PCB Substrate Quality Parameters

As the PCB is a structural support for electronic components, electrical, heat-resistant and other properties have strict requirements, large-scale electronics companies set up testing centers for comprehensive testing of its performance, now the main parameters and changes in these parameters on the

PCB performance is introduced as follows.

1 glass transition temperature (Tg)

In addition to ceramic substrates, almost all laminates contain polymers. Polymer is synthesized from organic materials, which is characterized by a certain temperature conditions, the substrate form will change, at this temperature the substrate is hard and brittle, that is, similar to the form of glass, usually referred to as glassy; if the temperature above, the material will become soft, rubber-like form, also known as the rubber state or leather state, at this time it is significantly lower mechanical strength, and therefore the decision to the critical temperature of the performance of the material is known as glass transition temperature (Tg). Therefore, the critical temperature to determine the performance of the material is called the glass transition temperature (GlassTranstion Temperture, referred to as Tg). Obviously, as a structural material, people hope that its glass transition temperature is as high as possible, the glass transition temperature is unique to the performance of polymers, it is a key parameter in the choice of substrate, this is because in the SMT welding process, the welding temperature is usually in the 220 ℃ or so, much higher than the PCB substrate Tg, so the PCB by the high temperature will be a significant thermal deformation, while the chip components are Directly welded to the PCB surface, when the welding temperature is reduced, the solder joints are usually cooled at 180 ℃ on the first solidification, and at this time the PCB temperature is still higher than the Tg, PCB is still in a state of thermal deformation, after a period of time in order to completely cool down, at this time, the PCB will inevitably produce a lot of thermal stress, the stress on the pins of components have been welded, and when the seriousness of the components will make the component is damaged, as shown in the figure.

 PCB thermal stress damage to components

Through testing, it can be seen that when the circuit substrate is in the substrate polymer glass transition temperature above the temperature, its expansion will be greater than in the Tg temperature below the same temperature rise in the amount of expansion, in the glass transition temperature below the amount of thermal expansion of the substrate material and the temperature approximate linear relationship, that is, the substrate material CTE approximate constant, and once the temperature exceeds the material's glass transition temperature, the amount of thermal expansion of the substrate material will be with the Temperature into an exponential relationship, that is, with the increase in temperature, CTE increases exponentially.

Therefore, in the selection of circuit substrate materials, the glass transition temperature Tg should not only be higher than the circuit operating temperature, but also as close as possible to the highest temperature occurring in the process.

Tg high PCB has the following advantages: drilling process, conducive to drilling micro-holes, low Tg plate drilling will be due to high-speed drilling to produce a large amount of heat energy, and cause the plate in the resin softens so that processing difficulties. Tg high PCB in the higher temperature environment still has a relatively small CTE, and the CTE of the chip components is close to the CTE, so it can ensure that the product works reliably. Especially with the introduction of FQFP, BGA, CSP and other multi-pin devices, PCB requirements are increasingly high. After the components are soldered at high temperature, the thermal deformation of PCB will produce high thermal stress on the components, therefore, when choosing PCB substrate for electronic products, the substrate with higher Tg should be chosen appropriately.

2 Coefficient of thermal expansion (CTE)

Any material will expand when it is heated, and the Coefficient of Thermal Expansion (CTE) refers to the amount of linear change in the size of the material triggered by each unit of temperature change.

The CTE of polymer materials is usually higher than that of inorganic materials, and when the expansion stress exceeds the material's tolerance limit, it will cause damage to the material. For a multilayer PCB, there is a difference between the CTE in the X and Y directions (i.e., length and width directions) and the CTE in the Z direction (thickness). Therefore, when the multilayer board is heated, the metallization holes in the Z-direction will be damaged due to the difference in expansion stress, and in severe cases, the metallization holes will be fractured. Because the multilayer board is made of several single-layer "semi-cured resin sheet" hot pressing, semi-cured resin sheet is made of fiberglass cloth impregnated with epoxy resin, heating and baking to make epoxy fiberglass cloth in a semi-cured state, and then the semi-cured sheet stacked layer by layer, such as the need to do the inner layer of the circuit, but also according to the requirements of the inner circuit should be placed on the copper foil, and finally will be stacked up several layers of semi-cured sheet. Stacked layers of half-cured sheet hot press molding, cooling and then drilling and plating in the desired location, and finally generate plated holes, also known as metallized holes.

Metallized holes are made, but also to achieve the interconnection between the PCB layer and layer. As the hole wall after drilling on the substrate is almost epoxy resin, it will not be very high with the combination of copper-plated layer. General metallization hole hole wall is only about 25 μm thick, and copper layer densification is low, the structure of the early multilayer board on the metallization of the hole to leave a certain amount of hidden trouble, that is, semi-cured sheet in the glass fiber cloth due to enhancement as well as the constraints of the multi-layer copper wiring, usually CTE significantly reduced to epoxy semi-cured board, for example, each layer of the CTE for (13 ~ 15) × 10-6 / ℃. And the multilayer board layer to layer mainly rely on the bonding force of epoxy resin itself to achieve bonding, so the epoxy resin in the absence of other material enhancement and constraints, its CTE will be significantly larger after heat, usually (50 ~ 100) × 10-6 / ℃. The semi-cured sheet layer is in the X-Y direction, while the semi-cured sheets are in the Z direction between them, so there is a significant difference between the CTE in the X-Y direction and the Z direction. Then, because the hole wall of the metallized hole is thin and the structure of the copper plating layer is not too dense, when the PCB is heated, the thermal stress in the Z-direction acts on the hole wall of the metallized hole, and after applying stress to its fragile part, it will lead to a fracture or partial fracture of the hole wall.

Such defects cannot be predicted in advance, and sometimes hidden defects occur after the electronic product has been in use for some time due to a variety of reasons such as fatigue, as shown in the figure.

In SMT products, the density of PCB wiring is increasing, the number of metallization holes is increasing and the diameter of the holes is getting smaller, and the number of layers in multilayer boards is also increasing. In order to overcome or eliminate the above hidden problems, the following measures are usually taken:

(1) Concave etching process to enhance the bonding force between the metallized hole wall and the multilayer board;

(2) Appropriate control of the number of layers of the multilayer plate, currently advocating the use of 8 to 10 layers, so that the diameter-depth ratio of the metal holes in the control of 1:3 or so, which is the most secure diameter-depth ratio, the most common diameter-depth ratio is about 1:6;

(3) the use of relatively small CTE materials or the use of CTE performance of the material stacked on top of each other, so that the overall CTE of the PCB is reduced; (4) in the PCB manufacturing process, the use of blind holes and buried holes in the technology, as shown in Figure 8-6, in order to achieve the purpose of reducing the diameter-depth ratio, which is the most desirable approach. Blind vias are interconnections between the surface layer and certain internal delaminations, without having to go through the entire substrate, reducing the depth of the hole; buried holes are only interconnections between internal delaminations, which can further reduce the depth of the hole. Although blind holes and buried holes in the production of difficult, but has greatly improved the reliability of the PCB, through the PCB light board test can be judged whether the line network is connected.

After taking the above measures, effectively prevent the product in the use of the phenomenon of metallization hole breakage occurred

3 flatness and heat resistance

1. flatness due to the process characteristics of SMT, the current PCB requires a high degree of flatness, so that the surface mount component pins and PCB pads closely match. Therefore, the PCB pad surface coating layer not only use Sn / Pb alloy hot air leveling process, and a large number of gold-plated process or preheated flux coating process to improve its flatness.

2. heat resistance usually SMT process is sometimes required by two reflow soldering, and thus after a high temperature, still require to maintain the flatness of the board, in order to ensure the reliability of the second patch; and surface mount components pads are getting smaller and smaller, the bond strength of the pads is relatively small, if the PCB using the substrate heat resistance is high, then the pads of the peeling strength is also high, the general requirements for the SMT process of the PCB can have a 250 ℃ / 50 s of heat resistance.

4 Electrical Performance and Characteristic Impedance

1. Electrical Performance

Due to the development of wireless communication technology to high-frequency direction, the high-frequency characteristics of the PCB requirements to improve, especially the expansion of mobile communication systems, the frequency used by the short-wave band (300 M ~ 1 GHz) into the microwave band (1 ~ 3 GHz).

(1 ~ 3 GHz). The increase in frequency leads to an increase in the dielectric constant (ε) of the substrate. Usually, the transmission speed V (m/s) of a circuit signal is related to ε. The frequency of a circuit signal is determined by the dielectric constant of the substrate:

where K is a constant, C is the speed of light, and ε is the dielectric constant of the PCB. When ε of the PCB increases, the transmission velocity V of the circuit signal decreases.

For example, the ε of the polytetrachloroethylene substrate is 2.6 to 3, and the ε of the epoxy substrate is 4.5 to 4.9, and the former is 35% to 47% lower than the latter, and if the former is used to make the PCB, the signal speed is 40% faster than the latter.

In addition, if analyzed from the perspective of signal loss, the dielectric material will consume energy due to heat generation under the action of an alternating electric field, which is usually expressed by the dielectric loss angular tangent (tg δ), and in general tg δ is proportional to ε.

If tg δ increases, the medium absorbs more energy and the signal loss is large; this relationship is more obvious at high frequencies, which directly affects the efficiency of high-frequency transmission signals.

In short, ε and tg δ is an important parameter to assess the electrical performance of PCB substrate, when the circuit's operating frequency is greater than 1GHz usually require substrate ε <3.5, tg δ <0.02. In addition, the assessment of the electrical performance of the substrate index of the electrical strength, insulation resistance, resistance to arcing performance.

Characteristic impedance when the pulsating electricity through the conductor, in addition to resistance, but also by the inductive resistance (XL) and capacitive resistance (XC) resistance, the circuit or component through which the AC current produced by the obstruction is called the impedance, and in the digital communication products such as computers, printed circuits are transmitted by a square-wave signal, usually also known as pulse signals, belonging to the nature of pulsating alternating current, so that the transmission encountered resistance Early PCB printed line, only play a PCB level between the interconnection between the components and parts of the function, but with the high-speed digital electronic products, such as CPU, the current mainstream products are in the 3.0 GHz or so, and the next five years of development plans to reach 10 GHz.

 PCB as a support for electronic components is no longer a simple electrical interconnection device, PCB demand is also not only satisfied with the PCB. PCB demand side is also not only satisfied with the printed line conduction function, but should be used as a transmission line, the need for ideal transmission characteristics.

Affect the Z0 value of a variety of factors, such as the dielectric constant of the insulating layer ε, the thickness of the insulating layer H, the printed wire width W, the thickness of the conductive layer T (including the thickness of the gold-plated layer), its ε, H, T and the characteristics of the PCB substrate itself. In the manufacture of multilayer boards in the insulation thickness of the accuracy of the Z0 precision control is the most important factor, followed by the width of the wire.

In actual production, first of all, through the improvement of the process to improve the accuracy of the thickness of the semi-cured sheet to control the thickness of the molding board H, the use of composite metal layer (Cu/Al) to control the thickness of the copper foil; to improve the formulation of the etching solution, as well as the location of the exposure process to control the width of the wire; the use of a new type of substrate in order to control the PCB's ε.

Through the improvement of the process mentioned above to make the PCB's Z0 accuracy has been significantly improved, and to achieve a controlled state. improved and a controlled state is achieved.