One, the introduction

With the emergence of large-scale integrated circuit products, the installation and testing of PCB has become more and more important. The general test of printed circuit board is the traditional test technology of PCB industry.

The earliest universal electrical testing technology can be traced back to the late 1970s and early 1980s. Since components at that time all adopted standard package (Pitch 100mil) and PCB only had THT (through-hole technology) density level, European and American test machine manufacturers 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 the universal test machine.

, thanks to the development of semiconductor packaging technology components begin to have a smaller package and SMT (SMT) encapsulation, universal test standard density began to no longer applies, then in the mid – ninety – s, us and European manufacturers also introduced a double density testing machine, combined with the use of a certain steel slope manufacturing grid connection machine and fixture to convert PCB test points, With the gradual maturity of HDI manufacturing process, double density universal testing can not fully meet the requirements of testing, so around 2000, The European test machine manufacturers launched a four-fold density grid universal testing machine.

Second, the key technology of general testing

1. Switching element

To meet the test requirements of most HDI PCBS, the test area must be large enough, usually with the following standard sizes: 9.6×12.8(inch), 16 ×12.8(inch), 24×19.2(inch), in the case of double density Full Grid, the test points of the above three sizes are respectively 49512, 81920, 184320, the number of electronic components is up to hundreds of thousands, Switching element is a core component to ensure the stability of the test, and it is required to have high pressure resistance (& GT; 300V), low leakage and other properties, and electrical properties such as resistance value should be balanced and consistent, so this kind of components must go through strict screening and detection, usually with transistors or field-effect tubes as switching components

Advantages and disadvantages of crystal triode:

Advantages: low cost, strong antistatic breakdown ability, high stability;

Disadvantages: current drive, complex circuit, need to isolate base current (Ib) influence, high power consumption

Advantages and disadvantages of FETS:

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

Disadvantages: high cost, electrostatic breakdown easily, need to add electrostatic protection measures, stability is not high, so it will increase the maintenance cost.

2. Independence of grid points

Full Grid

Each grid has an independent switching loop, that is, each point occupies a group of switching elements and lines, the whole test area can be four times the density of the needle.

Share Grid

Due to the large number of switching elements in full Grid and the complexity of the circuit, it is difficult to realize, so some test manufacturers use Grid sharing technology to make several points in different areas Share a group of switching elements and circuits, so as to reduce the difficulty of wiring and the number of switching elements, which is called Share Grid. One of the major defects of shared grids is that if the points in an area have been completely occupied, the points in the shared area can no longer be used, thus reducing the density of the area to a single density. Therefore, there is still a density bottleneck in HDI testing in a large area.

3. Structural composition

Modular construction

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 by the module, and can be interchangeable, low failure rate, simple maintenance and upgrade, but high cost.

Wound structure

The mesh is composed of winding spring needle and separation switch card, which has huge volume and no space for upgrading, and is difficult to maintain in case of failure.

4. Structure of fixture

Long needle structure fixture

Generally refers to the steel needle is 3.75″(95.25mm) of the fixture structure, the advantage of large needle slope, unit area can be scattered needle points than the short needle structure more than 20%~30%. But the structural strength is poor, fixture production should pay attention to strengthen.

Short needle structure fixture

Generally refers to the steel needle is 2.0″(50.8mm) fixture structure, the advantage of structural strength is good, but the slope of the needle is small.

5. Auxiliary software (CAM)

Proper CAM support is important in high-density universal testing and consists of two main components:

Network analysis and test point generation;

Fixture assistant production.

As a result of fixture production process of many parameters (such as fixture layer structure, hole aperture, safety hole distance, pillar structure, etc.) are greatly affected fixture test effect, this part must be assigned by the manufacturer skilled engineer training, and constantly sum up experience, in order to do better fixture.

Three, double density and four density comparison

First, we can finish four density double density board can’t test, the spring on the bed because needle lattice density and the density of test point on the PCB test fixture for different steel must have a certain slope, turn on the grid you can be off the grid, the Angle steel is, however, is limited by the structure, can’t be infinitely more, In general, double – density steel needles

The slope (the horizontal offset distance of the steel needle in the fixture) is up to 700mil, and the four-density is 400mil. Then, it is possible to produce the phenomenon of unable to plant the needle, how many such needles can be calculated.

In addition, can obviously improve test in the test results of false rate and creasing, four dimensional lattice density per square inch 400 points, double density at 200 points, the same points in a fixture on the bottom and needle area can reduce the half, so, using four density can reduce the Angle steel, the fixture under the condition of the same height, The same slope and needle four density test plate is basically half of double density, the Angle steel needle can effect has a great influence of the test, the slope is vertical distance is reduced, spring pin pressure will decrease, and fixture in each layer of steel in the vertical direction of resistance increases, lead to bad steel before contact with the PAD. In addition, in the process of up and down molding, the end of the inclined steel needle in contact with THE PCB will have a relative slide on the PAD surface. If the strength of the fixture is not good and deformed, the steel needle will be stuck in the fixture. At this time, the pressure of the steel needle on the PAD will be far more than the elastic force of the needle bed spring needle, which will cause indentation in serious cases. Four-density steel needle slope is smaller than double density, there is more space to install support columns on the fixture, so that the fixture structure is more stable. Another advantage of a smaller slope is that it reduces the hole size, thus reducing the possibility of hole breakage.

For BGA with PAD spacing of 20mil evenly distributed, the maximum slope of needle scattering is 600mil for double density test and 400mil for four density test. The number of points that can be arranged by double density test is 441, about 0.17inch2, and 896, about 0.35inch2, respectively. It is basically a double density, from a spot.