Hand-soldering a mobile phone motherboard is not a matter of relying on feel and guessing; it requires a 0.2mm diameter Solder Wire to melt stably at the tip of a 350℃ constant-temperature soldering iron, while ensuring the solder joint heating time is controlled within 2.5 seconds. Otherwise, the solder balls under the IC will collapse or oxidize due to overheating. We have repeatedly verified this window under the JJY JL-680A microscope; at 20x magnification, we can clearly see whether the Solder Paste wetting angle spreads evenly, and whether there are signs of pinholes or solder shrinkage at the edge of the solder joint.

The selection of fine-diameter solder wire must match the actual minimum pad size on the motherboard. For example, the pad pitch of the Wi-Fi module cable connector on a mobile phone motherboard is only 0.4mm; using 0.3mm Solder Wire allows single-point solder feeding without touching adjacent pads. If 0.5mm Solder Wire is used, bridging will occur frequently, while the finer 0.15mm solder wire is prone to breaking or unstable feeding during pushing. Therefore, the JJY F-020 series has become the standard configuration on our production line. It has evenly distributed rosin cores and low residue, and after continuously soldering 30 0201 package resistors, the soldering iron tip still shows no obvious carbon buildup.

The most common problem during hand soldering is the 0.3mm pads around BGA chips. These positions are often half-obscured by shielding cans. During operation, you must adjust the soldering iron angle to avoid adjacent components, while using your left hand to hold tweezers and gently press the solder joint to assist heat conduction, and your right hand to simultaneously complete the three actions of feeding solder, withdrawing solder, and finishing the tip. Any hesitation will cause the solder joint void rate to exceed the standard. We once used X-ray sampling inspection and found that the void area proportion of a batch of reworked boards reached 28%. After investigation, it was determined that the solder wire diameter was too large, leading to insufficient reflow. After switching to the 0.25mm specification, the void rate was reduced to within 7%.

The shape of the soldering iron tip must also be dynamically adjusted according to the solder wire diameter. For soldering 0.2mm solder joints, an I-type pointed tip with an edge width of no more than 0.15mm must be used; otherwise, the excessive contact area can easily burn the flexible circuit board substrate. Our production line stipulates that after every 50 solder joints, the soldering iron tip must be wiped with a damp sponge, and the oxide layer must be lightly polished with 1000-grit sandpaper. Otherwise, the oxide film will hinder solder wire wetting, and even if the temperature is accurate, cold solder joints will occur. This phenomenon is particularly typical when soldering the negative pad of a tantalum capacitor on the motherboard power supply module, because the copper foil on that pad is thick and has a large heat capacity, and a cold solder interface will easily form if you are not careful.

All hand soldering operations must be carried out under conditions where the anti-static wrist strap is well grounded and the ambient humidity is controlled at 45%～60%RH. Otherwise, the tiny spatter generated when the 0.2mm solder wire melts will easily adsorb dust and become trapped inside the solder joint, leading to intermittent open circuits during subsequent high-temperature aging tests. This type of defect cannot be detected at the functional testing stage; it only appears after 127 hours of HALT testing. Therefore, we now strictly limit the storage time of solder wire after opening to 72 hours. Any solder wire exceeding the time limit is scrapped, and even if it looks intact, it is not allowed to be used on the production line.