SMT technology introduction

Surface mount technology (SurfacdMountingTechnolegy, referred to as SMT) is a new generation of electronic assembly technology. Cost, and automation of production. This kind of miniaturized components is called: SMY device (or SMC, chip device). The process of assembling components on a printing (or other substrate) is called the SMT process. The related assembly equipment is called SMT equipment.

At present, advanced electronic products, especially in computer and communication electronic products, have generally adopted SMT technology. The output of SMD devices has increased year by year, while the output of traditional devices has decreased year by year. Therefore, SMT technology will become more and more popular with the passage of time.

SNT process and equipment

<1>Basic steps:

The SMT process has three basic operation steps: coating, mounting, and welding.

Coating

-Coating is to apply solder paste (or curing glue) to the PCB board. Coating-related equipment is: printing machine, paste dispenser.

-Coating-related equipment is printing presses and ointment dispensers.

—Coating equipment that our company can provide: precision screen printing machine, tubular multi-point three-dimensional precision printing machine.

Placement

—Mounting is to mount the SMD device on the PCB board.

—Related equipment placement machine.

—The placement equipment that our company can provide: automatic placement machine, manual placement machine.

Reflow soldering:

—Reflow soldering is to heat the component board to melt the solder paste to reach the electrical connection between the device and the PCB board pad.

—Related equipment: reflow oven.

—The company can provide SMT reflow soldering equipment.

<2>Other steps:

There are other steps in the SMT assembly process: cleaning, inspection, and rework (these process steps are also used in the traditional wave peak process):

Cleaning

—Wash the harmful residues in the welding process. If the solder paste is a no-clean solder paste, this step can be omitted.

—Related equipment: Gas-phase cleaning machine or water cleaning machine. Inspection—Inspect and test the electrical function and solder joint quality of the component board.

-Related equipment online instrument, X-ray solder joint analyzer.

Repair

—If the components are found to have quality problems during testing, they need to be repaired, that is, the SMD components with quality problems are removed and re-soldered.

—Related equipment: repair machine.

—The company can provide repair machine: hot air repair machine.

<3>Basic technological process and equipment:

Start --->

Coating: Use a printer to print the solder paste or curing glue on the PCB

Mounting: attach the SMD device to the PCB board

--->Reflow soldering?

Qualified<--

Qualified or not<-

Detect

Cleaning

Reflow soldering: perform reflow soldering

Unqualified<--

Wave soldering: use a wave soldering machine for soldering

Curing: heat the component to cure the SMD device on the PCB board

Rework: remove and re-solder the defective components on the component board

SMT related knowledge

Hot-press the laminated laminates and control them appropriately to avoid excessive exudation of the prepreg. During the hot pressing process, the prepreg solidifies. After the multilayer laminates are bonded, the laminates are taken out of the fixture to remove the exuded prepregs. Raw edges. Generate a program according to the diameter and position of the through hole required by the multilayer circuit board, control the numerical control drilling, and use compressed air or water to remove the debris in the hole. Before the electroless copper plating of the through hole, the residual epoxy resin on the end surface of the copper layer in the hole wall is cleaned with sulfuric acid to accept the electroless copper plating. Then a layer of copper is chemically deposited on the end face of the copper layer and the end face of the epoxy on the hole wall.

1. The process of covering the solder mask on the circuit pattern of the tin-lead alloy. As shown in Fig. 5-19, first cut the B-stage material, namely the prepreg, into pieces according to the size of the inner circuit board, and stack them in the order of Fig. 5-21 according to the number of layers of the multilayer board. For positioning pins, put the release paper into the positioning pins and place them on the bottom layer of the fixture, and then put the copper foil on the top of the copper foil, put the prepreg above the copper foil, and put the inner layer board with the corroded circuit pattern above the prepreg above the inner layer board Then put the prepreg, and put the inner layer board that corroded the circuit pattern on the prepreg until the required number of layers is stacked, then put a layer of copper foil and release paper on the prepreg, and insert the positioning hole on the top plate of the fixture Position in sale. The requirements for the positioning device of the fixture are very strict, because it is the guarantee of the pattern alignment between the layers of the multilayer printed circuit board.

For pattern transfer on the outer layer of the multilayer printed circuit board, the photosensitive film should be pasted on the surface of the copper manifold, and the photographic bottom plate of the outer circuit pattern should be flat and exposed to ultraviolet light. The exposed circuit board After developing, electroplating copper and tin-lead alloy and electro-film on the bare copper part that is not covered by the photosensitive film, and then using the tin-lead plating as a resist to corrode all the copper layer covered by the photosensitive film, then the multi-layer is responsible The surface of the circuit board is formed with tin-lead and plated through holes.

Many circuit boards are designed with connector graphics on the edge of the circuit board in order to connect with the system, commonly known as "golden fingers". In order to improve the performance of the connector, the nickel layer and gold layer are electroplated on the surface. In order to prevent the plating solution from contaminating other parts of the circuit board, a tape should be attached to the gold handle before electroplating. After electroplating, remove and heat to make the original tin plated. The lead layer reflows, and then the solder mask is covered on the parts that do not need to be soldered during assembly to prevent solder bridges or injuries in the wiring during soldering. Then print the character map (referring to the frame, serial number, model and polarity of the components, etc.) on the solder mask. After the character paint is cured, the circuit board is drilled on the circuit board to go through the on-off test, and the circuit wiring and interconnection should be ensured. There is no open circuit in the connecting hole and no short circuit between the wiring. Generally, program-controlled multi-probe needles can be used to visually check whether the circuit pattern, solder mask and character map meet the specifications.

2. SMOBC process

The SMOBC process is shown in Figure 5-20. The first part of the process is the same as the process of the multi-layer board where the solder mask is coated on the tin-lead layer. Different from the 19 processes, after the pattern is corroded, the tin-lead layer on the circuit pattern is eliminated, and the solder mask and the printed character pattern are coated on the circuit pattern of the bare copper. However, the pads and interconnects are exposed to copper. In order to prevent the surface oxidation of the copper wall and the iron wall from affecting the solderability and improve the reliability of the through-hole plating, it is necessary to have a tin-lead air leveling (HAL) process on the pad surface and the hole wall plating. Immerse the printed circuit board with the character map into the molten solder bath of the hot air leveler, and immediately lift the solder blown by the strong hot air beam from the beautiful surface of the pad and the plated through hole, so that the surface of the pad and A thin and uniform solder layer is left on the wall of the through hole, as shown in Figure 5-23. Then gold-plated the device, drilled non-conductive holes, conducted on and off tests and self-checks.

An important inspection index for printed circuit boards is the peel strength of the metal wiring on the board. For FR-4 laminates, the peel strength after treatment at 125°C for 1 hour is not less than 0.89Kg.

The circuit board for surface assembly should be manufactured by SMOBC process, because the tin-lead layer under the solder mask will produce during reflow soldering or wave soldering.

3. Solder mask and electroplating

(1) Solder mask

The assembly density of traditional printed boards is low, and barrier films are rarely used. The SMT circuit board generally uses solder mask. Solder mask is a kind of polythene material, which is mainly divided into two categories:

1) When the non-permanent solder mask is wave soldered, the peak of the wave passes through the tool hole of the circuit board and punches to the non-soldering surface. For example, the conductivity of the edge connector will affect the reliability of the socket, so before inserting the component Use non-permanent solder mask to cover the surface of the process holes and gold fingers, and then remove or dissolve them during the cleaning process.

2) Permanent solder mask The permanent solder mask is an integral part of the circuit board, and its role is to prevent the external surface of the solder bridge during wave soldering and also to avoid mechanical damage or chemical corrosion of the wiring.

Permanent solder mask is divided into two types: dry film and wet film. The dry film is a water-based or solvent-based polymer film. Generally, the dry film is attached to the liquid or paste-like polymer by a vacuum lamination process, which can be cured by ultraviolet or convection oven and infrared oven.

①Dry film solder mask The dry film solder mask has a high graphic resolution, which is suitable for high-density wiring circuit boards, and can accurately align with the wiring strips on the circuit board. Yes, it will not flow into the through-holes of the circuit board, and can cover the through-holes. When the circuit board is tested with a needle bed, the circuit board must be held by a vacuum to locate it, which is extremely helpful for the establishment of vacuum. In addition, the dry film is not easy to contaminate the pad and affect the reliability of soldering.

There are also some unfavorable factors in the use of dry film:

A: The dry film solder mask is pressed on the surface of the circuit board. There are pads and wiring on the surface of the circuit board, so the surface is not smooth, and the dry film has no fluidity. thickness. Therefore, there may be gas left between the dry film and the surface of the circuit board. After heating, the gas expands and the dry film may crack.

B. The thickness of the dry film is relatively thick, generally 0.08～0.1mm (3～4mil). The dry film covers the circuit board assembled on the surface, which will open the chip resistor on the surface of the circuit board, which may cause the solder at the end of the component to wet not good. In addition, the solder mask covers between the pads under the chip components, which may cause the component to shift during reflow soldering (that is, one end of the component stands upright on a pad).

C. The curing conditions of the dry film solder mask are strict. If the curing temperature is low or the time is short, the curing will be insufficient. It will be affected by the solvent during cleaning, the curing will be too brittle, and cracks may occur when subjected to thermal stress.

D. Poor thermal shock resistance. It is reported that the circuit board covered with dry film solder mask will crack in the solder mask 100 times at a temperature of -40～+100℃.

E. Dry film is more expensive than wet film

②Wet film solder mask: Wet film can be used for screen printing coating process and light pattern transfer coating process.

The wet film using the screen printing process can be closely attached to the surface of the circuit board, and there is no gas under the solder mask. Adjusting the printing parameters can control the thickness of the wet film layer to accurately align with the high-density fine wiring pattern, and it is easy to stain The surface of the pad affects the quality of the solder joint. Because it is liquid, it may flow into the through hole. Although it has the above shortcomings, its film layer is strong and cheap, so it is still widely used in low-density wiring circuit boards.

The wet film solder mask for optical pattern conversion combines the characteristics of dry film and wet film. The coating process is simple, the graphic resolution is high, it is suitable for high density, thin lines, and the price is cheaper than dry film. The light pattern conversion solder mask can be applied to the circuit board by screen printing or curtain process. The curtain process is to use the calendar curtain or suspension spring device of the high soldering film of the printed board to obtain a uniform solder mask.

The wet film exposure for light pattern conversion can be non-contact. The non-contact exposure device requires a set of aligned optical systems, which causes the dispersion of the light to be distorted, which requires a large investment. The contact exposure does not require an optical alignment system, and directly exposes under ultraviolet light, which can reduce costs.

(2) Electroplating

In the manufacture of circuit boards, a variety of metals are required to be electroplated. The quality of the electroplated layers such as copper, gold, nickel and tin plays an important role in the reliability of the circuit board.

1) Two copper plating methods are used in the manufacture of copper-plated circuit boards: chemical copper plating and copper electroplating. The interconnection between the layers in the multi-layer printed circuit board depends on the through hole. It is composed of the copper layer end face and the epoxy end face alternately. It is impossible to electroplate a border electroplating layer on such a surface, because the ring The oxygen end face is not conductive.

First, electroless copper plating is used to form a continuous copper deposition layer on the hole wall, and then a copper layer is electroplated on the hole wall by an electroplating process, so that the electroplated through-hole works.

The tensile strength of the copper coating, that is, in the case of tension, the maximum stress that the coating can withstand is about 20.4～34Kg/mm2. The higher the tensile strength of _____, the more solid. At the same time, it is also hoped that the plating layer has good extensibility, that is, it is allowed to be stretched longer before the plating layer is broken, so that a "yield" phenomenon can occur before the plating layer is broken. The type of residual stress in electroless copper and electroplated copper is also different. The residual stress in the layer is compressive stress, which can increase the impact of the electroless copper layer on the copper on the hole wall

The adhesive force of grease, and the residual stress in the electroplated copper layer is tensile stress, which is one of the reasons why electroless copper plating is used before copper electroplating.

Table 5-6 lists the initial and final weight of copper available on the circuit board. Note that the thickness of copper per ounce is 1.4 mils. So when using 1 ounce of copper, 1.4 mil copper plus 1 mil tin-lead plating, the total is 2.4 mil.

2) The conductive tape (gold finger) of the edge connector of the gold-plated printed circuit board should be plated with a layer of gold to improve the contact resistance on the surface of the copper layer

Even if the circuit works under high temperature and high humidity, the gold surface layer will not be oxidized, so that good contact between the circuit board and the system socket can be ensured. There are a variety of gold plating types that are classified according to the pH value of the solution, including acid gold plating solution, neutral gold plating solution, cyanide alkaline solution and cyanide-free alkaline solution. The performance of the plating layer is very related. For example, the hardness and porosity of the gold plating layer are closely related to the type of plating solution and the specific plating process parameters. The connector gold plating generally uses cobalt as a polishing agent.

3) The nickel layer of the nickel-plated circuit board is formed by an electroplating process. The nickel layer is used as the underlying metal of the gold-plated layer. Before the circuit board is gold-plated, the adhesion and wear resistance of a gold-plated layer must be plated on the conductive tape. At the same time, a barrier layer is also formed between the nickel and gold layers to control the intermetallic compound. The generation.

4) There are two processes for tin-lead solder to obtain the tin-lead layer on the circuit board, electroplating method and hot air leveling method.

The tin-lead layer obtained by the hot-air leveling process has good density and strong adhesion to the bottom copper foil because of the formation of intermetallic compounds between them. However, it is not easy to control the hot air conditioning, especially when the thickness of the tin-lead layer is relatively thick, the uniformity is relatively poor.

The thickness of the electroplated tin-lead layer is easy to control and uniform, but the density of the electroplated layer is poor, and the porous tin-lead layer after electroplating generally needs to be heated and reflowed and modified. Therefore, the electroplating process of lead-tin is still widely used in the manufacture of printed circuit boards.

4. Via hole, positioning hole and label

(1) Small vias

SMT circuit boards generally use small vias. Table 5-7 lists the range of via holes, the drilling method and cost used. The through hole opening ratio refers to the substrate thickness ratio, and the typical ratio is 5:1. The use of high-speed drilling machine can reach 10:1. The through-hole shape ratio determines the reliability of the multilayer board and the quality of the through-hole plating.

5-25 is the position of the through hole.

(2) Annular Rings (Annular Rings)

Annular ring refers to a pad whose size is larger than a drilled hole. It is used as a soldering area for leaded components to prevent drilling deflection. Through holes can also be used for interconnection and testing.

The layer pad size can be different. See Figure 5-26, Figure 5-27 and Table 5-8.

(3) Positioning hole

The positioning holes here are used for assembly and testing and fixing holes, most of which are non-plated through holes, which need to be made when drilling, and the size of the holes as far as possible with other holes on the board is usually 0.003". All positioning holes should be marked with the distance between each other And to the PCB reference point and the size of another plated through hole, the positioning error is generally

(4) Reference mark

There are three main types of benchmark labels: global (Globcl) and jigsaw (Local) as shown in Figure 5-29. The label is bare copper, usually with tin-lead plating at a distance from the solder mask, with a larger thickness of 2 mils. Need to use non-permanent solder mask coating on the label.

5. Jigsaw processing

In SMT, in addition to the multi-layer boards for large and medium-sized computers, most of the PCB surfaces