Tips for Soldering Station - Analysis of Special Conditions of Lead-free Solder Paste Wave Soldering
1. Deterioration of QFP second heated solder joint
When some QFP pins on the front side of the circuit board are first reflowed with lead-free solder paste firmly, when they enter the bottom surface again for the secondary high heat of lead-free wave soldering, sometimes it will be found that several pins will appear The undesirable phenomenon of melt-off and float-off (in fact, it is even worse when the reverse side of the board is reflowed).
Method: Completely eliminate any source of lead, avoid using bismuth-containing lead film or solder, and completely eliminate the occurrence of local low melting point is the right way.
2. Do not repeat wave soldering to avoid ring loss
For those who use SAC alloy for wave soldering, the tin temperature is usually as high as 260-265°C. After 4-5 seconds of strong heat wave contact, the edge of the PTH hole on the soldering surface has been severely corroded. Therefore, the best solution is to only Implement single wave soldering. When a second over-wave repair soldering is required, the copper layer at the edge of the hole will be corroded and thinned, which may even cause the copper ring on the bottom plate to be washed away by the tin wave, resulting in ring loss. Therefore, try not to perform secondary wave soldering to reduce scrap.
3. QFP wave soldering can also be performed on the base plate
The usual practice of the circuit board factory is to perform solder paste reflow on the front circuit board first, then turn the circuit board upside down, print solder paste on the bottom surface, and perform overhead reflow on all SMT and QFP components and wave soldering pins. Finally, partial wave soldering of the bottom surface is performed on the pin components under the protection of the tray. In this way, a total of three lead-free intense heat tortures will be required, and the circuit board and various components will be seriously damaged.
4. The top hole ring should be reduced
PCB design specifications and tools (Layout software) have mostly inherited the tradition of lead soldering for many years. In fact, due to the increased cohesion of lead-free solder, the soldering ability (referring to tin or loose tin) is poor. Under the normal pump speed, if you want to push tin waves~, the I/L top will even overflow and cover the front hole. For those in the ring, there are not many chances. 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. The size of the hole ring on the top surface of the OSP film does not have to be the same as the size of the bottom surface, otherwise there will be tin-free exposed copper on the periphery, so it is difficult for the damaged OSP film to ensure that the copper surface will not rust or migrate during subsequent use.
5. Filling tin in the porous area will cause explosion
In the old-fashioned design, many through holes are often densely arranged in the BGA backplane, as the interlayer interconnection function of multilayer wiring. When such a dense hole area is filled with tin wave, the influx of a large amount of heat energy will inevitably test the tolerance limit of the multilayer board in the Z direction, and often cause the board to crack or even break in the Z direction. In addition, there is a filler in the dense hole area for the pin insertion soldering of the connector. At this time, although the heat brought by the tinning is still large, some of it is absorbed by the pins, so the cracks in the Z direction of the plate are lower than the empty holes. As long as the hole copper thickness is sufficient (above 0.7mil), the elongation of the copper plating layer (Elongation) can still be maintained above 20%.
