PCB Tech - The special phenomenon of lead-free wave soldering of circuit boards

The special phenomenon of lead-free wave soldering of circuit boards

1. QFP 2. Deterioration of heated solder joints
(1) Some QFP pins on the front of the circuit board have been re-soldered firmly in the lead-free solder paste, and when they enter the bottom surface again for the second high heat of lead-free wave soldering, occasionally a few pins will appear to melt. The unfavorable phenomenon of detachment (in fact, it will be worse for the second reflow on the reverse side of the circuit board).

(2) Especially the QFP pins that are close to the high-heat tin-filled PTH are most prone to thermal cracking and floating. The reason is that the amount of tin and heat of wave soldering will escape from bottom to top (the larger the hole diameter, the worse), causing nearby The SMT pins are softened by heat, and the pin stress is applied to the pins to spring off (up to 201°C at this time).

(3) The main mechanism of this kind of QFP pin floating crack again is that if the original electroplating layer of the foot surface is a tin-lead alloy or tin-bismuth alloy film, although it has been soldered by SAC305 solder paste, it is because of the solder joints. A three-phase low melting point alloy of Sn36Pb2Ag (mp177°C) or even a three-phase alloy of Sn52Bi30Pb with mP98°C may be formed locally. Therefore, when it is heated again, it may crack due to the original stress of the lead and local melting.

(4) The preventive method is to use green paint to plug the holes, or install a special heat-insulating tray (Pa11etS) on the bottom surface of the wave soldering, and add a heat-resistant cover plate on the top surface, so as to reduce the recurrence of SMT solder joints. Heat, as well as the breakage of the plate and the hole copper caused by the thermal expansion of the plate.

(5) The fundamental solution is to completely eliminate any source of lead, avoid the use of bismuth-containing pin film or solder, and completely eliminate the occurrence of local low melting points.

2. No multiple wave soldering is allowed to avoid losing the ring
For those using SAC alloy for wave soldering, the tin temperature is usually as high as 260-265 degree Celsius. After 4-5 seconds of strong thermal tin wave contact, the edge of the PTH hole on the soldering surface has been severely corroded by copper, so the best solution is only Implement single wave soldering. Once the second wave repair welding is required, not only the copper layer at the edge of the hole will be eroded and thinned, and even once the bottom is broken, the copper ring on the bottom surface may be washed away by the tin wave and cause loss. ring. Therefore, do not perform secondary wave soldering as much as possible to reduce scrap.

After two lead-free wave soldering, the tin-filled holes will almost always be at the laminated film (B-Stae) of the multilayer board. The resin shrinkage problem occurs. Although it is not rejected by the specification, it becomes the overheating of the board. The evidence is out. In addition, a small area of the sheet is microcracked, and once the physical properties of the copper plating layer are poor, it will even cause a crisis of broken holes.

3. QFP wave soldering can also be performed on the bottom plate surface
When the circuit board requires double-sided soldering of SMT components, and there are also QFP active components on the bottom of the circuit board, and wave soldering of through-hole pins is also required, the general practice of the circuit board factory is to reflow the solder paste on the front surface first, and then the board Turn over and overhead, and then print solder paste on the bottom surface, and perform overhead reflow on all SMT components again. Finally, the pin components are locally wave soldered on the bottom under the protection of the tray. As a result, a total of three lead-free heat tortures will cause serious damage to the circuit board and various components.

At this time, if the active components such as QFP or SOIC on the bottom surface are also glued and positioned like small passive components, then the entire bottom surface is double-wave soldered with surge (spoiler waves) and advection waves, and the pin components can be connected All welded at the same time. In this way, not only the heat test of a reflow can be avoided, but also the method of dispensing wave soldering can kill two birds with one stone, which is much cheaper than solder paste reflow.

The trouble of large-scale QFP or SOIC wave soldering is that the close-pitch pins are often short-circuited due to the pull of the tin wave. Moreover, the surface tension of lead-free solder increases (that is, the cohesion becomes larger), and the disaster is particularly severe. At this time, you can add "Solder Thief" at the four corners or both ends of the pin pads at the beginning of the layout (Layout), so that at the moment of wave passing, the original pads can be dragged. The amount of tin can be absorbed by the tin thief at the tail, and various short circuits can disappear. However, it should be noted that the package body with IC facing downwards will pass through the tin wave at this time. Just like SMT reflow, it must be directly trapped in the heat source. Therefore, we must also pay attention to the J-STD-020C humidity sensitivity level (MSL). The key to prevent the package from cracking.

Fourth, the top hole should be reduced
Most of the current PCB design specifications or tools (Layout software) are the continuation of lead soldering conventions over the years. In fact, the lead-free solder has poor soldering ability (referring to tin or loose tin) due to the increase of cohesive force. At the speed of normal pumping pump, if you want to push the tin wave~, the I/L top may even overflow and cover the front hole. For those who are ringing, their chances are not much. OJ-STD-001D in its table 6-5 for Class 2 and 3 boards, only requires that the amount of tin in the hole reaches 75% to pass. Therefore, the top hole ring does not need to be the same size as the bottom surface, otherwise the top ring of the OSP film will leave the outer copper exposed in vain. The damaged OSP film is difficult to ensure that the copper surface will not rust and migrate during subsequent use. At this time, the top hole can be reduced (the area of other square pads should also be reduced, or the "Solder Maskon Pad" (Solder Maskon Pad), that is, "Solder Mask Defined (SMD)" (SMD) approach, One reduces the risk of copper exposure on the ring side.