Design for eXcellence

Unit 4: Design for test

Section 2: Test methods for fabrication and assembly


Section Contents


The test sequence

There are three levels of testing:

In this part of the unit we will be discussing PCB sub-assembly test. Component and system level testing are not considered here because it is unlikely the layout engineer will be involved in these areas. It could be argued the bare board is a component. After all, the copper tracks have resistance and capacitance. PCB test methods are considered here as well.

For an excellent discussion of component testing see the DTI Electronics Design publication A Guide to Testing and Design for Test, which is downloadable as a PDF file at this link.

Activity: PCB assembly process and test and inspection

Draw the PCB assembly process from goods-in, through process checks and final test to despatch. For each stage insert all the relevant test and inspection stages. You should have something similar to this.

Compare your answer with this one.

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Vendor testing – bare board fabrication

The board fabricator will carry out the first set of tests in the PCB assembly process. Presented here is an overview of a typical bare board fabrication process. It is divided into 4 parts and some test methods used during the process are highlighted. The following section gives more details of each test or inspection method.

  1. Front-end manufacture – the customer’s data is received and the relevant production data derived. DfT methods and software are used to generate test machine control data and any test fixtures for electrical testing.

  2. Multilayer plant manufacture – the copper traces are etched into each inner layer. The circuitry pattern is inspected against the original design data for various defects with Automatic Optical Inspection (AOI). Then the inner layers are laid-up and bonded together. Lastly, the holes required are drilled. X-ray inspection is performed following lamination, to determine the presence and degree of inner layer shifts.

  3. Wet plant manufacture – the traces and hole are plated with copper on the outer layers and solder mask is added and cured.

  4. Finishing plant manufacture – board legend is added with the silk-screen process and any finish such as HASL is added. The board is depanellised and each sub-panel electrically tested. Electrical testing uses ICT or Flying Probe testers. A final inspection completes the process.

Bare board inspection methods

X-ray

Figure 1, Figure 2 and Figure 3 show X-rays typically taken during the inspection of the inner layers for drill registration. X-ray equipment can give on-screen drill offset measurements so problems can be found easier and quicker (Figure 4). More information on X-ray inspection is given in the section describing the testing of printed circuit assemblies.

This set of three photographs was kindly provided by
A&D Group (Glenbrook Technologies)

Figure 1: Verification of drilling registration for small holes at 15X magnification

Verification of drilling registration for small holes at 15X magnification

 

Figure 2: Verification of drilling registration for small holes at 40X magnification

Verification of drilling registration for small holes at 40X magnification

 

Figure 3: On-screen measurement of drill offsets for all sizes of holes

On-screen measurement of drill offsets for all sizes of holes

AOI

More information is given on AOI in the section describing the testing of printed circuit assemblies. During fabrication the AOI equipment would be used during the inner layer fabrication, checking the copper circuitry against a ‘golden’ or ‘Known Good Board’.

Electrical test of bare boards

The purpose of electrical test is to check the electrical integrity of the circuit, that is, to make sure the circuits are complete and carry the correct currents. Using an electronic netlist, all boards are tested using a bed-of-nails and In-Circuit Test (ICT) machine or a flying probe test machine. The ICT will be considered in a later section and flying probe testers in this section.

Flying probe is very useful for small increments such as prototypes whereas for production, the bed of nails or fixture test is more appropriate. Both methods are equally reliable, just different in time and expense. Here are some examples of electrical tests carried out by PCB fabricators.

Open or continuity test

The continuity test is the application of a voltage at one point on the board and measurement of the current at another point. The voltages are typically between 10 V and 250 V. The current measured is used to calculate the resistance, which has been pre-calculated for pass and fail levels. Pass/fail thresholds are set at less than 5–200W. A pass will register as a resistance below the threshold, showing that there are no breaks in the circuit.

Figure 4 shows the board being probed from both sides and all copper including vias can be probed, although in some cases single-sided fixtures would be used (see later for fixture types). When the board is tested in one pass (instead of two test fixtures and two passes), the test is usually more thorough but fixtures and programming costs are higher.

Figure 4: A board being tested for any open circuit problems

A board being tested for any open circuit problems

Shorts or Insulation Resistance test

This is carried out the same as the continuity or open test but pass/fail threshold values are set at greater than 100 kW to 2 MW. A fail would register as a resistance measurement lower than the threshold showing there are no areas between the points tested where there is a short circuit or current flow occurs.

igure 5: A board being tested for any shorts

A board being tested for any shorts

High Pot test

The Hi Pot test (referring to High Voltage Potential test) is used to check for high resistance continuity (leakage) between power and ground layers in a bare board. The test equipment is essentially a high voltage DC power supply with control electronics for setting voltage, current, and voltage step/dwell times. This test generally involves placing one probe on a connection to the power plane and one probe on a connection to the ground plane (or alternate plane) and then testing at a voltage of 500, 1,000 or an even higher voltage for a sustained dwell time. Any current flow indicates a failure.

Flying probe testers

This refers to a method of electrical test whereby the circuits are checked for open and short circuits by a tester that uses moving test probes that are guided to specific XY locations on the surface of the PCA (see Figure 6). This is similar in concept to how a circuit board drill drills holes in the PCB. Flying probe testers usually have at least two and as many as sixteen moving probes. The opens and shorts tests are conducted on the circuits by employing various methods of test such as resistance (Ohm’s Law), capacitive, field effect, phase differential, etc. These tests can be carried out on both bare boards and assemblies.

Figure 6: Close up of a flying probe tester showing four probes testing discrete components on an assembly

Close up of a flying probe tester showing four probes testing discrete components on an assembly

Activity

In the year 2000, the flying probe market grew by 40%, a much faster rate than the test market as a whole. This technique’s rise in popularity is due to a number of reasons. Can you find out why this is the case?

Compare your answer with this one.

Advantages and disadvantages of flying probe testers

A problem with ICT is it requires full nodal access (a ‘bed of nails’ test fixture). This adds significant NRE cost, as well as the time needed to build and debug fixtures and tests. There have been many improvements in the ICT method of test, such as BIST, boundary scan and cluster testing that has allowed for a higher level of PCB fault coverage without full nodal access. ICT with full nodal access offers pin level diagnostics, complete process testing and limited performance testing with test time almost always dictated by the desired ‘beat rate’ of the manufacturing line.

Flying probe test has the advantage of quick and easy set-up and cost savings when testing circuit boards since there is no test fixture that must be designed and manufactured. However, Flying probe testers are significantly slower in testing a given circuit board than a fixture-based test. This is due to the fact that a test fixture has all points of contact with the circuit board being made simultaneously while a flying probe tester has each point of contact being made consecutively. Even though flying probe testers are slow, they can generally test circuit boards that have very small features and dense circuitry that cannot be tested reliably or easily using a test fixture.

Test coupons

A coupon is a region within the bare board, where non-functional layout items are created to test the manufacturing process. It is on the same fabrication panel as the PCB, but separate from the electrical circuits and outside the actual board outline. It is cut away from the PCB prior to assembly and soldering of components (Figure 7).

The coupon will typically consist of the narrowest possible track, tracks that are as close together as allowed by the design technology, the smallest allowable holes, and so on. The internal nets of a test coupon would normally have no intended functional impact on the printed circuit board. This region of the bare board can be subjected to a series of (possibly destructive) tests. If this region fails the tests, then it is likely that the board as a whole has not been manufactured correctly. If it passes the tests, it does no more than increase the tester’s confidence in the state of the rest of the bare board.

Figure 7: An example of test coupons placed on a two-board panel

An example of test coupons placed on a two-board panel

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Testing of printed circuit assemblies

The tests and inspection methods and tools have been touched upon in Background to test and inspection. The X-ray and AOI inspection techniques and the electrical tests (using ICT and Flying Probe testers) are the same as those using during board fabrication.

AOI

Initially, AOI was used primarily in the assembly process for post-reflow inspection of the quality of solder joints by checking for:

More recent trends have seen the placement of AOI systems also in post-paste and post-placement stations to inspect for paste coverage and placement of components.

However, AOI has limitations, because it only analyses visible features. With Ball Grid Arrays (BGAs), µBGAs, Chip Scale Packages (CSPs), flip chips and other hidden-connection devices, manufacturers have to opt for X-ray systems to analyse the critical solder joints of these new-generation packages.

X-ray

X-ray inspection relies on the X-ray blocking characteristics of the lead content in the solder to assure the integrity of solder joints. They also screen for solder splash and solder flow shorts as well as open joints, even beneath BGAs. AXI is the only method than can quantitatively, as well as qualitatively assess solder joint integrity.

The following figures show typical images from X-ray equipment on an assembly line. These photographs were kindly provided by Dage Precision Industries.

Figure 8: Top view of insufficient solder paste on a QFP

Top View of insufficient solder paste on a QFP

 

Figure 9: Oblique view of insufficient solder paste on QFP

Oblique View of insufficient solder paste on QFP

 

Figure 10: Misaligned BGA

Misaligned BGA

 

Figure 11: Misaligned SOIC on Pad

Misaligned SOIC on Pad

 

Figure 12: SOIC bridges

SOIC Bridges

 

Figure 13: Non-wet balls in µBGA

Non-wet Balls in µBGA

Test contractor sequence

When assemblers plan the assembly process they have to include the test process as well. The test fixture must be ordered and test software produced by the fixture manufacturer. If we take an ICT machine as an example, can you describe the process after the assembler has supplied the data for the fixture manufacturer?

Compare your answer with this one.

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