Factors affecting the quality of re - flow welding(2)

- Aug 28, 2018-

(iv) printing quality of solder paste

According to statistics, on the premise of correct PCB design and guaranteed quality of components and printed boards, 70% of the quality problems of surface assembly are caused by printing process.The correct printing position (printing accuracy), the amount of solder paste, the uniformity of solder solder, the clear adhesion of solder paste, and the adhesion of solder paste on the surface of printed board directly affect the welding quality of surface assembly board.

There are many factors affecting the quality of printing, mainly including:

1. First, template quality

Stencil printing is contact printing, so the stencil thickness and opening size determine the solder paste print.Too much solder paste will lead to bridge, too little solder paste will lead to insufficient or virtual solder.The shape of the opening of the template and whether the opening is smooth will also affect the demoulding quality.The opening of the formwork plate must be downward, otherwise the solder paste will be taken out from the chamfer of the horn.

2. Second, solder paste viscosity, printing (rolling, transfer), thixotropy, and service life at room temperature will affect printing quality.If the solder paste printing is not good, serious solder paste is only sliding on the template, in this case is not printed on the solder paste.

3. Printing process parameters

The solder paste is a thixotropic fluid with viscosity.When the scraper to move forward at a certain speed and Angle, butt paste produce certain pressure, promote the solder paste before the scraper roller, will produce solder paste into the mesh or the leakage hole pressure, viscous friction force of solder paste make the solder paste in the scraper and halftone junction produces shear, shear force make the solder paste viscosity decreased, is advantageous to the solder paste smoothly into the mesh or leak.

The speed of the scraper, the pressure of the scraper, the Angle between the scraper and the plate and the viscosity of the solder paste all have certain restrictive relations.For example, the blade pressure is too large, printing will cause the solder paste graph adhesion;Too fast printing speed may cause insufficient solder paste.If the residual solder paste at the bottom of the template is not polished in time, and the solder paste is stuck outside the solder pad during printing, these factors will cause the welding defects such as bridge, false solder, tin bead, etc.

4. Equipment accuracy

High density narrow spacing in printing products, printing machine printing accuracy and repeat precision can also play a role, if the printer without visual for the system configuration, even artificial graphics on time is very fine, PCB solder graphics graphics coincide with the template leakage holes, but still can't solve the machining error of PCB.

5. The use and management of recycled solder paste, environmental temperature, humidity, and environmental sanitation all affect the quality of solder joints.Recycled solder paste and new solder paste should be stored separately. Excessive ambient temperature will reduce the viscosity of solder paste, excessive humidity will absorb moisture in the air, humidity over the hour will accelerate the volatile solvent in the solder paste, dust in the environment mixed into the solder paste will create pinholes.

(v) sticking components

Three elements of mounting quality: correct component, accurate position, suitable pressure (patch height).

1. Correct component -- the type, model, nominal value and cabinet polarity of each assembly bit number component shall be marked in accordance with the assembly drawing and detail list of the product, and shall not be mislabeled.

2. Accurate position -- the terminals or pins of the components and the welding plate shall be aligned and centered as far as possible.

The placement of components and components should meet the technical requirements.Because two end Chip components since the orientation effect is bigger, the effect of SMT components length direction two end as long as received the corresponding welding plate, width is 1/2 lap on the bonding pad, can the position during reflow soldering, but if one of the end did not take to the bonding pad, reflow soldering will produce a shift or when the suspension bridge, and for the SOP, SOJ, QFP, PLCC relatively small role in the positioning device, such as mount offset is not through the reflow soldering to correct.Therefore, it is necessary to make sure that 3/4 of the width of the pin is on the pad. The toe and heel of the pin should also be on the pad.If the mounting position exceeds the allowable deviation range, manual alignment must be performed before entering the reflow furnace for welding.Otherwise, it must be repaired after re-flow welding, which will result in waste of time, materials and even affect product reliability.In the production process, if the mounting position is found to exceed the allowable deviation range, the mounting coordinates should be corrected in time.

For manual installation, the position of the mounting should be accurate, and the pin should be aligned with the pad. In the center, do not put the pin on the pad.

3. Pressure (patch height) -- the patch pressure (height) shall be appropriate, and the welding end of components or pins shall be not less than 1/2 thickness and shall be soaked in solder paste.The amount of solder paste extrusion (length) should be less than 0.2mm for the patch of general component, and less than 0.1mm for the patch of narrow spacing components.If the patch pressure is too small, the solder end or lead pin will float on the surface of the solder paste. The solder paste will not stick to the component, and the position will move easily during transfer and re-flow welding.In addition, since the z axis is too high, the placement of the patch will be offset when the element is dropped from the top.Too much pressure on the patch and too much extrusion of solder paste can easily cause adhesion of solder paste, bridge can be easily produced during reflow welding, and even damage components in serious cases.