Closing Inspection loopholes - SMT Workcell Machine Setups
August 19, 2010
Introduction
This document highlights the inspection techniques and strategies commonly used in the setup of SMT loaders using First Article Inspection as the key to closing the loop in setup verification.
Current established guidelines for setting up, loading feeders and verification of programs are discussed along with the common strategy of performing an inspection of the loaded PCB to verify all SMT equipment is setup correctly. Improper SMT setups cause of a great deal of rework due to incorrectly, reversed or skewed parts after reflow. Since most SMT loaders do not test parts they can and do load the wrong parts just as quickly as the correct ones.
In order to assess all of the risks associated with the methodology behind the process of machine setup within the SMT area we need to examine the way in which the documentation is produced and presented. For the purposes of this article we will examine a typical Contract Manufacturing situation.
The flow of the documentation starts from the Customer. The documentation would include but limited to:
Bill of Materials.
Loading Diagram (reference designators included).
PCB files and/or Gerbers that are used for the generation of the SMT solder paste stencil.
Normally the Contract Manufacturers Production Engineering department would input the Bill of Materials into the MRP System and place the additional documents on file for use within the manufacturing area.
The SMT area supervisor would ensure that a stencil is ordered. He would also convert the BOM and PCB file into the CAD files required depending on the types of SMT loaders used. These would normally be a PCB part location (X/Y/Rotation) file and a feeder file. These are required for the SMT loaders. These would be either automatically generated or manually created.
Once all of the parts are placed into the feeders and the first PCB loaded the First Article Inspection Process would begin.
First Article Inspection Process
The First Article is built by the SMT machines and sent to the QA department to check the product assembled against the Bills of Materials (BOM) to ensure that the programs contained in the Pick and Place machine are correct as well as the parts are placed into the feeders in the correct locations. There are several methods used however the most common will be described here. T
he program that designates which feeder part is required as well as its location on the PCB (X/Y/Z) is prepared, normally automatically using the CAD Data supplied from the customer or engineering departments and loaded into the SMT Loader. This CAD Data shows the positions (X/Y) and the Reference Designators. This information is automatically compiled normally using 3rd party conversion software specifically designed to create pick and place CAD data. This software would also split the data for use on several machines in a SMT line if required. This software is generally referred to as Line Control or line Balancing Software. Its’ main function is to ensure that the time loading parts for each of the machines in a SMT line (chip-shooters, IC inserters) is equal so that the line runs at maximum efficiency.
If changes have been made to the parts on the PCB’s that are not reflected in the CAD Data, then the BOM is required to show the current part numbers associated with the particular reference designators. The CAD data is modified either on the machine itself (only if machine is a single machine not in a line), in the source Pick and Place file, or within the 3rd party conversion or line control software that controls a line of Pick and Place machines.
Feeders with the parts installed on them are then loaded onto the Pick and Place machines in designated slots pre-established by either a manual method or the 3rd party Conversion program specifically designed for this purpose. This feeder location information is programmed into the Pick and Place machine.
Once this is complete the first PCB, referred to as the First Article, is screen-printed and populated using the Pick and Place machine. Normally prior to reflowing the PCB is initially inspected by the operators. This minor inspection is limited to IC’s and polarized parts and generally is limited to checking for correct values and direction of parts that are either expensive and/or complicated to rework.
The PCB is then reflowed using the designated profile in the Reflow Oven.
This First Article is then turned over to the QA department within the SMT area for full inspection. To perform this function QA is required to search the PCB using the BOM and Loading Diagram (This is because most SMT PCB’s do not have reference designators marked on the PCB) for each of the reference designators looking under a magnifying lamp or microscope to visually inspect the parts to ensure the setup of the Pick and Place machine. The QA inspector must manually cross off the parts one by one until all parts are found and inspected. The Pick and Place machine program is also crossed off to ensure that no additional parts are loaded that are not contained on the BOM.
The Pick and Place machines are probably not run during this process as it could potentially be loading incorrect parts. Normally the Pick and Place machines start running once the major parts (IC’s, capacitors, and any polarized parts) are checked. This is to minimise the length of downtime associated with the Pick and Place machines. The rest of the parts are then inspected.
Sometimes inspection aids are used in conjunction with these processes such as scanners (Mylar sheets with locations and polarities marked), which are overlayed on top of the PCB that indicate particular areas of interest and the feature (Location and Rotation data) to be inspected. These scanners can take significant time to construct.
Risks associated with this method are:
The process of inspection can take up to 3 hours normally to completely inspect 100% of the parts on the PCB that contains 200-300 SMT parts on it. Generally this is too long to allow the Pick and Place machines to lay idle so they are generally started after major parts are inspected.
It is very difficult to locate all of the parts on the PCB using the loading diagram so it is generally limited to most of the different part values. If most of any one type of part is located it is assumed that if they are on the SMT loader program then they will be correctly loaded. This is a risk, however small.
Along with the general risks previously noted, there are specific risks that should be discussed. There are several areas that are manually driven that can introduce massive errors. In actual fact almost all methods however automated can introduce errors. Sometimes these processes are so refined with automation that they can lull the operators into a false sense of security and cause massive error at random times. An analysis of the flow of information is required to understand the potential for error.
Customer CAD eData
This data is the actual PCB data file that is normally the same file used to manufacture the raw PCB and generate the Gerbers used in the manufacture of the solder paste stencil.
Due to the fact that the manufacture of blank PCB’s is not dependant at all on the accuracy of the values and part numbers of the parts loaded on the PCB the BOM style information is normally not up to date or not contained in this file. Most engineering departments that design PCB’s would control the BOM information on a separate database or spreadsheet. This information must be merged back together either manually or by 3rd party conversion software. Either way the resulting information has potential for error and must be manually checked to ensure its accuracy.
3rd Party Conversion Software
This software is used to create the actual CAD and feeder data for the SMT loaders. It can be used as a standalone as well as a line control configuration that is actually linked to several Pick and Place machines in a line.
Stand-alone
In the standalone configuration the resulting file is created and taken to the SMT loader to be used. Normally the information is split into two parts. These being the loading information containing the X/Y/Rotation/Part data as well as the feeder file that tells the operators where to put the feeders with the parts onto the machine. After installing the data on the machines any changes that are to be made to the data would normally be updated using the Pick and Place machine manually. These would include modifications to the original program as well as any ECO’s that affect part values or where they are loaded in the future.
Line Control
In the line control configuration the software normally controls the machines and divides the information into several parts in order to balance the line (so that all machines spend equal time loading PCB in order to maximize output). The computer that does this splitting of data is normally connected via RS232 or network cable to all of the machines being controlled. This way any changes that are made to the program have an immediate effect on the loading of the PCB’s.
Associated Risks
Both methods described for the 3rd party software configurations have inherently the same potential for problems with the accuracy of the data. The conversion software can automatically convert the X/Y/Rotation data with a high degree of accuracy, however the problem of the component part number accuracy is in question because as discussed earlier this data is normally not contained in the CAD data with the certainty of 100% accuracy. This is normally a manual process to enter and update the part number/description data process with the 3rd party program and has a high potential for errors.
Manual Programming of Pick and Place machines
In some manufacturing plants the Pick and Place machines are programmed manually. Placing a blank PCB onto the machine and individually digitizing the locations where the parts are to be loaded is the normal method. Using this method the Pick and Place machine must also be programmed with the feeder locations containing the parts as well as the part numbers of all the parts individually.
Associated Risks
This method obviously has the highest risk of error due to the manual method of transposition of data. Any number of problems can exist in the data including missing parts, rotation errors, addition of parts and incorrect part numbers just to name a few.
Loading Feeders
The loading of the feeders is the manual process of taking the individual parts and placing them into the feeders for use on the Pick and Place machines. They can take the form of reel, tubes, trays or loose parts. Normally the operator would follow a prepared list that is generated from the Pick and Place machine or the 3rd party conversion software. It tells the operator what part to load onto what kind of feeder and place it on the machine in a particular location. This process is the same for both the Stand-alone and the SMT Line Control configurations, the only difference being that the operator needs to be told which machine to locates the part.
Associated Risks
The associated risks are that an incorrect part can be placed on the machine or possibly in the wrong location on the feeder platform. Normally an inspection sheet is used to double check the operator against error, however this inspection sheet by the same token may have been generated by the same software that generated the ambiguous or potentially incorrect feeder loading sheet information.
Pre-Reflow Inspection
This inspection was discussed earlier as the one performed on the first article only checking major parts for polarity, etc. This is only a basic check to ensure that the parts that would be difficult to rework are correctly placed prior to reflowing the PCB. The operator would be given a list of suspect parts and/or potential problem areas to inspect. This inspection would only take about 5-10 minutes.
First Article Inspection
This inspection is performed post reflow of the SMT parts loading process. This manual process usually involves a highly skilled inspection person or SMT lead hand operator to perform properly. It involves the understanding and cross-checking of the following information:
Loaded and reflowed PCB
Bill of Materials
Pick and Place CAD Data
Pick and Place Feeder
Loading diagrams/sheetsLoading diagram for component location
The operator must step through the Bill of Materials (BOM) one part at a time verifying that the following information is accurate:
Locate the part on the Pick and Place CAD data to ensure it is in fact being loaded and the part specified matches the BOM
Locate the part on the Loading diagram. Most PCB’s do not have reference designators marked on them so this is the drawing that tells them where the part is.
Locate the part on the Feeder setup sheets and verify the part is the correct part.
Locate the part on the PCB and verify that it is the part specified on the BOM and CAD data matches the part actually loaded. Also verify polarity is correct.
Due to the nature of the very tiny parts contained on an average SMT PCB this operation must be performed under a microscope or Mantis. This makes the entire process very time consuming and confusing, as there are too many reference documents to keep track of. This usually means that the whole process is best done 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 SMT Pick and Place machines lying idle. Compounded to this is that most Contract Manufacturers deal with many different customers and each one has a different way of documenting their individual projects or PCB’s.
The kinds of problems that can be missed are potentially far reaching not least of which are:
Missing parts
Incorrect parts
Backwards parts
Additional parts
The most dangerous impact of poor first article inspection is the potential for singular problems to be compounded so that to affect the entire run of thousands of parts. The effect of an incorrect part on the first article means that all parts of that type on all PCB’s assembled are going to be incorrect. This would mean 1000’s of parts to be reworked. A single mistake at this point would be catastrophic to the production effort not to mention the bottom line in dollar terms.
The only way to combat this is to have an extremely well controlled and repeatable first article inspection process that includes some type of computer aided assistance. This is where the Cluso Vision Systems First Article Inspection System provides the solution.
Automated First Article Inspection
This section describes the same first article inspection process using the Cluso Vision Systems First Article Inspection system to aid the inspection.
The process for handling the Customer data would be the same as the manual method previously decribed. The main difference comes in during the First Article Inspection process.
With the Cluso system the loaded and reflowed PCB is scanned into the system. Then the CAD data is imported into the system, either directly from the pick and place or the line control software. The operator now has all of the relevant data required to perform the inspection. Because the X and Y coordinates are associated with the pick and place data, the Cluso system automatically displays an exploded view of the part with its associated data on the screen. There is no requirement to locate the parts manually as Cluso does this automatically.
The BOM data can be imported for cross checking the pick and place data. This is done automatically using an algorithm that determines associations between BOM data and pick and place data using the reference designator and part number relationships. The operator simply establishes the fiducials on the PCB and then steps through the PCB inspecting each part displayed on the screen along with its associated data (part numbers, reference designators, etc).
For the first PCB ever inspected of this type, 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 so it is 100% accurate. Also the location on the PCB is automatic as well. This enables the operator to concentrate on the visual inspection of the parts loaded on the PCB.
The system tracks all parts inspected to ensure that the inspection is 100% complete. Once the inspection is complete the operator saves this known good sample and data for subsequent runs. On subsequent runs the operator loads the sample and the system automatically compares the PCB currently being inspected with a previously known good sample with respect to the BOM and CAD Data. This virtually guarantees that the setup of the pick and place machines is the same, giving 100% confidence that the same parts are being loaded as before. It ensures that once a PCB has been manufactured correctly, all subsequent runs are guaranteed to be the same, providing unparalleled consistency in manufacturing. Consistency is the name of the game in manufacturing and the Cluso system guarantees this.
As an added feature the system can archive any inspection for later use. This is especially important in batch run military or medical applications. The archive process saves the scanned PCB along with the CAD and BOM data for 100% traceability to the job. At any time in the future if a problem arises with the customer, this archived data can be reviewed or exported to any Windows program.
There is also a notes feature connected to the Sample PCB which can be used by the Engineering or QA departments to feed forward information relating to future builds of this product. For instance, if the customer provides an ECO that affects future builds, the Engineering department can place a note in the Sample files notes field, and when the time comes to build the PCB the note will be there at the right time. This effectively gives advance notice at the exact time required, during the first article inspection for that product.
Risk analysis:
If a company produces 10 million parts a month on PCB’s (medium contract manufacturer levels) the first article inspection process would be checking approximately 20,000 parts per month. This is because only the first PCB’s are checked in this process. If human error leads to 1 problem in 1,000, which would be considered extremely good, there would be 20 problems each month. Using the manual method, given that the probability of inspecting 100% of parts is low and likely to be only 70%, then this would reduce this to 6 problems per month. If each of those 6 problems were associated with reels of 5000 parts, which is common to reel sizes, then 30,000 parts are likely to be incorrect every month. As entire reels are unlikely to be used up on any given run then the figures are likely overstated by potentially 80%. Even at these levels there would exist 6,000 parts incorrect each month on average.
The problem is that it will not be a consistent level per month and will arrive at various times which would lead one to believe it to be a random event. This is far from the case and it is a fairly predictable event, not when but how it happens. Murphy’s law would predict that it would happen at the most inconvenient time possible, guaranteed.
To further exacerbate the problem, within most manufacturing companies there is a distinct move away from ICT style of testing and a move towards functional testing. This itself is not necessarily a problem providing QA are 100% sure of the part content on the PCB’s. If the inspection procedures are not in place, like the conventional methods in SMT first article inspection, then a proportion of the mistakes made in SMT will not be found prior to shipment to the field.
With the Cluso First Article Inspection System 100% of the parts are inspected. This would virtually eliminate all problems as described above.
Conclusion
The Cluso system provides an invaluable aid to the accurate setup of the SMT Pick and Place Machinery. It dramatically reduces the time spent performing the vital First Article Inspection Process getting the SMT area producing faster. The accuracy of the setup is 100% when using the Cluso System, eliminating errors in manufacturing due setup issues.
Using the Cluso First Article Inspection System the SMT Workcell now becomes an integrated department capable of verifying its output is setup 100% correctly to the customer requirements. Once the setup of machines has been covered it is simply a matter of process control to ensure output consistency. The Cluso system allows the staff within the SMT Workcell to concentrate on process control issues and leave the setup of machines to the Cluso First Article Inspection Machine.
Greg Ross
Cluso Vision Systems