or the array drawing? That’s where you can give it a personal touch. …or dive off the deep end. That collaboration should track ahead of fabrication, while you’re early in the design.

Cleanliness or no-clean should be covered in the workmanship manual. Solder paste composition is part of the soldering process document. The characteristics of the solder joints are part of IPC-A-610. From that one document, the entire body of IPC knowledge is linked by references from one document to another. I’ve never traced it out but it sure seems that way.

First article inspection, sampling plans, Certificate of Compliance notes are common here but really belong to the Purchase Order. These are tactical and dynamic attributes that do not want to be set in the stone-work of the Assembly Drawing. You want the ability to relax or tighten them based on the vendor’s track record.

Lead-free, RoHS compliance and other environmental aspects are part of the contract. Final packaging, ESD handling, or any other concern that can be handled by a universal document should be done that way. Testing is its own world. While we should have a test-plan for any new product introduction, it’s well beyond the scope of assembly notes.

The takeaway is that fewer is better. My litmus test is that unless you can look at the board and see that the requirement was met, it probably doesn’t belong on the Assembly Drawing. There’s always an exception for unusual processes but that’s what word-docs are for. Go down that rabbit hole and both you and Elon will be in “Production Hell.”

So that’s it, lesson over. If you want to know why I feel this way, here’s the War Story.

It was 1992, maybe ’93. The job was Operation MILStar. The objective was winning the sub-contract for an RF amplifier about the size of a microwave oven. (they were bigger back then, you know, so was everything else) The crucial step at that point in time was to pass an audit. A squad of ham-fisted Army Sergeants descended on the Mountain View headquarters of Microwave Modules and Devices Inc.

Ole Sergeant Ham-Fist was measuring the height of the text and the space between on the drawing. It’s an eighth of an inch minimum for C size, a hundred and forty thousandths for D and above. Then he actually started reading it. Huston!!! Are you there? We’ve discovered workmanship instructions on the Assembly drawing!

So in the final analysis, we passed the audit with two dings on my entire package of drawings. One was that we called the hardware thread sizes by commercial names, ie 4–40 UNC 2-B Thru and Sgt. Fist wanted to see 0.112–40 UNC 2-B Thru where the .112 drill/shaft diameter.

AND THEN, my Manager asked me, “John, What are ‘methodized’ drawings”? And I said, “Merrill, (his name was Merrill but you guess that much) so yeah, Merrill, you know those extra notes that Manufacturing Engineering keeps asking for and I keep advising against? Yeah, those. Those turn a drawing into a process document.” I guess, we have to stop doing that.