PCB substrate

Brief introduction

PCB substrate is the substrate material used. The substrate has a great influence on the finished PCB withstand voltage, insulation resistance, dielectric constant, dielectric loss and electrical properties such as heat resistance, moisture absorption and environmental protection. The right choice is an important part of the substrate PCB design, which is more important for high-speed PCB design.

Insulating Backbone

PCBs consist of two basic parts: a substrate (the board) and printed wires (the copper traces).
Multi-layer boards require substrates that separate the various layers.

The substrate provides a structure that physically holds the circuit components and printed wires in place and provides electrical insulation between conductive parts.

A common type of substrate is FR-4, which is a fiberglass-epoxy laminate. It is similar to older types of fiberglass boards but is flame resistant.

Substrates are also made from Teflon, ceramics, and special polymers.

PCB Substrates are specialized materials that do not conduct electric currents.For this purpose, they serve as a laminated electrical insulator between circuitry,An electrical insulator is a material whose internal Electric charge do not flow freely, and which therefore does not conduct an under the influence of an Each layer of circuitry, consequently, is connected through plated through holes to connect traces on opposite layers.

Substrate Types
Substrates can be classified into five types, each having a unique set of characteristics for specific applications.

FR-2
This lower grade of substrate is a made of impregnated paper, also called Phenolic, that is easy to machine over a fiberglass material substrate. The “FR” refers to the term Flame Resistant. This type of substrate is usually found on more inexpensive consumer electronics.

FR-4
Fiberglass substrates are made up of a woven fiberglass material and impregnated with a flame resistant material. The material is rigid and can also be drilled cut or machined but due to the abrasive nature of the fiberglass, tungsten carbide tools are needed. An FR-4 is a stronger substrate compared to an FR-2 and is more resistant to cracking or breaking and are usually found in higher end electronics.

RF
RF substrates are comprised of low dielectric plastics and used in printed circuit boards for applications in high power radio frequencies. Although the substrate has poor mechanical properties it has exceptional electrical performance properties.

FLEX
Not all circuit boards use rigid core materials. Some are designed to be very flexible or slightly flexible called flex circuits. Thin and flexible plastics and or films are employed as substrates. The manufacturing process is more difficult than using rigid substrates, but offers benefits that cannot be achieved with rigid substrates such as saving space by bending the circuit board to fit a particular space or where repetitive movement requires a flexible layer. METAL
Power electronics demand a low-thermal resistivity substrate. A ceramic core or metal core substrate provides the necessary characteristics to handle larger copper tracks and the high electrical currents used with these type of circuit boards.

Classification of the substrate

PCB Processing Z arts have an impact on specific indicators above characteristics (such as resistance, voltage, insulation resistance brush warpage, etc.), but basically the base substrate does not meet the requirements of the decision will cause lots of quality problems, Depending on the PCB manufacturing process and the requirements of different varieties · printed substrate with extremely divided into the following types.

(1) copper-clad laminate (CCL abbreviated) of the insulating material -. Rang or both sides with a copper foil laminates for printed circuit board manufactured according to the subtractive method is a rigid insulating material, or soft, divided into rigid plates and rocker plates.

A reinforcing material impregnated with a resin solution, the pre-formed prepreg drying, according to the required thickness with a plurality of prepregs are stacked in one or two outermost surface covered with copper foil, heating, pressing forming The plate-shaped composite material.

CCL is currently the most widely used at home and abroad, the largest amount in subtractive (etching copper) used in the production of printed circuit board substrate.

(2) a prepreg (adhesive sheet). (Semi-cured resin pre-dried state) by default crushed material reinforced resin material for the manufacture of a multilayer printed adhesive grip intermediate insulating material.

Skim reinforcing material impregnated with a solution of a resin, after pre-baking sheet made of pre-impregnated material, the surface is not coated copper foil, a variety of thicknesses, an intermediate layer for multilayer bonding material.

(3) resin coated copper (RCC). Foil side is coated with resin for manufacturers printed pull-density interconnect (HDI)

After the electrolytic copper foil surface treatment, coated with a layer of organic resin prepreg resin B stage with foil structure is the material developed in the late 20th century, mainly for the manufacture of high-density multilayer interconnect method PCB, suitable for miniaturization and thinning of the PCB required, such as a communication device and high-speed circuits for different purposes HDI board.

(4) a photosensitive resin or film. Green with trees Ba or film photosensitive agent. Additive method for rent HDI PCB manufacturing.
The most widely used is the subtractive method (etching copper tube) used for manufacturing a printed base laminate sensitive layer sensitive arrows · It is the largest amount of PCB substrate segment currently abroad. Such material will hereinafter a brief introduction.

The main performance

Key Features

Jifeng electrode printed on the substrate is reflected in the performance of the performance characteristic has the following child.

(1) Mechanical properties of copper tube and the matrix resin adhesion (peel strength) and mechanical strength (e.g., flexural strength), impact resistance, dimensional stability, flexibility, heat distortion resistance, the base processing (punching, drilling performance) and the like.

(2) electrical characteristics of insulation resistance, resistance to the electric field strength (voltage resistance), the dielectric properties (dielectric constant, dielectric loss factor Wei, etc.), ion migration resistance, resistance to leakage and trace quality copper resistance.

(3) the physical characteristics of thermal expansion coefficient (CTE), flammability (flame retardant), a base that is generally warped by the degree of flatness (bow and twist), moisture absorption.

(4) chemical properties. Heat resistance, breaking glass transition temperature (Tg), weldability, resistance to chemicals capacity (acid, alkali and solvent resistance) and the like.

(5) environmental resistance. Fungus resistance, moisture resistance, cooking, resistance to thermal shock properties.

(6) Green living substrates suitable for RoHS Directive inauspicious lead and other harmful substances halogen chapter though - .. Some high reliability requirements of the product is not currently required · main lead solder must be used, but from the perspective of environmental protection jealous consideration. When conditions are ripe to make the use of non-polluting or less polluting materials.

Exterior

Refers to the appearance of the substrate to be able to directly observe the irresolute substrate quality status, into the substrate surface and the substrate surface under two conditions described below.

A substrate surface

(1) allows to see the resin-covered fabric fiber textures · But Lucy (substrate exposed braid reinforcement and resin in a state) can not make the remaining area of ​​the spacing between the wires is less than the minimum wire spacing requirements [Figure 12-2
(A)], three products should not be exposed printed fabric pull [GJB362A a 1996 provision in Figure 12-2 (b)] because the board table will reveal the fabric and rough, dirt and moisture sorption easily affect grip Table insulation resistance and voltage resistance.

(2) halo is caused due to mechanical processing and the base table or on an internal layer, destruction phenomenon usually occurs around the hole edge of the plate or other mechanical or machined parts.

Presented whitening phenomenon on the substrate. Range halo edge or edges of the panel can not make the hole nearest unaffected between the conductive pattern of more than 50%, or decrease the distance 2.5mm. Whichever is the smaller value (see Figure 12-3) if compared with halo large distance from the conductive pattern close. Plate will reduce the insulation resistance and the welding base material due to heat is easy to extend the conductor delamination or warped.

(3) of the exposed fibers or fiber breakage bridging defects generated neither the wire, and the wire is not less than the minimum distance required to ask, it refers to the defect between two adjacent wires of the wire can not be filled in the wire of the Distance * There is no room for a period of at least or greater than the forces at the smallest spacing of the defects (see Figure 12-4), which also affect the insulation defect between the wires, so the minimum wire insulation is not required in the presence of this distance Q. Class defects.

(4) holes and pitting diameter of not more than 0.8mm, and did not ask the conductor to produce a bridge, every shadow area by less than 5% of total board area (see Figure 12-5). Emphasize here that the two points, that defects The diameter and empty, the total surface pitting job if larger voids and pitting. both affect the quality plate appearance of the Methodist board will reduce the electrical performance in high-speed circuits, high-frequency dielectric Road also empty and hemp When points are above or below the transmission conductor will affect the characteristic impedance of the transmission line.

2. substrate surface state

(I) from the substrate surface under the spot. Except for the high voltage field report, the products are all white levels are acceptable (see Figure 12-6), which means that as long as the situation is not at a high voltage board using self spots are not affected because the PCB substrate generally also high withstand voltage up to 1200V/mm more severe since the spots will reduce electric strength plate, but even reducing the dielectric strength, withstand voltage value is higher than the general will use it in case of the absence of the two voltages - like can be used, but in this white spot welding or thermal shock test if allowed to expand from the spot welding or expanded after the test .. . Indicates that the internal substrate delamination, generally can not be used. Especially for 2,3-level high reliability requirements of PCB are not allowed.

(2) Since the filaments inside the base reinforcement material separation occurs microcracks twisted Lai formed (within the base table and the next point is connected or "cross" pattern). Defect shall be less than the minimum distance between the wire conductor spacing value and after the heat stress testing should be expanded, the very edge of the micro-pattern does not reduce the minimum required distance between the edge of the conductive pattern sensitive, if it is not required to be greater than or equal spacing 2.5mm. products for a printed circuit board Diffusion microcracks area allowed to exceed 50% of the conductive pattern spacing between the conductive pattern, but you can not bridge the receiver; For two, three printed products hit the premise of meeting the above requirements, the emblem of the region can not crack more than 50% of the distance between the adjacent conductive pattern (Figure 12-7). emblem as excessive cracking roar affect strength properties of the substrate and the Units will expand when heated and some microcracks, so if a crack in said emblem within the specified range, and no extension test after thermal stress, it can be received.

(3) from the tank and the surface of the substrate is layered with a separation phenomenon between the inner base insulating layer is not more than the area of ​​each surface defects into the plot of 1%. Defects of the conductive pattern Q is not reduced to less than a small section from the wire spacing of the following requirements, and does not extend a small distance from the section edge of the defect and the subject is not smaller than the predetermined conductive pattern between the plate edge and in the thermal stress test defects. If you do not defect to the provisions of the implant edge is not less than 2.5mm; under the premise of the above requirements are met, a very defective products span a greater distance between adjacent wires can receive 25% of the time, two, three products planting. span defects are not allowed more than 20% between adjacent conductive patterns minister (see Figure 12-8).

The defect if it continues to expand in the heat stress tests, indicating that poor quality laminated substrate. Unavailable if you are using this board, wave soldering may cause a large area of ​​delamination or blistering, resulting printed pole assembly scrap, will result in greater losses.

(4) in the insulating material entrapped metal or nonmetal inclusions of foreign matter (see Figure 12-9), if the micro-inclusions are translucent control (generally not metal) can be received, if the inclusions are opaque ( may be metal), the distance from the nearest grain emblem conductive pattern should not be less than 0.125mm particles that did not make the distance between adjacent conductors ask reduced to below the minimum spacing requirements; If not specified, then the paragraph spacing should be small not less than O.125mm., and should not affect the electrical properties of the printed circuit board, because metallic inclusions between the conductor insulation material inside is opaque, easy to reduce dielectric strength between the wires, so the requirements should ensure that the conductive pattern The minimum spacing between conductive.

3. surface scratch insanity, indentation plate surface pits and scratches, pits, dents and other defects should not result in enhanced fiber fabric base material is cut and exposed fabric or disturb the insulation spacing between the wires small dry prescribed minimum (equal to or greater than a predetermined value O. 125mm) the defect may exceed the standard grade defects exist in the easy absorption of moisture caused by dust or insulation resistance or withstand voltage performance.

Rigid PCB substrate introduction

1, phenolic paper laminate

Phenolic paper laminate can be divided into different levels, most capable high level of 70 ~ 105 ℃ temperature use, long-term working temperature is higher than this range may result in reduced number of properties. And it could cause charring in the affected area, the insulation resistance may be reduced to a very low value. Placed under high humidity conditions can cause the insulation resistance of the substrate is drastically reduced, but when the humidity decreases, the insulation resistance will increase.

2, epoxy paper laminates

Compared with phenolic paper laminate, epoxy laminates paper has greatly improved the performance of electrical and non-electrical aspects, including better machining and mechanical properties. According to the thickness of the material, the use of temperature up to 90 ~ 110 ℃.

3, polyester glass mat laminate

Mechanical properties of the material below the glass cloth, but higher than the low-quality material. Has good impact resistance, good electrical properties, it is possible in a wide frequency range of applications in high humidity environments, to maintain a good insulating properties. Temperature up to 100 ~ 105 ℃.

4, epoxy glass cloth laminated sheet

Lower mechanical properties than the base material, bending strength, impact resistance, X, Y, Z axis dimension stability, warpage and good resistance to soldering heat shock than low quality materials, electric performance is better, the temperature can be used up to 130 ℃. affected by the harsh environment (humidity) little affected.

About PCBWay