PCB Tech - The method of PCB board inspection and testing?

PCB inspection and testing refers to the inspection and testing of quality control, final product performance and service life (lifetime) reliability during the PCB production process. Through these inspections and tests, the defective or defective PCB products are removed to ensure the reliability of the PCB products during the service life.

1. Evaluation of PCB product quality and reliability

The quality and reliability evaluation of PCB products generally uses the PCB board or test sample used in the complete machine to inspect and test the following items, and then evaluate them.

(1) Visual inspection.

Use visual inspection or magnifying glass to inspect the surface of the product (raw and auxiliary materials and PCB, etc.) for abnormal appearance, such as scars, colors, contaminants, residues, obvious open circuits and short circuits.

With the development of high density and refinement, it is necessary to use AOI (Automatic Optical Inspection Machine) to inspect the appearance of products, and even use scanning electron microscope (SEM) to inspect and measure copper foil surface micro-corrosion, inner surface oxidation treatment, and drilling Hole wall roughness, etc.

(2) Inspection of microsection cut surface.

Use a metallurgical microscope to observe whether there are any abnormalities or sizes in the plated through holes or via holes, such as the inside and outer layer patterns, etc., for evaluation, such as the roughness of the drilled hole wall, the de-drilling condition of the hole wall, the thickness distribution of the plating layer and the defect condition, and the layer Alignment and structure, as well as the situation after various aging tests, and so on.

(3) Dimensional inspection.

Use tool microscope, coordinate measuring instrument or various measuring tools to measure the shape, hole diameter, hole position, wire width and spacing, land size, position relationship and flatness (warpage, deformation) of the board surface. Evaluation.

(4) Electrical performance test.

Various electrical performance test equipment are used to test the "on" and "off" (or "open", "short") of the loop (line), conductor resistance (conductor/via/inner connection) measurement, Insulation resistance (loop and loop, layer and layer, etc.) test, current resistance (wire, via or plated through hole) test, and voltage resistance (surface layer, layer and layer) test.

(5) Mechanical performance test.

Various test devices and jigs are used to measure the peel strength of copper foil, the peel strength of the copper plating layer (adhesion), the pull-off strength of plated through holes, ductility, bending resistance, bending resistance, solder resist and marking symbols. Adhesion and hardness test.

(6) Aging (lifetime reliability) test.

Various test devices are used to test and evaluate high and low temperature cycle resistance, thermal shock resistance (gas/liquid phase, such as float welding test), temperature and humidity cycle resistance, and interconnect stress test (IST).

(7) Other tests.

Various test devices are used to conduct tests and evaluations of combustion resistance, solvent resistance, cleanliness, solderability, soldering heat resistance (reflow soldering, reflow soldering, etc.), migration resistance, etc.

In recent years, due to the rapid development of high-speed signal transmission, digitization and multi-functionalization of electronic products, significant changes, progress and diversification have taken place in the substrate, PCB product use environment and installation technology. Therefore, the conditions and methods of testing and evaluation must be adjusted and changed accordingly. Such as fine pattern (or fine line width/spacing) and microelectrode (connection pad) bonding strength and insulation characteristics test, thin multilayer board characteristic impedance control and measurement, migration resistance test, high-frequency characteristics (substrate Test and evaluation of high-frequency characteristics, insulation resistance of high GHZ tape, copper foil treatment layer, etc., and test conditions and evaluation of heat resistance (bonding strength) using lead-free solder, etc.

It is also worth noting that due to the obvious shortening of the production cycle of PCB products, it has become more and more important to shorten the test and evaluation time and reduce the cost of the test and evaluation when performing reliability evaluation. To this end, the development of new test methods or accelerated test methods and evaluation has become an urgent task.

The conditions and methods of the above tests and evaluations will be in the PCB production process, the final product and the product aging (service life) test and evaluation, and the relevant items will be selected for the test and evaluation.

2. Electrical testing of PCB products

The electrical test referred to here is the "on", "off" or "open" or "short" circuit test in the PCB product to check whether the network status in the PCB product meets the original PCB design requirements. Due to the rapid increase in density of PCB products, the needle-bed contact test has reached its limit, and it will inevitably move towards non-contact test methods in the future.

2.1, contact test

2.1.1 Needle bed test with fixture

(1) General needle bed test. Test using grid matrix needle bed structure, each grid node is equipped with gold-plated spring

Needle and pogo pin seat, one end of the pogo pin seat is a circular groove to facilitate the hard needle in the test fixture to push in and contact. The other end is connected with the switch circuit card. The contact pressure between the needle tip and the test point on the board surface is required to be greater than 259 grams to ensure a good contact.

The grid node size has changed from 2.54mm to 1.27mm, 0.635mm, 0.50mm, and even as small as 0.30mm. The failure rate is high and has reached the limit.

(2) Dedicated needle bed test. The test points required by the PCB are connected to the switch circuit card, thereby eliminating the need for a grid of test needle beds, but a special test fixture must be made.

There are also the test limits and damage test points caused by high density.

2.1.2 Test without fixture

(1) Move the probe (flying probe) test.

It tests the "on" and "off" conditions of each grid by moving probes (multiple pairs) on both sides. Because it is a "series" test, it is slower than the "parallel" test of a needle bed, but it can test high-density PCB boards. Such as BGA and µ-BGA, even the pitch as small as 0.30mm can be competent. But there is also the problem of bumping the test point.

(2) Universal fixtureless test (UFT). The test heads are alternately arranged in an array to form a double-density test substrate. Such a high density can ensure that no matter where the PCB is placed on the test platform in any direction, the test points can be tested by more than two test heads. The density of this test head can reach 11,600 test heads per square inch. At present, this method has not been promoted and applied.

2.2 Non-contact testing

(1) Electron beam test. This is a test point that distinguishes between charging and non-charging by collecting secondary emitted electrons, so as to judge the "open" and "short" paths. The steps are as follows:

1. Charge the test disk of a node in the N network (that is, the N network is charged to a certain voltage value);

2. Use electron beam to detect other nodes of this network. If this node cannot test the second emission electrons, there is an open circuit in this network;

3. Test the nodes of the N+1 network at the same time. If the secondary emission of electrons is tested, it indicates that the N+1 network and the N network form a short circuit.

(2) Ion beam test.

(3) Photoelectric test or laser beam test.

In short, the quality of PCB products is produced, more precisely, it is produced by quality control during the production process. PCB products are produced through many processes, so the quality of PCB products is the comprehensive result of the production quality of each production process, such as the final product qualification rate is the result of the product of the semi-finished product qualification rate of each production process. That is to say, the quality of PCB products is mainly determined by the worst production process, equipment and operators, which fully illustrates the importance of PCB products in the production process.