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Differences in Loaded Board Test
|System Test / Hot Mock-Up Summary:|
A Functional Test system has almost all of the advantages of the System / Hot Mock-Up test method and solves many of the disadvantages.
A Functional test system can typically test the board at functional system speeds, unless very fast data rates are involved.
Fixturing is limited as the board is connected to the test set up just as if being hooked up to the final system. Functional and system / hot mock-up test methods do not normally have access problems due to edge card interface techniques. This is also the very reason why functional and hot mock-up systems require a high level technician to debug a failed board. Diagnostics are very vague, typically only specifying a general operational error as opposed to a specific component or specific location on the board for solder opens and shorts. Normally by the time the technician does find out what the failure is, many boards have already been produced with the same failure.
Failure analysis of the composite board can be very complete due to the software programming effort. Functional systems provide many software tools to assist in failure reporting.
Individual component faults can be missed if the fault does not impact the overall functionality of the board.
The Functional system is isolated from the board under test, and is therefore not susceptible to damage induced by a faulty board under test.
As with the Hot Mock-Up, identifying board process and assembly faults at this late level in production can be very expensive to correct as damage to the board may have occurred.
Process problems at the Functional Test level are difficult to correct as several days of production may have already been completed.
|Functional Test Summary:|
The In-Circuit Tester differs significantly form the Functional and Hot Mock-Up tests in both architecture and testing philosophies.
While the Functional / Hot Mock-Up tests verify the board as operational, the In-Circuit test system verifies each individual component as operational. This is accomplished by using various techniques to electrically isolate the components. “Back Driving” digital pins and “Guarding” analog circuits are two methods used extensively.
The In-Circuit tester normally tests the board in a static, non-powered state for manufacturing and process faults first. This Analog In-Circuit sequence detects faults such as opens, shorts, resistors, diodes, capacitors, inductors, etc… Once the board passes these tests, the sequence moves to the next stage which is to apply power to the board.
Once power is applied to the board, additional tests are performed. Voltage and current measurements are made as well as dynamically exercising the digital IC’s.
Fault isolation and error reporting are very thorough with In-Circuit systems. This is due to each component begin tested individually.
Test program development can be extensive and take between two to six weeks. Some boards will take shorter, others longer. A good deal of software tools are provided to facilitate the programmer.
Costs are moderate as compared with full functional systems.
Fixturing is normally accomplished through the use of a Bed of Nails vacuum fixture. There are many different versions available. Many manufacturers of test system utilize their own unique fixture interface. However, 3rd party fixture houses can provide the fixture for almost any system.
A vacuum system is normally needed in association with the tester. The vacuum pump is typically supplied through a 3rd party vendor.
Using the In-Circuit system to help control assembly processes is very successful. This is due to the test system’s ability to report specific component failures. This ability is dependent on test probe access. Surface mount technology has forced test point accessibility or design for testability to become something that must be considered during the design of the PCB. That is if the manufacturer’s are going to be able to receive the full value from their test system.
|In Circuit Test Summary:|
The Manufacturing Defects Analyzer uses the same test philosophy as the In-Circuit tester and is very similar in architecture. Both systems focus on testing components individually. However, the MDA normally does not power up the board. For this reason, dynamic testing such as digital IC operation and function is typically not performed.
The Manufacturing Defects Analyzer will provide the ability to detect and isolate process faults such as opens and shorts, wrong, missing, or reversed components. Resistors, capacitors, diodes , inductors, etc…are all tested.
Testing for digital IC orientation and presence is possible by learning the protection diode signature of each pin of the IC. When the IC is missing or reversed, the system can normally detect and identify this fault. There are several algorithms currently being used from different manufacturers that offer a high degree of coverage.
There are two situations where the IC diode characterization method does not work. The first situation is when a reversed IC’s diode signature measures the same as the correctly oriented part. This is not a very common situation. The second situation is on bussed IC pins. When several IC’s are placed in parallel, their protection diodes are also in parallel. If one pin is open, the test system will not see it as the test current is flowing through one of the other IC’s on the bus. The open pin situation shows up mostly on a digital SMD board assembly. For this situation, HP’s Test Jet technology, and a few others like it on the market offer a solution. Most MDA systems today offer the Test Jet technology as an option.
If the digital SMD IC is misoriented, the Manufacturing Defects Analyzer will normally detect the fault as there are usually one or two pins not bussed together. The IC diode characterization is very effective in this circumstance.
Receiving Inspection can also be a factor in developing a test methodology. Making sure the components are functional prior to installing them onto the board can impact overall product yield significantly. Most of the time it’s best to have the manufacturer test the parts thereby using only pre-tested parts. In the late 90’s, it is almost unheard of for IC vendors to ship untested parts. The yields are much higher today than they were in the late 70’s and 80’s. This provides the opportunity to the board assembler to save on test expenses by not testing the IC’s functionally at the MDA or ICT level. Furthermore, in many cases the IC models change so rapidly that the ICT software test libraries are unable to keep up with them without expensive maintenance contracts. Also, a large percentage of the IC’s used today are ASIC’s which are difficult to test with generic ICT test systems.
Test program development is very short as compared to In-Circuit or Functional. Typical programming time is between 1 to 3 days. A good rule is that 1 day is needed to program between 50 to 100 components.
Software tools and input screens are used extensively to reduce the skill level needed to program the system. Most Manufacturing Defects Analyzers do not require any programming skill. Electrical or schematic knowledge is only needed if active guarding is required. Some systems use automatic guarding to reduce the technical programming requirements to that of operator level.
Manufacturing Defect Analyzers tend to cost 1/2 to 1/3 that of In-Circuit testers.
Fixturing is normally accomplished through the use of a Bed of Nails vacuum fixture, a mechanical press, or pneumatic press. There are many different versions available. Many manufacturers of test systems utilize their own unique fixture interface. However, 3rd party fixture houses can provide the fixture for almost any system.
A vacuum system, or compressed air is normally needed in conjunction with the tester. The vacuum pump is typically supplied through a 3rd party vendor.
The Manufacturing defects Analyzer is used almost exclusively to test for process faults. The test system’s failure reporting is very specific and can dramatically assist in process control.
Many Manufacturing Defect Analyzers are located on the production floor at the end of the assembly line, or even farther up in the process. This placement provides immediate feedback of the process. If the wrong reel of parts are set up in the automatic inserter, the system will identify the faults before a large number of boards are manufactured.
|Manufacturing Defects Analyzer Summary:|
Some form of Visual Inspection also needs to be factored into the test strategy.
Current generation optical inspection systems are high speed and are often used in-line early in the process. Pre-reflow, post reflow, and post wave applications enable quick and reliable process control feedback
The in-line systems usually consist of a conveyor, PC controlled vision processors, a high speed x,y stage, one or several cameras, and xenon strobe lighting or LED lighting array. There are many variations of components used in the industry, all having cost/performance advantages and disadvantages.
Automated visual inspection is especially useful for today’s high density SMT boards. Many times there is simply not enough real estate for test points therefore visual inspection is the only way to insure part presence, orientation, and solder joint integrity. Lifted leads on fine pitched components, excess solder, insufficient solder, and solder bridging are a few more examples of defects that can be found with an AOI system.
Automated visual inspection can also be used for through hole and mixed technology PCB’s as well as complex loaded backplanes. Bent and missing connector pins can be also be detected.
Off-line manual or semi-automatic bench top vision inspection systems can also be used for the same purpose. The bench top system cost is typically lower than the in-line system and the only sacrifice is a slower throughput.
There is no mechanical fixturing necessary. In many cases, fixturing costs can be reduced when AOI is combined with electrical test due to the opportunity to cut down on the number of probes needed.
Many of today’s PCB’s, especially for the communications industry are full of high frequency circuitry which cannot be tested with even the most sophisticated ICT systems. Automated optical inspection systems can inspect the difficult to detect high frequency components.
Though electrically impossible verify, reversed bypass capacitors can be detected optically.
Most AOI systems diagnose the failures very specifically by displaying the failed reference designators and the reason the component or solder joint failed. This makes failure analysis of the boards is easy to perform
Most systems can be programmed by technician level personnel. The programming time can vary tremendously between systems depending on program generation technique, software tools, and operating system.
Some systems “self learn” a golden board comparing (or correlating ) the original bit images to newly acquired bit images. This technique can provided quick programming, however the failure diagnostics may not be as specific, exact component placement and rotation measurements cannot be made, specific numerical tolerances cannot be set for component placement or solder joint shape, and first article inspection cannot be done.
Systems that use either original board build CAD data or pick and place data combined with IPC standard component libraries can quantify component translation, rotation, and solder joint shape. Programming time can also be very quick for these types of systems if the input pick and place data or CAD data is clean and correct and if the system provided part libraries are comprehensive enough to include all components loaded on the board.
|In-Line Automated Optical Inspection (AOI) Summary:|
|Off-Line, Bench Top Automated Optical Inspection(AOI) Summary:|
Manual visual inspection consists of an individual human identifying manufacturing and process faults with the use of a magnifying lens or sometimes even a video camera system. Manual inspection can be very tedious and prone to errors. Also, labor costs tend to make easy justification for a Manufacturing Defect Analyzer and/or an automated vision inspection system. Typical return on investment for an MDA and/or automated visual inspection vs. Manual inspection can be as little as 6 to 18 months based on board volume.
Many of the best test strategies are developed from using a combination of electrical test equipment, vision inspection equipment, and/or combinational test equipment. One popular technique is to use an MDA tester and/or an automated vision inspection system early in the process, preferably “in-line”. Many of these systems have automatic failure alarms and real time statistical feedback that give the production crew the ability to correct process problems on the fly, preventing numerous boards being manufactured improperly.
For example, an automated vision inspection system could be placed pre-reflow to verify the correct placement of the components. A repair loop can be installed right there to make sure no boards are re-flowed with missing, reversed, rotated, skewed, or wrong components. Later in the process all solder connections and component values can be checked by using an MDA. This technique will ensure very high yield at the hot mock-up or system level test. In many cases the manufacturer will have such a high yield that system testing is only necessary on a random sample basis. This takes the burden off the most expensive and slowest stage of testing.
Many test equipment manufacturer’s have addressed the fault coverage issues by offering Hybrid or Combinational Systems. These are systems that cross the typical category boundaries and offer test strategies found with different methodologies.
A few examples would be:
- MDA + Functional
- MDA + In-Circuit
- In-Circuit + Functional
Significant differences in fault coverage can be achieved by using a combinational test system, but there are sacrifices. Cost can be slightly higher or significantly higher. The additional functions added to the base system may not be as effective as a stand alone dedicated system. However, by addressing the overall test strategies and determining the test requirements, combinational systems may offer significant value for the percentage of fault coverage.
The first example, MDA + Functional is a great alternative for an OEM that runs a high volume of each product. Many MDA systems can combined with some functional instrumentation that will perform some simple necessary functions without slowing the throughput down and without adding significant cost. The only disadvantage is that the added functional instruments may be only versatile enough for one or two types of products and further integration or development may be necessary to bring on new products to be tested.
The second example is really just a traditional ICT system. This system will perform all un-powered MDA testing first with the same level of specific diagnostics as the MDA. It will have the added capability of board power up and digital back-driving for functionally testing the IC’s. MDA is the lowest cost of all strategies and by adding the ICT capabilities the cost can double or triple. The level of personnel needed to program the test system is also higher, usually being an engineer level as opposed to a technician level for MDA programming.
The third example, is a system that can perform MDA, ICT and Functional. It will also give you the same fault coverage as an MDA with the same diagnostics, however, this system will be very expensive and a full time test engineer will be needed to program it. This test system may also be quite a bit slower than just MDA or ICT alone.
Now that we have identified several of the differences within each of the general test system categories, we can look at the typical test coverage in respect to the fault distributions.
|Chart 1: Loaded Board Fault Distribution|
When we look at the fault distribution, we can see the impact SMD technology has on the fault distribution. The shift is based on Through Hole vs. SMD technology boards. This is due to opens on SMD solder joints. Whereas, in the past, with 100% through hole boards the most prevalent fault was solder bridging and solder shorts, today’s mixed technology boards and 100% SMT boards suffer primarily from cold solder joints, and insufficient solder.
The test system coverage is not greatly affected by this shift in fault distribution. Primarily due to the Vectorless Tools that have been developed to detect the SMT opens. These tools consist of analog testing of each protection diode inside the IC’s and HP’s Test Jet technology. Both technologies are available on most MDA’s and ICT’s. Of course standard continuity and component testing will be able to pick up solder joint opens on all other types of components.
Based on the industry standard fault distributions displayed above, it is shown that 92% of loaded board defects are caused by manufacturing process faults. Some of the newer SMT production equipment is able to cut down on these problems with internal system verification tools, such as on board visual verification, etc. However, these tools are usually only able to sample verify placement due to fast line speeds. Although the newer production line equipment has given process engineers the ability to more closely control their process, the human element is still a factor as well as equipment inconsistency and malfunction.
Please note that for these examples, we are not taking into consideration board accessibility. Some very dense SMD boards may not have accessible test nodes. If this is the case, both the Manufacturing Defects Analyzer and the In-Circuit System’s coverage will be reduced. Accessibility will typically be a function of the bed of nails fixture.
Within this paper, we have discussed some of the various differences between system types and how they can be used to develop an overall testing philosophy. Many alternatives exist, each with their own strengths, weaknesses, and economic considerations. The above chart compares typical test & inspection system in relation to fault coverage. There are many other variables that should be considered in relation to test strategy. The next two charts demonstrate throughput requirements or volume of product to be produced in relation to types of test, as well as in relation to board type variability.
For purposes of this examination we will use the following definitions listed in Table #1 for production volume. We have also created a scale in Table #2 for rating each type of test.
|Chart 2: Test Coverage/Cost|
A brief look at Chart #2 shows that for production quantities of 400 + (High Volume) for each board type per month, the best test and inspection strategy would consist of a combination of In-Line AOI early in the process, MDA towards end of process and possible sample Hot Mock-Up testing at final assembly. The higher the rate of production the more controlled your process should be to prevent large amounts of scrap. Higher volume also requires faster machines which accounts for an in-line vision system as opposed to a bench top solution.
Medium and low volume situations are always more difficult to size up. However, the low cost bench top vision system is a clear choice for reasons including but not limited to; no fixturing necessary, quick programming time, quick product change over time, ability to reduce labor costs by automating a manual process, and increasing workers efficiency.
Chart #3 takes into consideration board type variability. The two most important qualifiers in this analysis are fixturing costs and programming time. In the high volume, high mix category ICT is rated lower than MDA due to the increase in fixturing and programming costs as compared to MDA testing. This board variability can also be analogous to product stability. There are some situations where MDA or even ICT make sense in a low volume – high mix situation due to the longevity of each product. Each particular product may only be produced in lots of 25 per month, however if the product doesn’t change over periods as long as 5 to 10 years it is still cost effective to build fixtures and write test programs.
|Chart 3: Throughput/Volume|
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