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SMT micro-pad leakage, less tin improvement and sharing
As electronic products become lighter, thinner, and smaller, component solder terminals and PCB pads are getting smaller and smaller, which brings great challenges to solder paste printing. It is well known that the quality of solder paste printing is the core of SMT process control, and the quality of solder paste printing affects 60-70% of SMT quality problems. The biggest experience in the past 15 years is that the quality of products is often determined by individual tiny solder joints, so the printing quality and soldering quality of tiny solder joints must be controlled as the core.
1. Device, PCB pad traces and bad pictures
Two, analysis
1. Enlargement and analysis of poorly printed pads
2. Analysis of the gap between the bottom of the stencil and the solder mask, pad, and substrate around the pad during printing:
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3. Analysis of the causes of undesirable phenomena
1. The principle of solder paste printing
The solder paste is squeezed into the steel mesh through a squeegee, so that the solder paste touches the surface of the PCB and adheres to the PCB surface. The solder paste that adheres to the PCB surface during demolding overcomes the resistance of the steel mesh wall and is transferred to the PCB surface.
2. Observe, think, compare
a. Although part of the substrate around the pad is covered by the stencil opening during printing, the solder paste at the bottom of the stencil opening is difficult to contact the PCB pad and the surrounding substrate, and it is not enough to overcome the hole wall during demolding. Resistance (only a small amount of solder paste on the pad)?
b. As shown in Figure 7, there is a 35 um deep annular deep pit between the pad and the solder mask. Does the solder paste with the stencil opening on the pit not touch the bottom of the pit?
c. Why are other pads connected to the circuit not easy to miss?
3. Verification of bare copper plate printing
The solder paste of 5 different brands of 4# powder can be stably tinned (laser + electro-polished steel mesh) on a circular hole with a thickness of 0.1 and an opening diameter of 0.28.
Four, summary
1. The above inferences a and b can be proved by printing on bare copper board (the printing gap caused by no solder mask on bare copper board). Although the opening of the stencil is larger than the pad, it is difficult for the solder paste to contact the PCB pad and the substrate around the pad during printing. The small tin sticking area causes difficulty in printing transfer (IPC7525 stencil opening area ratio requirement is based on zero Under the premise of gap printing).
2. It can be confirmed by printing on bare copper plate and Figure 8 that there is a clear gap between the frequently missed and less tin pad and the bottom of the stencil. The largest gap is the area above the substrate (deep pit area) where the opening of the stencil is located.
Summary: The solder mask on the real PCB is not flat, the height of the solder mask on the circuit copper foil is higher, and the height of the solder mask on the substrate is lower. If there are NSMD pads near the circuit, the solder mask on the circuit will top up the stencil, making it difficult for the nearby NSMD pads to touch the bottom of the stencil when printing. When the stencil opening area is larger than the pad area, the gap between the opening area above the substrate and the bottom of the stencil is the largest.
In the final analysis, this case is caused by the printing gap caused by solder mask, and this kind of problem is most easily overlooked. Eliminating tiny pad printing gaps, especially the elimination of printing gaps caused by PCB traces and solder masks, is the key to solving this type of problem.
Five, improvement direction
1. Find all the pads that are not connected to the outer circuit, change the size of these pads from the original circle with a diameter of 0.27 to a circle with a diameter of 0.31, reduce the area of ​​the deep pit around the pad, and make the original opening area on the deep pit It becomes on the copper foil of the pad, so that the gap between the opening area on the deep pit and the bottom of the steel mesh is reduced. After the small batch verification is OK, the original stencil is used for mass production, and the soldering of the soldering pads that are difficult to solder is good (the area of ​​the soldering pad is increased, and no poor soldering is found in the batch verification).
2. Reduce the thickness of the PCB solder mask and reduce the influence of the higher height solder mask layer on the circuit near the pad. It is recommended that the PCB solder mask thickness be less than 25um (the sixth item in this article is explained).
3. The new PH steel mesh is used to eliminate printing gaps to the maximum extent.
6. Industry Standards and Improvement Methods for Excellent Enterprises
1. Solder mask thickness standard
At present, most PCB manufacturers appraise the solder mask thickness according to the American civil standard IPC-SM-840C. In this standard, the solder mask thickness of level 1 products is not limited; the minimum solder mask thickness of level 2 products is 10 µm; the minimum thickness of solder mask of level 3 products is 18 µm.
2. Friends in the mobile phone industry reported that their BGA pad traces below 0.4 Pitch all use HDI technology (as shown in Figure 12), and the PCB pads use NSMD pads to avoid solder masks on the traces around the pads. A printing gap appears on the top of the net.
3. Aiming at the printing gap problem caused by PCB, the improvement method of excellent companies (such as 0.4 Pitch BGA): cancel the white oil silk screen, replace it with exposed copper, control the solder mask thickness to less than 25um, and use NSMD pads for PCB pads , Adopt the hole-in-disk routing (micro-hole filling, grinding, electroplating) method, which can ensure a good printing effect from the design side.