SMT simple knowledge of solder paste printing

In reflow soldering of surface area mount assembly (SMT), solder paste is accustomed to join pins or terminals of SMT factors to pads. There are lots of variables. This sort of as solder paste, display screen printing machine, paste application method and printing system. Within the course of action of solder paste printing, the substrate is placed on the workbench, mechanically or vacuum clamped and positioned, aligned with positioning pin or vision, and pasted printing is performed with stencil.

From the process of stencil paste printing, printing press would be the vital to attain the desired printing high-quality.

SMT solder paste printing

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For the duration of the printing course of action, the solder paste is dispersed instantly. The printing scraper presses down within the template to make the underside from the template speak to the very best surface on the circuit board. Since the scraper passes from the total length of your etched sample region, the solder paste is printed onto the pad as a result of holes while in the stencil / monitor. Just after the solder paste has actually been deposited, the display will snap off promptly soon after the scraper and return to its primary position. This separation or separation length is set with the devices design and style, about 0.020 "~ 0.040".

The separation distance and scraper pressure are two significant machines similar variables to obtain superior printing top quality.

If not, the process is called on-contact printing. Use get hold of printing when making use of all metal templates and scrapers. Off speak to printing is useful for flexible wire mesh.

There are three essential elements in paste printing, which we get in touch with 3S: solder paste, stencils, and squeegees. The correct blend of the three factors is the critical to sustainable printing top quality.


When the scraper is printing, the scraper will roll the solder paste in front of it to circulation into your template gap, and after that scrape from the surplus solder paste, leaving the solder paste as thick given that the template within the PCB pad.

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There are two widespread varieties of scrapers: rubber or polyurethane (polyurethane) scrapers and steel scrapers.

The metallic scraper is made of stainless-steel or brass, which has a flat blade shape along with a printing angle of thirty ~ fifty five ??. When working with greater tension, it's going to not dig out the solder paste through the opening, and since it is actually metal, they are really not as easy to use as rubber scrapers, so that they will not should be sharp. And they may be a great deal more high priced than rubber scrapers. Rubber scraper, utilizing 70-90 durometer hardness scraper.

When making use of much too higher stress, the solder paste infiltrating in to the bottom of the template may possibly cause tin bridge, which needs frequent bottom wiping. It could even injury the blade plus the template or wire mesh. Also significant force also tends to dig out solder paste with the huge opening, resulting in insufficient solder fillet. The small strain in the scraper triggers lacking and tough edges. The wear, pressure and hardness of your scraper blade determine the printing excellent, which should really be very carefully monitored. For suitable print high quality, the blade edge really should be sharp and straight.

Template (stencil) variety

At the moment, you'll find mostly chrome steel templates, and you'll find a few varieties of manufacturing procedures: chemical corrosion, laser reducing and electroforming.

Since the solder paste printed by metallic template and metallic scraper is complete, from time to time the printing thickness is too thick, which can be corrected by decreasing the thickness on the template.

Furthermore, the solder paste region about the pad can be minimized by lessening the duration and width of the wire hole by 10%. Consequently, the sealing condition with the frame among the template and also the pad caused by the inaccurate positioning with the pad may be enhanced, as well as "explosion" from the solder paste amongst the underside from the template as well as PCB is usually minimized. The cleaning times on the base area from the printing template may be lessened from the moment each and every five or 10 printing cleansing times to as soon as each individual fifty printing cleaning periods.

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Solder paste

Solder paste is really a combination of tin powder and rosin. The function of rosin is usually to take out oxides from component pins, pads and solder beads in the very first stage of reflow furnace. This phase lasts for about 3 minutes at a hundred and fifty ??. Solder can be an alloy of guide, tin and silver, reflow at about 220 ?? during the second stage of reflow furnace.

The lower the viscosity with the solder paste in to the PCB, the better the circulation features in the solder paste. Just after printing, the solder paste stays around the PCB pad, and its viscosity is superior, so it retains its filling form without having collapsing.

The common viscosity of solder paste is about 500kcps ~ 1200cps, and 800kcps is good for template display screen printing.

There is a functional and economical solution to choose whether or not the solder paste has the right viscosity. It is as follows: stir the solder paste inside the container that has a spatula for approximately 30 seconds, then pick up some solder paste, that is three or four inches better than the container, and permit the solder paste fall down by itself. At first, it should slide down like a thick syrup, after which break and fall to the container. When the paste won't slide, it can be way too thick and way too small viscosity. If it falls constantly without the need of breaking, it's as well slender and the viscosity is too lower.

(one) Method parameter placing

The separation speed and separation length (snap off) of your template and PCB. Following silk display screen printing, the PCB is divided in the display printing template, leaving the solder paste on the PCB in lieu of inside the monitor printing hole.

To the most wonderful screen printing holes, the solder paste may well be a lot easier to stick to the hole wall than for the pad. The thickness on the template is rather critical, and two aspects are favorable:

For starters, the pad is usually a continuous area, as well as inner wall from the wire hole is generally divided into four sides, which aids to release solder paste;

Next, gravity as well as the adhesion power using the pad can pull the solder paste away from the wire gap and cling into the PCB in just 2 ~ six seconds of monitor printing and separation.

To be able to optimize this useful result, the separation delay is often delayed, as well as PCB separation is sluggish in the starting. Several equipment let a hold off right after display printing, and also the journey velocity with the initial 2 ~ 3 mm in the table falling might be altered bit by bit.

(two) Printing pace

All through printing, the speed of your scraper about the printing plate is vital since it takes time for the paste to roll and move into the die hole.

If your time will not be plenty of, the solder paste are going to be uneven about the pad while in the route on the scraper. When the speed is higher than twenty mm for each second, the blade may perhaps scrape by means of smaller die holes in fewer than tens of milliseconds.

(3) Printing force

The printing strain have to be coordinated with the hardness with the scraper. In the event the tension is too little, the scraper will never clean up the solder paste on the template. In case the pressure is simply too superior or even the blade is simply too gentle, the blade will sink into the greater hole during the template and dig out the solder paste.

(4) Stress empirical formulation

To be able to get the right strain, implement one kg force on each individual 50 mm blade size for the commencing, for instance, six kg force for three hundred mm scraper, slowly lessen the tension until eventually the solder paste begins to stay to the display, and after that increase the stress by 1 kg. There ought to be a suitable array of 1-2 kg amongst enough time when the solder paste can't be scraped cleanse until the scraper blade sinks in the wire hole to dig out the solder paste.

As a way to accomplish great printing results, a mix of the proper paste content (viscosity, metallic articles, optimum powder dimensions and minimum amount flux activity possible), the best equipment (printer, template and scraper) along with the right method (excellent positioning, cleansing and wiping) are required. In line with distinct solutions, set the corresponding printing approach parameters from the printing program, this kind of as doing work temperature, functioning strain, scraper velocity, template computerized cleaning cycle, and many others., and concurrently, it truly is needed to formulate rigid procedure management and course of action.

¡é¨´ The solder paste shall be utilised strictly in accordance along with the specified brand in just the validity period. The solder paste shall be stored within the refrigerator for additional than 6 several hours prior to use. The solder paste shall be stored separately and its quality shall be determined when it is reused.

¡é¨² Prior to generation, the operator makes use of a special chrome steel bar to stir the solder paste to produce it uniform, and frequently utilizes the viscosity tester to sample the paste viscosity.

¡é? Right after the adjustment from the initially printing board or gear on duty, the solder paste thickness tester shall be utilized to measure the solder paste printing thickness. The test details shall be selected at five details, like higher, decreased, left and ideal and center on the exam surface with the printed board. The value shall be recorded. The solder paste thickness shall be between - 10% from the template thickness + 15% from the template thickness.

¡é¨¹ From the manufacturing procedure, 100% inspection of solder paste printing top quality is carried out, the principle articles is whether the solder paste sample is comprehensive, whether the thickness is uniform, no matter if there may be solder paste suggestion phenomenon.

¡éY Following the operate on responsibility is finished, the formwork shall be cleaned in keeping with the process specifications.

¡éT Right after the printing experiment or printing failure, the solder paste over the printed board shall be completely cleaned and dried with ultrasonic cleaning products, or cleaned with alcohol and high-pressure gas, so as to prevent solder balls following reflow soldering because of residual solder paste to the board all through reuse.

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