Learning objectives
- Explain why design for manufacturability review happens before any board is loaded on the line
- Verify that the bill of materials revision, the assembly drawing and the bare board revision all agree
- Check centroid data for rotation and polarity errors before programs are released
- List the machine and inspection programs that are generated offline before production starts
Lesson
### Lesson 1: The DFM review
Every good build is decided before the first board reaches the printer. In pre production, engineering works from the design data package: the Gerber or ODB++ files that describe the board, the bill of materials that lists every part, the assembly drawing that shows where parts sit and which way they face, and the centroid file that gives the placement coordinates and rotations. The design for manufacturability review, called DFM, tests whether this design can actually be built well. Reviewers check that the footprints on the board match the real component packages in the bill of materials, that large pads and ground planes have proper thermal relief so joints can wet and flow, and that fiducial marks are present and placed so the vision systems on the printer, the placement machines and the inspection systems can all find them. Panelisation is part of the same review. Boards are grouped into panels with rails and break off tabs so they can travel through the line, and the reviewers confirm that rails and tabs do not clash with components and that the panel is strong enough to stay flat. Test access is checked here too. If the design carries no test points, no later stage can fully test the board, and that failure is designed in, not built in.
### Lesson 2: Documentation control and offline programming
The second half of pre production is programming and paperwork discipline. The stencil is ordered from the same design data, and programs for the printer, the solder paste inspection system, the placement machines, the optical inspection system and the electrical test equipment are generated offline from the CAD data, so expensive machines are never standing idle while someone types in coordinates. The classic disaster of this stage is a revision mismatch. The bill of materials says one revision, the board in the rack is another, and the line builds the wrong product perfectly, hundreds of times. A single wrong rotation value in the centroid data does the same damage in miniature, placing one part turned around on every single board. In his book on lean efficiency in PCB manufacturing, James Onsomu teaches that design decisions dictate manufacturability and assembly efficiency, and that understanding the design is what makes the move from design to production seamless. That is the spirit of this stage. Nothing proceeds on memory or assumption. Revisions are compared in writing, polarity marks are checked against the drawing, and the released data pack travels with the job through every later stage.
Key numbers
- Revision agreement: bill of materials, assembly drawing and board revision must match exactly, with zero mismatches released to the line
- Fiducials: required on the board or panel so printer, placement and inspection vision systems can align
- Test access: every circuit node the test plan must reach needs a designed test point
- Panel design: rails and break off tabs must clear every component on the panel
Defects to spot
- Wrong revision built: revision mismatch between documents and board missed at review
- Rotated components on every board: wrong rotation values in the centroid data
- Untestable board: no test points designed in for electrical test
- Component clashes at the panel edge: panel rails or break off tabs placed against tall parts
Stage check, self test
Answer each question, then open the answer to check yourself. Passing standard: 80 percent, at least 4 of 5 correct.
1. What is the main purpose of the DFM review?
A. To speed up the placement machines
B. To confirm the design can be built well before production starts
C. To inspect finished boards
D. To train new operators
Show answer
Answer: B
2. The line builds the wrong product perfectly, hundreds of times. What is the most likely cause?
A. A worn squeegee blade
B. A revision mismatch between the bill of materials and the board
C. A slow conveyor
D. A dirty stencil
Show answer
Answer: B
3. One component is placed turned around on every board. Which data should be checked first?
A. The paste expiration date
B. The oven profile
C. The centroid rotation data
D. The packing list
Show answer
Answer: C
4. Why must test points be considered during pre production?
A. They make the board look professional
B. They reduce the weight of the panel
C. They help the stencil align
D. A board with no test points cannot be fully tested later
Show answer
Answer: D
5. What is the correct practice for machine programs at this stage?
A. Generate them offline from CAD data so machines are not idle during programming
B. Type coordinates in by hand at the machine
C. Copy the program from the previous job without checking
D. Skip programming checks when the job is urgent
Show answer
Answer: A
Next: Stage 1: Incoming inspection, storage, ESD and moisture control