First, please confirm you are talking about a bare PCB and not an assembled PCBA.
By your reference to IPC 6012, I am assuming you are auditing a bare PCB manufacturer. PCB manufacturing is extremely complex. You really need someone with experience.
Below are some things that are easier for someone without experience to audit.
If the supplier has not already successfully made the boards, pay close attention to how they convert requirements into the manufacturing process. The biggest source of quality problems is part setup.
Inspection areas should have access to IPC-A-600, current version unless the procurement documentation specifies the use of an older revision in which case they may need the older revision. Use of a different revision in an area may be OK if the requirements that apply to a particular inspection have not changed. The recent revisions have often made no change to many sections and requirements, or the wording changed slightly without changing the actual requirement (or better pictures were substituted). If necessary, ask your supplier to justify the use of a different revision. For actual board inspection, IPC-A-600 is far more useful than 6012 or other 60xx series IPC documents.
Training: Are inspectors trained to IPC-A-600? Preferably inspectors and the quality department are IPC certified. Ideally anyone who handles boards is trained at least in the sections that apply to their work or what they may observe.
Process control is extremely important.
- Have they characterized processes so they understand the process limits?
- What lab capability do they have? (most tests should be performed in house)
- How do they track lab results? A typical plant tracks hundreds of different tests, so some sort of dedicated software should be used.
- How do they react to out of spec results?
- How do they track out of spec responses?
- Do they have action limits in addition to spec limits for many processes?
- Do they periodically verify the performance of important on-line sensors? (Due to the strong chemicals used, online sensors in many areas periodically fail.)
Note: If a process is well characterized, for a poor test result, a well run PCB plant may be able to continue running easy work while stopping difficult work. At the plant I worked at, this gave some customers fits even though we could find no change in yield when this was done.
If your product involves laser microvias, is the machine use one or two laser types? One laser machines are outdated. Two laser machines have one laser to remove metal and a different laser to remove laminate. Two laser machines are faster and allow more precise control because the laminate laser can be used for extra time to ensure no resin remains on the metal while with a one laser machine the copper will be damaged if the laser is run too long.
For drill and rout operations, high quality resharpened bits are acceptable for many purposes. Ask if they have limitations on when they use resharps. If no limits on resharps, that is a possible source of yield problems with some types of work.
Do the drill and rout machines have automated broken bit detectors?
Do the drill and rout machines verify the bit size before using the bit?
Inner and outerlayer imaging should be laser based for all newer work.
Cleanrooms: The following work should be done in cleanrooms
- Inner and outer layer imaging
- Inner and outer layer resist application
- Layup of inner layers and prepreg before press (not always done, but it increases yields)
- Soldermask imaging and preferably application (note this assumes the traditional apply to the entire surface, image, and develop method.) If the supplier has a direct large "inkjet" type that applies the soldermask only where it is needed, check what the machine requires.)
Assuming the end customer allows, are all PCBs coded with a unique serial number that also allows the position in the panel to be determined? Do processes have a standard orientation for panels? Knowing the position in the panel and the orientation in a machine makes narrowing down the source of a problem much easier in many cases.
Manual vs automatic plating lines is not significant. What matters is how well they are run. At the plant I worked in we had mostly automatic plating lines but the best performing line was one of the manual lines, despite this line running one of the more difficult plating processes.
Do they have one or more XRF machines for checking plating thickness? Not having at least one is a big red flag.
Do they have enough AOI capacity to AOI most signal layer inner and outer layers? For things not inspected with AOI, do they have clear criteria that seem to make sense? (Ground layers are often not AOI inspected, and sometimes signal layers with a large trace and space are not AOI checked.)
Legend marking should be san "inkjet" type machines for most work. Only non routine work, such as non-standard colors, should be screen printed.
Do they have enough electrical test capacity, and do they generally electrical check all boards?
Do they have the capability to properly dry pack bare PCBs in moisture barrier bags? Moisture barrier bags are preferable to baking at the assembly house because baking can have a negative effect of solderbility.
Do they outsource any o
