Many customers of PCB Trace Technologies Inc., especially those who are new entrants in the field of electronics and printed circuit boards, want to know how to certify acceptance of PCBs they have received. With advances in technology, printed circuit board designs are increasingly more complicated involving intricate footprints and fine-pitch components. This makes incoming inspection of printed circuit boards an important factor. An optimal inspection process for printed circuit boards must include:
- Preliminary Inspection
- Reviewing incoming paperwork
- Inspecting cosmetic aspects
- Dimensional Checks
- Assessing the final documentation from the supplier
- Advanced Board Tests
- Electrical tests
- Solder samples
- Micro-Section analysis
- Impedance values
Preliminary Inspection
Reviewing Incoming Paperwork
Primary aim of reviewing incoming paperwork is to assess if the PCBs received are the ones ordered. The product must match the contents of the online order receipt. The documentation must be clear on what the customer should expect.
Inspecting Cosmetic Aspects
Once the customer has determined that the PCBs received are indeed the ones they had ordered, they must inspect each PCB for its cosmetic aspects. These must include:
- Packing quality
- Identification and traceability code
- Presence of surface damages
- Presence of solder mask
- Presence of surface finish
- Legend or silk screen quality
Packing Quality
The packing must provide a high-quality sealing against moisture and dust ingress. The sealing must be adequate for storing the PCBs for at least six months if not opened.
Identification and Traceability Code
Each packet must contain at least the following information for identification and traceability:
- Lot number
- Date of manufacture
- Number of PCBs in the packet (each packet must have an identical number of boards)
- Any other information the customer may have required printed on the packet
Presence of Surface Damages
If everything is acceptable so far, the customer may randomly open a few packets to examine individual boards. This may be a statistical sampling based on the total number of boards received by the customer. The visual inspection may involve a hand held or table-top magnifier. The inspector must look for:
Any type of scratches on the surface—the PCB surface must be free of scratches of any kind.
Extra material on the surface—there must be no dust or chemicals present on the surface of the PCB.
PCB breakage—the PCB must be intact and without cracks and breakages.
Corner integrity—there must be no damage to any corner of the PCB.
Presence of Solder Mask
The supplier may have masked the board on one side or both sides depending on the design. Solder masks are available in different colors like green, red, blue, black, and white. The inspector must confirm whether the mask is glossy or matte finish according to the online order receipts.
Some applications require application of two coats of solder mask on the PCB. This may be difficult to confirm immediately, and may require magnification. Some customers require a solder mask certification stating the type of mask, its lot code, and date code.
Using magnification also clarifies whether there is registration issue with the solder mask application. Often presence of two coats of solder mask may not cause an issue, unless the board as components with critical height considerations. Two coats of solder mask may make the board surface too thick for critical components, causing assembly headaches.
Presence of Surface Finish
Surface finish attributes are easy to see and assess. It is easy to see the type of surface finish the supplier has applied and whether the surface finish is HASL, ENIG, immersion gold, immersion silver, lead free HASL, hard gold, or OSP. The finish should match the order in type and thickness.
Legend or Silk Screen Quality
This is another easy to see and assess attribute. The inspector must check the documentation for the color of the legend the supplier has offered on the board. In addition, the legend must be legible, crisp, and clear.
Dimensional Checks
The board received must match with the fabrications drawings the customer had supplied. If dimensional tolerances are not present in the fabrication drawings, the supplier can default to common industry standards of ±0.005 inches (0.13 mm) for routing and ±0.003 inches (0.08 mm) for plated holes.
The inspector must inspect the product received against the supplied fabrication drawing for all physical dimensions including length, width, overall thickness, and all cutouts. The drawing must define each dimension along with its tolerance and the product must meet this. Checking with a pair of calipers or a micrometer is essential.
The inspector must check the PCB against the drill drawing for all hole sizes and locations, whether they need a plating or not, and their tolerance. The drawing must show different symbols designated for each hole size. Checking the hole sizes with a set of pin gauges helps the incoming inspection process.
Final Documentation from the Supplier
The inspector must ascertain the received items match the order. This must not include anything close to or similar to the order. They must retain a copy of the online order receipt and verify the items against it. A clear documentation from the customer is a great help to those inspecting incoming material. A proper set of tools with the inspector is also a must.
Advanced Inspection
Electrical Tests
The preliminary inspection verifies most of the external aspects of the printed circuit board, but cannot verify the quality of the inner layers. One way to check the integrity of the inner layers quickly is with an electrical test. Although this may not test the inner tracks in total, the electrical continuity and shorts test can weed out many faults.
An electrical test report must accompany each board, and the board must carry an ET mark to denote it has undergone electrical test.
Solder Samples
Customers can ask for inclusion of a solder sample along with the documentation. This is generally a 2-inch x 2-inch board section that the fabricator has solder analyzed. Solder wetting on the sample must be proper—solder must have covered all pads smoothly and there must not be any surface finish left exposed.
Micro-Section Analysis
Customers can also ask for a micro-section along with the documentation. But this analysis requires a microscope. An analysis of the micro-section using a microscope allows determination of the copper weights of the inner layers of a multi-layered PCB. The inspector will notice a difference in thickness of different sections when the outer layer is 1½ oz finished copper while the inner layers are ½ oz copper.
Impedance Values
The fabrication drawings may call out impedance values. This makes it very important to verify the copper weights and the stack-up of the layers. If the customer is using many PCBs with impedance control, then it is necessary to identify a process for verifying the stack-up the fabricator is using. The customer can also ask the fabricator to supply impedance testing reports.
Conclusion
The purpose of the incoming inspection process for PCBs is to assure a smooth functioning of the subsequent assembly and soldering processes. PCB Trace Technologies Inc. suggests the customer acquire a copy of the IPC-A-600 acceptability standard for printed circuit boards. Packed with photographic examples of non-conforming and acceptable attributes, this standard is invaluable when conducting incoming inspections.
