First Article Inspection: Understanding the Problems

Estimated reading time: 11 minutes

In these days of lean manufacturing in a high-mix/low-volume (HMLV) environment, it is becoming increasingly important that all efficiency potentials are explored. One area that warrants investigation in the SMT realm is that of first article inspection procedures. This article discusses the benefits of an automated first article inspection system in the setup of a pick-and-place machine.

By Greg Ross, Cluso Vision Systems

In electronics manufacturing, the first article is built by SMT lines and sent to the inspection department to check the product assembled against its bills of materials (BOM), ensuring that the programs contained in the pick-and-place machine are correct, and that parts are placed into the feeders in the correct locations. Common ways of performing this task are riddled with potential quality problems and are a perennial waste of valuable manufacturing resources within the SMT assembly process. It is not uncommon for contract manufacturers (CMs) to changeover their machinery 3-4 times a week to keep up with the growing range of PCBs required to manufacture in small volumes. Pressure to be accurate and quick while getting SMT lines up and running after change overs means taking a risk, and not fully inspecting all parts on the PCB.

Figure 1. Cost of manual vs. automated first article inspection processes.

Improper setups cause a great deal of rework due to incorrectly loaded, reversed, or skewed parts after reflow. Because most loaders do not test parts, there is a possibility of loading the wrong parts - just as quickly as the correct ones. To assess all of the risks associated with the methodology of the first article inspection process, it is important to examine the way in which documentation is produced and presented.

Customer documentation would include, but is not limited to:

• BOM;
• Loading diagram (reference designators included);
• PCB files and/or Gerbers used to generate a pick-and-place program and solder paste stencil.

A CM’s production engineering department would input the BOM into the MRP system, and place additional documents on file for use within the manufacturing area. They would ensure that a stencil is ordered and would convert the BOM and PCB files into the computer-aided design (CAD) files required for the SMT loaders. The program that designates which feeder part is required, as well as its location on the PCB (X/Y/Z), is constructed. Normally, this is done automatically using the CAD data supplied from the customer or engineering departments. Generally referred to as line-control or line-balancing software, this would also split the data for use on several machines in a SMT line, if required. Its main function is to ensure that the parts-loading time for each machine in a SMT line is equal so that the line runs at maximum efficiency.

If changes have been made to the parts on the PCBs that are not reflected in the CAD data, then the BOM must show current part numbers associated with the particular reference designators. The CAD data is modified either on the machine itself - if it is a single machine and not in a line - in the source pick-and-place file, or within the third-party conversion or line-control software. Full feeders are then loaded onto pick-and-place machines in designated, pre-established slots.

Once this is complete, the first PCB (first article) is screen-printed and populated. Prior to reflow, operators inspect the PCB. This minor inspection is limited to ICs and polarized parts, and checks for correct values and parts direction for expensive or difficult-to-rework parts. The PCB is then reflowed.

This first article is then handed to the inspection department. To perform a full inspection, quality assurance (QA) is required to search the PCB using the BOM with reference to the loading diagram. This occurs because most PCBs do not have reference designators marked on them. Each part is inspected to ensure correct setup of the pick-and-place machine. The QA inspector must cross off the parts one-by-one until all parts are found and inspected. The pick-and-place machine program also is crossed off to ensure that parts not contained in the BOM are not loaded. This process can take up to three hours to completely inspect all of the parts on a PCB containing 300-400 SMT components. This may be too long to allow the pick-and-place machines to lay idle, so they are started after major parts are inspected. It is very difficult to locate all of the parts on the PCB using the loading diagram, so this is limited to most of the different part values. If almost all of one part type is located, it is assumed that this part will be loaded correctly. But this is risky.

There also are specific risks. There are several manually driven areas that can introduce massive errors. These processes can be so refined with automation that they can lull the operators into a false sense of security - causing major errors at random times. An analysis of the flow of information is required to understand the potential for error.

Customer CAD Data

This data is the actual PCB data file, which is the same file used to manufacture the raw PCB and generate Gerbers files used in the manufacture of the solder paste stencil. Because board boards manufacture does not depend on the accuracy of the values and part numbers of parts loaded on the PCB, BOM information is not up-to-date or contained in this file. Most engineering departments that design PCBs would control the BOM information on a separate database or spreadsheet. This information must be merged either manually or by third-party conversion software. In this instance, the resulting information has potential for error, and must be checked manually by the CM to ensure its accuracy.

Third-party conversion software is used to create the actual CAD and feeder data for SMT loaders. It can be used as a standalone and in a line-control configuration that is linked to several pick-and-place machines. In the standalone configuration, the resulting file is created and taken to the SMT loader for use. The information generally is split into two parts - the loading information containing the X/Y/rotation/part data and the feeder file. After installing data on the machines, any changes that must be madewould be updated using the pick-and-place machine. These include modifications to the original program, as well as any engineering change orders (ECOs) that affect part values or where they will be loaded in the future.

In the line-control configuration, the software controls the machines and divides the information into several parts to balance the line. The computer that splits the data is connected to all machines. Therefore, any changes made to the program have an immediate effect on the PCB loading.

Both methods described for third-party software configurations have the same potential for problems with the accuracy of the data. The conversion software can convert the X/Y rotation data automatically and accurately; however, the problem of component part-number accuracy is in question because the data are not contained in the CAD file with 100% accuracy certainty. Entering and updating part number/description data with the third-party program is a manual task, and has a high potential for errors.

Loading Feeders

When loading a feeder, an operator would follow a prepared list, generated from the pick-and-place machine or the third-party conversion software, that explains what part to load onto what kind of feeder, and where to place it on the machine. This process is the same for standalone and line-control configurations; the only difference is that the operator needs to be told onto which machine to load the part.

The obvious risk is that an incorrect part will be placed on the machine, or in the wrong location on the feeder platform. Normally an inspection sheet is used to double-check the operator against error; however, the same software that generated the potentially incorrect feeder-loading-sheet information most likely creates the inspection sheet as well.

This inspection is performed post-reflow, and is a manual process involving a highly skilled inspector or SMT hand operator. First article inspection involves understanding and cross-checking:

• Loaded and reflowed PCBs;
• BOMs;
• Pick-and-place CAD data;
• Pick-and-place feeder loading diagrams and sheets;
• Loading diagrams for component locations.

Due to the nature of small components, this operation must be performed under a microscope or magnifying lamp - making the entire process time consuming and confusing, as there are too many reference documents to track. This means that the whole process is best accomplished with two operators - one locating and verifying the parts on the PCB and the other verifying and keeping track of the document information.

The inspectors are working under extreme pressure due to the confusion and overload of a variety of information, as well as the added impact of expensive pick-and-place machines lying idle. Most CMs also must deal with different customers, each with a different way of documenting individual projects or PCBs. The effect of an undetected incorrect part on the first article would mean that all parts of that type on all PCBs assembled would be incorrect - resulting in thousands of PCBs that must be reworked. A mistake at this point would be catastrophic to the production effort, not to mention the bottom line. The only way to combat this is to have a well-controlled and repeatable first article inspection process that includes some type of computer-aided assistance. Several automated optical inspection (AOI) systems can perform this process, but it is best left to dedicated systems.

Automated First Article Inspection

With a computer-assisted (CA) first article inspection, the process of handling customer data would be the same as the manual method; however, the difference occurs during the inspection process itself. With a CA system, the loaded and reflowed PCB is scanned into the system. The CAD data is then imported into the system, either directly from the pick-and-place system or the line-control software. The operator has all relevant data required to perform the inspection. Because the X and Y coordinates are associated with the pick-and-place data, the CA system displays an exploded view of the part with its associated data on the screen. There is no requirement to locate parts manually.

The BOM data can be imported for cross-checking the pick-and-place data. This is done using an algorithm that determines associations between BOM data and pick-and-place data with the reference designator and part-number relationships. The operator establishes the fiducials on the PCB, and then inspects the PCB as each part is displayed on the screen with its associated data (part numbers, reference designators, etc.). Any differences are brought to the operator’s attention automatically.

For the first PCB of any type ever inspected, all of the data on the BOM is stepped through with the image on the screen, showing a greatly expanded view of the part being inspected. The operator does not need to worry about checking off parts in the system, as the data contained in the BOM came from the SMT loader - making it 100% accurate. The location on the PCB is displayed automatically, enabling the operator to concentrate on the visual inspection of parts loaded on the PCB.

The system will track all parts inspected to ensure that the inspection is complete. Once the inspection is complete, the operator saves this known-good sample and data for subsequent runs. On these runs, the operator loads the sample, while the system compares the PCB being inspected against a known-good sample with respect to the BOM and CAD data. This would virtually guarantee that the setup of the pick-and-place machine is the same.

Risk Analysis

If a company produces 10 million parts a month on PCBs, the first article inspection process would check about 20,000 of them. If human error leads to one problem in 1,000, which would be considered very good, there would be 20 potential problems each month. If each of those 20 problems were associated with reels of 5,000 parts, then 30,000 parts are placed incorrectly every month. As entire reels are unlikely to be used up on any given run, these figures are most likely overstated. Even if only 10% of reels were used each time on average, there would be 3,000 parts incorrectly placed each month. The problem is that this will not be a consistent level per month, and it will arrive at various times. To further exacerbate the problem, within most manufacturing companies there is a distinct move away from in-circuit testing (ICT), and toward functional testing. This is not necessarily a problem provided the QA department is 100% sure of the part content on the PCBs. If inspection procedures are not in place, then a portion of the mistakes made in SMT will not be found prior to shipment. Using an automated first article inspection system ensures 100% of the parts are inspected, and could eliminate problems.

In a recent return-on-investment (ROI) study at a medium-size CM, it was shown that cost savings could be achieved by automating the first article inspection process. Because their customers are in the medical and military markets, it was required that their first article be completed before manufacture. For an average-size PCB with 400 surface mount components, it was taking 15 seconds to locate and check off each part. This equals 1 hour, 40 minutes during which the line is not producing - with a lost opportunity cost of $1,650. Considering that they have five SMT lines with changeovers two times a week, the total lost-opportunity cost was calculated at $16,500/week. Once automated inspection was implemented, the average time to perform the same task was reduced to 15 minutes, a 65-minute savings per inspection (Figure 1). This calculated to $10,830/week in savings, or 10.83 hours of lost opportunity on SMT loaders.

Conclusion

An automated first article inspection system aids in the accurate setup of a pick-and-place system. It can reduce the time spent performing this process - speeding production and eliminating errors in manufacturing. Cost savings also are difficult to ignore. Using an automated first article inspection system allows the SMT work cell to become an integrated department that verifies its output according to customer requirements for overall process control.

Greg Ross, chief executive officer (CEO), Cluso Vision Systems, may be contacted via e-mail: gregr@cluso.com.au.