E-Test

At the end of PCB fabrication process, Electrical Test (E-test) is used to verify the interconnectivity of the bare PCB is correct against the board netlist file as per IPC-9252 Guidelines and Requirements for Electrical Testing of Unpopulated Printed Boards. MADPCB performs 100% PCB E-test on all bare PCBs produced, including rigid PCB, FPC, Rigid-flex PCB, at its facilities before packaging and shipping to your hand.

 

PCB E-test Required Netlist File

 

For PCB E-test, we require the netlist file (formed as IPC-356), which is nothing more than an ASCII text file that includes instructions for the board CAM software such as net names, pin, and X/Y coordinates of start and end points for each net or node. If you can supply an IPC-365 netlist then it is ready in during the initial Gerber files loading; If you cannot supply, we can create a reference netlist from your original Gerber files for the purpose of electrical test, but we do not consider this to fall under best practices. We much prefer to have original netlist that was generated after schematic design, since this can help to catch any errors that might occur during either PCB layout or Gerber file generation. For this reason, we do not include the netlist among our essential PCB design files, and we can of course develop your quotation in the absence of the netlist, but we do strongly recommend keeping this file on hand if at all possible.

PCB designers will likely know of the netlist file through its function in the design process, which is to translate the connections made during schematic design into nets for PCB layout. This purpose might seem somewhat limit, but your PCB’s netlist is also quite useful during the production of your boards, particularly during the PCB E-test phase of the process.

The nets defined by your netlist files are simply collections of pads on the PCB that should be electrically connected to one another. For example: most every design will have a GND net to define all of the pads should be connected to ground. Every pad on the board is included in the netlist file, which makes it an invaluable tool for testing your completed bare PCB boards against your intended design. PCB E-test makes use of your netlist file to compare test results against intention.

 

Key Terms of PCB E-test

 

• Net: an entire string of points or connections from the first source point to the last point including component lands and vias.
• Netlist: a list of alpha-numeric locations that are used to describe a group of two or more points that are electrically common.
• Shorts Test: check to make sure that no current flows between separate nets by measuring the amount resistance between them.
• Opens Test: check to make sure there is current flow from one “node” to the next for every net on the PCB board.
• 100% Netlist Test: check of every “node” on every net on the PCB board.
• Optimized Net List Test: check every “end of net” for all nets on the PCB board and selected intermediate nodes in the PCB board.

 

PCB E-test Capabilities and Types

 

Our electrical test capabilities are listed in the following table:

 

Min Continuity Resistance	0.1 ohms
Max Test Voltage	1,000 volts
Max Isolated Resistance	25M – 2G ohms
Electrical Test Pitch (Fixture)	20mil (0.5mm)
Electrical Test Pitch (Flying Probe)	4mil (0.1mm)

 

Depending upon both the design and the cost, our testing methodology will either be:

• Fixture
• Flying Probe

 

Plying Probe vs. Fixture

 

Sometimes customers are unclear about the difference between Fixture and Flying probe, as well as the costs associated with them. The following example will clarify the difference between the two methods, as well as the cases in which one or the other might yield a more efficient PCB assembly process.

For this example, let’s assume there are 3,500 test points with 750 nets.

 

Test Fixture E-test

 

• Electrical Test Pitch=20mil (0.5mm)
• Setup time: 30 minutes
• Learn time: 0 minutes
• Debug /check for missing pins: 10 minutes
• Test time per board: 40 seconds
• All points are tested simultaneously. However, this process involves set-up to create the fixture itself and additional costs associated with the cost of materials.

 

Flying Probe E-test

 

• Electrical Test Pitch=4mil (0.1mm)
• Setup time: 0 minute
• Learn time: 20 minutes
• Test time per board: 6 minutes
• The points are tested one at a time. There s no setup time because the machine learns the program as it operates.

 

How Bare PCB Should Be Tested?

 

According to Section 4.4 States of IPC-9252 Guidelines and Requirements for Electrical Testing of Unpopulated Printed Boards, 100% continuity and isolation electrical test is the confirmation that the actual electrical interconnect of conductive nodes matches a proven reference source, including but not limited to CAD/CAM digital data, master pattern artworks, or released drawings.

Basically, what it is saying is that an “Optimized Net List Test” is sufficient to meet the guideline for the testing of a bare printed circuit board. We test for opens and shorts based on optimized data from the Gerber files. Ideally, when a bare board is tested you can be assured that it is a good board. However, that relies upon the assumption that the board has been designed correctly. A board that has problem with the design can still pass a bare board electrical test and not function correctly. This does not happen very often but is always a possibility.

We measure closely how many boards do not pass electrical test as a measure of the quality from our manufacturing team. If, for some reason, we see an increase in the number of PCB test failures, we can go bac and review our processes to improve the quality.

But how to confirm whether the batch PCB manufacturing is qualified, or not? Upon request, we supply bare PCB test reports, which should include but not limit to:

 

• PCB E-test Report
• Micro-section Sample
• Solderability Sample
• Solderability Report
• Impedance Coupon
• TDR Test Report (for impedance-controlled PCBs)
• Certificate of Compliance (COC)