PCB Tech - Key Technology Analysis of PCB General Test

1. Introduction

With the continuous emergence of products using large-scale integrated circuits, the installation and testing of the corresponding PCB circuit boards have become more and more important. General testing of printed circuit boards is a traditional testing technique in the PCB industry.

The earliest general electrical testing technology can be traced back to the late 1970s and early 1980s. Because the components at that time were all in standard packages (Pitch is 100mil), and the PCB only had THT (Through Hole Technology) density levels, so European and American testing machines The manufacturer has designed a standard grid test machine. As long as the components and wiring on the PCB are arranged according to the standard distance, each test point will fall on the standard grid point, because all PCBs can be used at that time, So it is called a universal testing machine.

Due to the development of semiconductor packaging technology, components began to have smaller packages and surface mount (SMT) packages, and standard density general testing became no longer applicable. So in the mid-1990s, European and American test manufacturers introduced double-density testing. The machine, combined with the use of a certain steel needle slope to make fixtures to convert the PCB test points and the machine grid connection, with the gradual maturity of the HDI process technology, the double-density general test can not fully meet the test needs, so around 2000, European testing machine manufacturers have introduced a quadruple-density grid universal testing machine.

2. The key technology of general test

a. Switching element

To meet most of the HDI PCB test requirements, the test area must be large enough, usually in the following standard sizes: 9.6*12.8(inch), 16 X12.8(inch), 24*19.2(inch), in double density full In the case of Full Grid, the test points of the above three sizes are 49512, 81920, and 184320 respectively. The number of electronic components is as high as hundreds of thousands. The switching element is a core element to ensure the stability of the test, and it is required to have high voltage resistance (> 300V), low leakage and other properties, and electrical properties such as resistance value must be balanced and consistent, so such components must undergo strict screening and testing, usually transistors or field effect transistors are used as switching components

The advantages and disadvantages of transistors:

Advantages: low cost, strong anti-static breakdown ability, high stability;

Disadvantages: current drive, the circuit is more complicated, the influence of the base current (Ib) needs to be isolated, and the power consumption is large

The advantages and disadvantages of FETs:

Advantages: voltage drive, simple circuit, not affected by the base current (Ib), low power consumption

Disadvantages: high cost, electrostatic breakdown is easy to occur, electrostatic protection measures are needed, stability is not high, so it will increase maintenance costs.

b. Independence of grid points

Full Grid

Each grid has an independent switching circuit, that is, each point occupies a set of switching elements and circuits, and the entire test area can be scattered with four times the density.

Share Grid

Due to the large number of switching elements in a full grid and the complicated wiring, it is difficult to implement. Therefore, some test manufacturers use grid sharing technology to share a set of switching elements and wiring at several points in different areas, thereby reducing the difficulty of wiring. And the number of switching elements, we call it the Share Grid. The shared grid has a big flaw. If the points in an area have been completely occupied, the points in the shared area can no longer be used, and the density of the area is reduced to a single density. Therefore, there is still a density bottleneck in HDI testing of a larger area.

c. The composition of the structure

Modular structure

All switch arrays, driving parts and control components are highly integrated into a set of switch card modules. The test area can be freely combined and interchanged with this module. The failure rate is low, maintenance and upgrades are simple, but the cost is high.

Wire-wound structure

The grid is composed of winding pogo pins and separate switch cards. It is bulky, has no upgrade space, and is difficult to maintain in case of failure.

d. The composition of the fixture

Long needle structure fixture

It generally refers to the fixture structure with a steel needle of 3.75" (95.25mm). The advantage is that the needle spreading slope is larger, and the number of needle spreading points per unit area is 20%~30% more than the shorter needle structure. But the structural strength is poor, and the fixture is made Need to pay attention to strengthening.

Short needle structure fixture

It generally refers to the fixture structure with a steel needle of 2.0" (50.8mm). The advantage is that the structure is strong, but the slope of the needle is small.

e. Auxiliary software (CAM)

In high-density general-purpose testing, proper CAM support is very important, and it mainly consists of two parts:

Network analysis and test point generation;

Assisted production of fixtures.

Since many parameters of the fixture manufacturing process (such as fixture interlayer structure, drill hole diameter, safety hole distance, pillar structure, etc.) greatly affect the fixture test effect, this part must be trained by skilled engineers appointed by the manufacturer, and continuous Only by summing up experience can we make the fixture better.

3. Comparison of double-density and four-density

First of all, four-density can complete the board that cannot be tested with double-density. Because the density of the pogo pin lattice on the needle bed is different from the density of the test points on the circuit board, the steel needle of the test fixture must have a certain slope in order to transform the on grid. Become an off grid, but the inclination of the steel needle is limited by the structure, and it is impossible to increase it indefinitely. Under normal circumstances, the double-density steel needle

The slope (the horizontal offset distance of the test steel needle in the fixture) is 700 mils at the maximum, and the four-density is 400 mils. Then, it is possible that the needle cannot be planted. How many such needles can be calculated.

In addition, the test effect can significantly improve the false dot rate and indentation of the test. The four-density dot matrix density is 400 dots per square inch, and the double density is 200 dots. The area of needle sprinkling on the bottom layer of the fixture can be reduced for the same number of dots. Therefore, the use of four-density can reduce the inclination of the steel needle. In the case of the same fixture height, the four-density of the same test board is basically half of the double-density, and the inclination of the steel needle will be It has a great influence on the test effect. The larger the slope, the smaller the distance in the vertical direction, the pressure of the pogo pin will decrease as a result, and the resistance of each layer of the fixture to the steel pin in the vertical direction increases, causing the pin to contact the PAD. bad. In addition, the end of the inclined steel needle that is in contact with the PCB during the press-fitting process of the upper and lower molds will relatively slide on the surface of the PAD. If the strength of the clamp is not good and deformed, the steel needle is stuck in the clamp. At this time, the steel needle is on the PAD. The pressure is far more than the elastic force of the needle bed pogo pin, and it will produce indentation in severe cases. The slope of the four-density steel needle is smaller than that of the double-density, so there is more space to install the support column on the fixture, which makes the fixture structure more stable. Another advantage of the small slope is that the hole diameter can be reduced, thereby reducing the possibility of hole breakage.

For a BGA with an evenly distributed PAD pitch of 20 mils, the maximum slope of the needle spread is 600 mils for double-density and 400 mils for four-density. The number of points that can be arranged by double-density testing is 441, about 0.17 inch2, while four-density testing is used. The number of points that can be arranged at a time is 896, about 0.35 inch2. It is basically twice the double density, as can be seen from it.