IPC 9252A Electrical Test Considerations & Military Specifications Versus Electrical Test

Estimated reading time: 10 minutes

Abstracts

IPC 9252A Electrical Test Considerations

The PCB industry has advanced significantly in the recent millennium. OEM specifications and requirements have also advanced due to maturing technologies. With this, the requirements of electrical test of these higher technology products has advanced as well. Long gone are the pin-in-hole technology PCBs now surpassed by large multilayer, blind/buried, and rigid-flex technologies. For suppliers of electrical test, be it in-house or sub-contracted the industry specifications can be confusing, and at times non-comprehensible. OEMs direct the IPC specification (6012, 9252A, AS9100, etc.) for their fabrication to the manufacturer, but do OEMs and contract manufacturers (CMs) really understand what they are asking? Many variables are associated with these specifications and requirements to their designated classes regarding electrical test. OEMs decide what IPC class to which they wish their product manufactured due to performance requirements, but overlook the electrical requirements associated with those requirements. Manufacturing, plating, etching and all those processes may be within the class requirements they require, but they overlook the electrical requirements associated with their required IPC class. This paper will outline and define what requirements must be met for the OEM community to truly achieve the IPC class product from the electrical test standpoint. This will include the test point optimization matrix, isolation (shorts) parameters, and continuity (opens) parameters. This paper will also address IPC Class III/A additional requirements for aerospace and military avionics. The disconnect exists between OEMs understanding the requirements of their specific IPC class design versus the signature that will be presented from their design, resulting in many Class III builds failing at electrical test.

Military Specifications Versus Electrical Test

Many PCB manufacturers are now gaining contracts from military OEMs. From these new specifications are a required prerequisite above and beyond the standard IPC consortium. Military specifications have different requirements than the standard IPC requirements. With electrical test the manufacturer or sub-contractor must test this product in accordance with the designated military specification.

The disconnect here between the OEM and the manufacturer or CM is that this information is not readily supplied to the electrical test department or sub-contractor. Just because the board is to be built to MIL 31032, Mil 50884, or MIL 55110 is not enough. The Defense Logistics Agency Land and Maritime (DSCC) has for a long time required that not only shall a military OEM state to what specification the product should be built, but also state the specific revision of that specification and amendment level. This paper will outline the electrical test requirements for the specifications 55110, 50884, and 31032 up to the current revision and amendment level as of publish of this document. Electrical test requirements have changed depending on the revision and amendment of each of these documents, and the use of flying probe test versis fixture test.

Introduction

Over the years, many aspects of testing printed circuits have changed. We have come a long way from single-sided breadboards, double-sided boards, and PTH-only builds to the massive layer counts, blind-buried vias, strict TDR requirements and even buried passives such as capacitance and resistance. The industry has expanded tremendously over the last 25 years and so has the technology. Time to market is crucial for many OEMs. They need their product out to the consumers and perform to the modeling the designers intended. This causes when constraints building and testing the product for conformance. I shall explore:

1. IPC Classes Defined;
2. Test Methods: Definitions;
3. Requirements When Testing Class I-III and 3A;
4. Design Concerns Versus Class Requirements;
5. Knowing Your Specifications (31032, 50884, and 55110);
6. DLA Requirements for the Suitability Lab; and
7. OEM/Manufacturer DLA Expectations.

1. IPC Class I, II, and III Defined

Class 1 General Electronic Products — Includes consumer products, some computer and computer peripherals suitable for applications where cosmetic imperfections are not important and the major requirement is function of the completed printed board. Class 2 Dedicated Service Electronic Products — Includes communications equipment, sophisticated business machines, instruments where high performance and extended life is required and for which uninterrupted service is desired but not critical. Certain cosmetic imperfections are allowed.

Class 3 High Reliability Electronic Products — Includes the equipment and products where continued performance or performance on demand is critical. Equipment downtime cannot be tolerated and must function when required such as in life support items or flight control systems. Printed boards in this class are suitable for applications where high levels of assurance are required and service is essential.

2. Test Methods

Bare Board Electrical Testing today is done primarily by one of two methods: A universal grid test machine using fixtures or a flying probe machine. Both options provide electrical test, but with some trade-offs, primarily speed versus cost. Also within the IPC-9252A specification definitions exist outlining continuity, isolation, indirect, signature comparison, and resistive isolation testing. Confused? Continuity, isolation, resistive continuity, and resistive isolation are used by grid test machines. Indirect, signature comparison, and adjacency are all terms used with flying probes.

• Continuity: The value in ohms Ω that may not be exceeded on any circuit or the machine will fail the circuit as “open.”
• Isolation (discontinuity): The value in ohms Ω (usually Meg ohms) that two circuits should have in resistance between them as a minimum or the machine will fault the two circuits as “shorted.”
• Adjacency Value (used in flying probe machines): This value is 1.27 mm (0.050 in.) What this means is that any two circuits that are 1.27 mm in distance or less from one another at any point, they will get tested for possible “short” conditions. The number can be adjusted as agreed between user and supplier. (AABUS.) The IPC standard here states that only horizontal or “line of sight” adjacency is required, vertical, or Z-axis adjacency is not required unless specified. Adjacency both horizontal and vertical is outlined below in Figures1 and 2.

Figure 1: Horizontal adjacency. (Click here to enlarge image.)

Figure 2 :Vertical adjacency. (Click here to enlarge image.)

• Indirect testing by signature comparison: This is how most flying probe machines gain their speed in testing along with adjacency. The first board of the order goes through the signature gather (capacitive discharge) and will receive a full resistive continuity and isolation (using adjacency) test. If the board passes the master value will be retained. The second and all subsequent boards will also receive the capacitive signature gather. However, only nets that have deviated beyond machine preset threshold against the master will receive the resistive verify for possible opens or shorts. Flying probe machines can provide full resistive continuity and isolation testing. Again the isolation test is using adjacency. In this case, all boards will take the same time to test as the first board.
• Voltage: This parameter is probably the most misunderstood parameter in the electrical test arena. How much should be applied and when? Most Eeectrical test machines use voltage in both the continuity and isolation phases of the test. However they usually do not use the same in each. On many machines the continuity test voltage is not configurable. This voltage is very low, usually around 12-15 VDC. The reason for this is that higher voltage is not necessary when checking for “open” conditions. An “open” will be open at 12 Volts as much as it will be at 250 Volts. Further, the low Voltage is used to protect the circuit. Running high Voltage on a point-to-point network would cause high current runaway and most likely damage or scrap the circuit. The higher Voltage is desired when checking for “shorts.” Since theoretically there should not be a current path between isolated circuits the higher voltage is used to “sniff” the possibility of a “short.” If the machine identifies a leak or a “dead short” it will identify the nets shorting, but also shut down the test on those nets so as not to damage them. Electrical test machines are not designed in principle to “blow” shorts. So the question is: How much should be applied to check for shorts? Figure 2-3 is an excerpt from the IPC-9252A standard.

Figure 3: Test voltage.

Still confused? This is why many test contractors have developed standard parameters that will meet and/or exceed requirements of the IPC standard. There are many times where a master drawing doesn’t say anything about electrical test, but it is required. In such a case. the test machine only needs to use a minimum of 40 Volts. This is why many test facilities use voltages such as 100VDC as their standard. This is a better test and exceeds requirements.

3. Requirements When Testing Class I-III and 3/A

Minimum electrical test requirements are different between the IPC classes. This does not mean that opens or shorts are allowed! The electrical thresholds to identify these conditions are different. In the electrical test industry we are not only looking for the extreme cases (infinite open or direct short) we are also looking for indicators of what may be a potential problem. This is why the minimum requirements are different between classes (Table 1).

Table 1: Requirements by test level. (Click here to enlarge image.)As you can see from Table 1, the resistive continuity and resistive isolation requirements are much different between Class I and Class III. You will also notice that indirect continuity and isolation testing by signature comparison is allowed in Classes I and II, but AABUS in Class III. This does not mean that it cannot be used, but it does require authorization from the OEM or customer for that method to be used on a Class III product.

Class 3/A Exception: Space and Military Avionics IPC-6012 Appendix A

This is an exception beyond the standard Class III requirement for electrical test. In this case there are parameters specified when testing product under this class. These requirements need to be on the master drawing and communicated from the OEM to the manufacturer or this type of test may be overlooked. The differences with Class 3/A can be found in Table 2.

Table 2: Class 3/A. (Click here to enlarge image,=.)

4. Design Concerns Versus Class Requirements

From an OEM or designers perspective, one must be aware of constraints they may build into the board design which may cause difficulty during electrical test. We find this most prominent with Class III product designs. From the information presented in Table 2, we see that for continuity resistance there must be no circuit on the board with a resistance greater than 10 Ohms. That is the standard. Now there are a couple problems that come into play here. If you will look back to Note 4 below Table 1, you will see a statement regarding referee calculations for circuit length. With that said, a board may be out of tolerance to Class III with any net having a length exceeding 500 mm (19.68 in.) Compounding this is that some OEMs not only want Class III, but continuity resistance to not exceed 5 ohms and have net lengths in their board exceeding 750 mm (29.53 in.) This usually leads to delays in final inspection and or shipping of product until a waiver or master drawing deviation is obtained.

5. Know Your Specification: 31032, 50884, and 55110

Many manufacturers are building military products. The military, of course, has its own specifications for building and testing products. Two main specifications with an impact on electrical test are MIL-PRF-31032 and MIL-P-50884. The former is the performance specification covering a full range of product builds. This covers multilayer, blind/buried vias, flex, rigid-flex, and thermoplastic with or without plated holes for high frequency. The latter is the general performance specification for flex. I also noted above the 55110. Some military POs and prints may still call on this one, but it was superseded by 31032 for any new builds after December 31, 1997. It is still an active document for builds performed with designs prior to that deadline.

6. DLA Requirements for the Suitability Lab

The disconnect that the Independent or even in-house electrical test organization has with regard to the military specification is which specification of MIL-PRF-31032 are we supposed to test? Folks there are seven of them! MIL-PRF-31032 B, 1C, 2B, 3B, 4B, 5, and 6! For the test certification to be compliant with DLA requirements the specification, revision, and amendment level must be stated on the certificate of compliance. Table 3 outlines the current requirements.

Table 3: Military requirements. (Click here to enlarge image.)

7. OEM/Manufacturer DLA Expectations

Test centers and manufactures continue to move to gain suitability via the DLA. With this come requirements on reporting, record retention, and traceability. The certificate of compliance needs to state what specification, revision, and amendment level of the specification was used to test the product. This information must be given to the manufacturer and also to the electrical test organization, whether they are independent or in-house to guarantee the product was built and tested to the required specification.Todd L Kolmodin is vice president of quality at Gardien Services USA Inc. located in Hillsboro, Oregon.