PCB Assembly Manufactures SMT Auxiliary Materials
In SMT production, we usually refer to patch glue, solder paste, and steel mesh as SMT auxiliary materials. These auxiliary materials play an important role in the quality and production efficiency of SMT in the whole process of SMT. Therefore, as SMT staff, you must understand some of their properties and learn to use them correctly.
Ⅰ. Common terms
1. Shelflife
Under the specified conditions, the material or product can still meet the technical requirements and maintain the storage time for proper performance.
2. Workingtime
The maximum time that the patch glue and solder paste can maintain the specified chemical and physical properties before being exposed to the specified environment.
3. Viscosity
Adhesive properties of the drop ductility of the patch glue and solder paste when they drop naturally.
4. Thixotropic ratio
SMD adhesives and solder pastes have fluid properties when they are extruded under pressure, and quickly recover to have solid plastic properties after extrusion.
5. Slump
After the solder paste is printed, the slump phenomenon is caused by the effect of gravity and surface tension, temperature rise or too long parking time, etc., and the bottom area exceeds the specified boundary.
6. Spread
The distance that the patch glue spreads out at room temperature after the glue is dispensed.
7. Tack
The size of the adhesive force of solder paste to components and the change of its adhesive force with the storage time of solder paste after printing
8. Wetting
The molten solder forms a uniform, smooth and non-breaking thin layer of solder on the copper surface.
9. No-clean solder paste
After soldering, it contains only a small amount of harmless flux residue without cleaning the solder paste of the PCB
10. Low temperature paste
Solder paste whose melting temperature is more than 20°C lower than 183°C.
Ⅱ. Patch glue (red glue)
The patch glue used in SMT is used to fix chip components, SOT, SOIC and other surface mount devices on the PCB to prevent them from falling off or shifting during the plug-in and wave soldering process.
Patch adhesives can be divided into two types: epoxy type and acrylic type. In general production, epoxy resin thermosetting glue is used, and its characteristics are:
l Fast heat curing speed
l High connection strength
l Better electrical characteristics
Instead of using acrylic glue (need to be cured by ultraviolet radiation).
SMT's basic requirements for patch glue:
l No impurities and bubbles in the package
l Long storage period
l Can be used for high-speed/or ultra-high-speed dispensers
l The glue dot shape and volume are consistent
l High point section, no wire drawing
l The color is easy to identify, which is convenient for manual and automated machines to check the quality of glue dots
l High initial adhesion
l High-speed curing, low curing temperature of glue, short curing time
l When heat curing, the glue dot will not collapse
l High strength and flexibility to withstand sudden temperature changes during wave soldering
l Excellent electrical properties after curing
l Non-toxic
l Has good repair characteristics
1. Production quality problems caused by patch adhesive
l Missing parts (with or without traces of patch adhesive)
l Component skew
l Poor contact (brushed wire, too much patch glue)
2. Standards for the use of patch adhesives:
l Storage
After receiving the glue, register the arrival time, expiration date, model, and number each bottle of glue. Then store the glue in a refrigerator with constant temperature and humidity at a temperature of (2-10) ℃.
l Access
When using the glue, the principle of first-in-first-out should be implemented. It should be taken out of the refrigerator at least 1 hour in advance. Write down the time, number, user, and applied product, and seal it at room temperature. When the glue reaches room temperature, press it for one day Use the glue gun to inject the glue into the dispensing bottle separately. When injecting glue, inject the glue bottle carefully and slowly to prevent air bubbles.
l Use
Put the glue bottle filled with glue into the refrigerator again, take it out from the refrigerator 0.5 to 2.0 hours before production, indicate the time, date, and bottle number of the take-out, fill in the glue (solder) thawing, use time record table, and use the glue Clean the water bottle with alcohol or acetone and put it away for next use. The unused glue should be stored in the refrigerator at the time indicated.
Ⅲ. Solder paste
Defects caused by solder paste account for 60%-70% of defects in SMT, so it is particularly important to regulate and use solder paste reasonably.
In the reflow soldering of surface mount components, solder paste is used to connect the leads or terminals of the surface mount components to the pads on the printed circuit board.
Solder paste is a homogeneous mixture of alloy solder powder, flux and some additives. It has a certain viscosity and good thixotropy. It has good printing performance and reflow soldering performance, and is stable in storage. Paste.
Alloy solder powder is the main component of solder paste, accounting for about 85%-90% of the weight of the solder paste. The commonly used alloy solder powders are as follows:
Tin-lead (Sn-Pb), tin-lead-silver (Sn-Pb-Ag), tin-lead-bismuth (Sn-Pb-Bi), etc. The most commonly used alloy composition is Sn63Pb3.
The shape of alloy solder powder can be divided into spherical and elliptical (amorphous). Its shape and particle size affect the degree of surface oxidation and fluidity, so it has a great impact on the performance of the solder paste.
Generally, the size and shape of the solder powder particles are determined by the opening size of the printed steel plate or screen or the diameter of the syringe. Different pad sizes and component pins should use different grain sizes of solder powder, not all small particles, because small particles have a much larger surface area, which makes the flux more burdensome when dealing with surface oxidation.
In the solder paste, the flux is the carrier of the alloy solder powder, and its main function is to remove the surface oxides of the welded parts and the alloy solder powder, so that the solder quickly diffuses and adheres to the surface of the soldered metal. The composition of the flux is: active agent, film-forming agent and adhesive, wetting agent, thixotropic agent, solvent and thickening agent, and other various additives.
The activity of the flux: the activity of the flux must be controlled. Too little active dose may affect the welding effect due to poor activity, but too much active dose will increase the residual amount and even increase the corrosiveness, especially for the halogen in the flux The content needs to be strictly controlled,
In fact, according to performance requirements, the weight ratio of flux can also be expanded to 8%-20%. The composition and content of the flux in the solder paste have a great influence on slump, viscosity and thixotropy.
When the metal content is higher (greater than 90%), the slump of the solder paste can be improved, which is conducive to the formation of full solder joints. And because the amount of solder is relatively small, it can reduce the flux residue and effectively prevent the appearance of solder balls. The disadvantage is that the printing and soldering process requirements are strict; when the metal content is low (less than 85%), the printability is good, and the solder paste is not easy to stick. The squeegee has a long life of missing plates, good wettability, and is easy to process. The disadvantage is that it is easy to collapse, and defects such as solder balls and bridging are easy to appear.
The classification of solder paste can be done in the following ways:
According to the level of melting point: high-temperature solder paste has a melting point greater than 250°C, and low-temperature solder paste has a melting point less than 150°C. The commonly used solder paste has a melting point of 179°C-183°C, and its composition is Sn63Pb37 and Sn62Pb36Ag2.
According to the activity of the flux: it can be divided into inactive (R), medium active (RMA) and active (RA) solder paste. The most commonly used is medium activity solder paste.
SMT has the following requirements for solder paste:
1. It has a long storage life and can be stored at 0-10℃ for 3-6 months. There will be no chemical changes during storage, and no separation of solder powder and flux, and its viscosity and adhesion will remain unchanged.
2. It has a long working life. After printing or dripping, it is usually required to be placed at room temperature for 12-24 hours, and its performance remains unchanged.
3. After printing or coating and during the preheating process of reflow soldering, the solder paste should maintain its original shape and size without clogging.
4. Good wetting performance. The activator and wetting agent components in the flux should be selected correctly in order to meet the requirements of wetting performance.
5. No solder spatter occurs. This mainly depends on the water absorption of the solder paste, the type, boiling point and amount of the solvent in the solder paste, and the type and content of impurities in the solder powder.
6. It has good welding strength to ensure that components will not fall off due to vibration and other factors.
7. After welding, the residue has good stability, no corrosion, high insulation resistance, and good cleaning.
Selection of solder paste
Mainly based on process conditions, use requirements and properties of solder paste:
1. It has excellent storage stability.
2. It has good printability (flowability, release property, continuous printability), etc.
3. It holds certain adhesiveness to SMD for a long time after printing.
4. A good joint state (solder joint) can be obtained after welding.
5. Its welding composition has high insulation and low corrosion.
6. It has good cleaning ability to the flux residue after welding, and no residue should be left after cleaning.
Precautions for the use and storage of solder paste
1. To receive solder paste, register the arrival time, expiration date, model, and number each can of solder paste. Then store it in a refrigerator with constant temperature and humidity at a temperature of about (2-10) ℃.
2. When the solder paste is used, the principle of first in first out should be followed. It should be taken out of the refrigerator at least 2 hours in advance. Write down the time, number, user, and applied product, and seal it at room temperature until the solder paste reaches Open the cap at room temperature. If it is opened at low temperature, it is easy to absorb water vapor, and it is easy to produce tin beads when reflow soldering. Note: The solder paste cannot be placed next to the heater, air conditioner, etc. to accelerate its heating.
3. Before opening the solder paste, a centrifugal mixer must be used for stirring to make the components in the solder paste uniform and reduce the viscosity of the solder paste. After opening the solder paste, in principle, it should be used up within the same day. Solder paste beyond the time limit must not be used
4. When the solder paste is placed on the drain version for more than 30 minutes and not used, it should be used after re-stirring. If the interval is long, put the solder paste back into the jar, close the lid tightly and store it in the refrigerator.
5. According to the size of the printed board and the number of solder joints, determine the amount of solder paste to be added to the missing plate for the first time. Generally, add 200-300 grams for the first time, and then add a little after printing for a period of time.
6. After the solder paste is printed, the mounting should be completed within 24 hours, and the PCB solder paste should be cleaned and printed again after the time has passed.
7. The best temperature for solder paste printing time is 25℃±3℃, and the relative humidity is 60%. If the temperature is too high, the solder paste will easily absorb water vapor, which will produce tin beads during reflow soldering.
