2 printing operations
2.1 scraper edge
It is generally recommended to use a metal scraper that is better than a polyurethane scraper. However, extruding the solder paste from the opening of the polyurethane squeegee is a problem. It is recommended that the squeegee angle is 45° to 60°, and generally the squeegee is pushed by 45°.
Press the squeegee down so that the squeegee of the metal squeegee bends to form an optimal angle. When choosing the printing force, a vector parallel to the template pushes the solder paste so that the solder rolls. The other vector is an opening directly under pressure applied to the stencil, pushing the solder paste into the opening. Change the pressure of the template and change the angle of the squeegee.
Another factor that is often overlooked is the sharpness of the scraper. In general, the more blunt the squeegee, the greater the pressure required to push the entire top of the stencil; the sharper the edge of the squeegee, the less force required. A simple microscopy scan of the squeegee blade shows a large difference. A lower squeegee pressure usually means that the stencil pusher has a lower frequency, which is the deciding factor.
2.2 push speed
When printing is first started, it is recommended to set the squeegee speed at 0.5"/sec. A lower squeegee speed usually causes the solder paste to deposit approximately the calculated amount. (Normally, the actual amount of solder paste deposited is less than the theoretical ration. Because of the cycle time requirement, you have to operate at a higher rate of squeegee, but after a successful slower squeegee you will gradually increase the squeegee speed and quickly measure the print results. In order to increase the speed of pushing and pushing, it is necessary to increase the force applied to the squeegee.
2.3 Contact and paste distance
In general, the more popular printing method is contact printing, which helps to reduce the "bleed-out" of the solder paste at the bottom of the template. For a small area ratio printing such as μBGA, achieving a consistent template solder paste miss is the focus of competition. One method is to release the solder paste process in advance. This method is achieved through a very small paste removal distance (0.002 to 0.003′′. Programmatically, at the beginning of printing, the template is separated from the printed board with a small amount of force with a small angle. However, this distance is not enough to release the template, at least at the beginning of the printing process.
3 Solder paste
The choice of powder is a key factor in the release of solder paste. To avoid clogging of the openings, standard 3.5 particles can be used throughout the opening, and square openings are helpful for maintaining consistency. Table 4 lists the mesh size and powder size.
Table 4 Solder paste classification and mesh size classification Head size I class -100/+200
100 (150μm) 0.0059
Class II-200/+325
200 (75μm) 0.030′′
Class III-325/+500
325 (45μm) 0.0018′′
IV class-400/+500
400 (38μm) 0.0015′′
Class V-500/+635
500(25μm)0.0010′′
635(20μm)0.0008′′
Solder paste rheology: There are many factors that make the printing process out of control. For example; sometimes the solder paste will dry on the surface of the template, forming a large clot that can block the opening. If the press is idle for 5 minutes, it is very likely that the control of the next print will not meet acceptable requirements. In addition, the delay of the printing cycle will also cause the residual solder paste or flux to dry on the sidewall of the aperture, which will reduce the size of the opening and reduce the amount of solder paste. The empirical analysis alone shows that when the solder paste on the stencil is idle for more than 5 minutes, the bottom of the stencil should be wiped before implementing the next printing.
Traditionally, people have focused on the stickiness or thixotropic index of the solder paste. The thixotropy is defined as the quality of a certain paste or similar gel-like material that will become liquid under static conditions. With a high thixotropic index, the solder paste is easily diluted and can flow into the template. The disadvantage is that it is easier to generate “heat-lubricated sludge†in the reflow furnace unless the manufacturer believes that the necessary reformulation of the formulation should be performed.
3.1 Solder paste amount
A simple equation is used below to describe the amount of solder paste required, as listed in Table 5. It should be preferable to make the ring pad around the through hole smaller as much as possible. Also make the tolerance between pins and vias and the length of the pins as small as possible. In practice, it is necessary to apply a small amount of solder paste. Apply the following three types of stencil designs to the through-hole solder paste on the board:
(1) Stepless template. image 3
(2) Template with steps. Figure 4
(3) Template for two printing modes. Figure 5
Table 5 Dipping tin through hole equation V = Ts (Lo × Wo)
1
= s{TB(AH-AP)+(FT+FB)+VP}-VH
Where: V is the amount of solder paste required VP is the amount of solder remaining on and/or at the bottom of the board pad S is the factor for solder paste shrinkage AH ​​is the cross-sectional area of ​​the through-hole AP is the cross-sectional area of ​​the through-hole pin TB Is the thickness of the printed board FT+FB is the total amount of filler needed Ts is the thickness of the template Lo is the length of the overprint opening
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