Detailed explanation of the solder paste process in the soldering defect phenomenon

In the solder paste process, soldering defect phenomenon is a common soldering defect that manifests itself as a solder joint that appears to be connected on the surface but actually has poor electrical contact or insufficient mechanical strength. Soldering defect phenomenon may lead to product functionality failure, reliability degradation and even short circuit risk. The following detailed analysis from the causes, performance, impact, detection and preventive measures:

1. the definition of the soldering defect phenomenon and harm

Dense foot device soldering defect phenomenon refers to the component pins and PCB pads do not form an effective electrical connection between the phenomenon, in which the open soldering (Open Soldering) is the most common type of defect. As shown in Figure 1-1, the typical soldering defect phenomenon is caused by the pin warping local solder joints missing, or poor wetting of solder paste to form cold solder joints. The defects are highly hidden and may cause intermittent equipment failure, which is a high-hazard process problem. Some manufacturers use toothpick flicking inspection method for initial screening, but over-reliance on manual operation carries the risk of quality fluctuations.

Figure 1-1 Dense-pin device soldering phenomenon

2. the cause of soldering defect phenomenon analysis

According to the process chain, the causes of soldering defect phenomenon can be divided into five categories, with strong randomness and process sensitivity:

2.1. Solder paste printing defects

Specific performance: leakage of printing, less printing leads to insufficient solder (such as Figure 1-1 in the area of the solder joint is too small area); stencil blockage or improper parameter design (such as the opening is too small).

Percentage of soldering defect phenomenon: about 35% (industry statistics, the actual proportion may vary due to differences in process conditions).

2.2. Pin coplanarity problem

Specific performance: pin stilts, deformation leads to local pins out of contact with solder paste (such as Figure 1-1 in the pin warping area).

Percentage of soldering defect phenomenon: about 28%.

2.3. Through-hole siphoning effect

Specific performance: pad through-hole design is not appropriate, molten solder flow to the hole leads to the root of the pin solder shortage (such as Figure 1-1 in the pad through-hole area).

Percentage of soldering defect phenomenon: about 18%.

2.4. Solderability decline

Specific performance: pin plating layer oxidation (such as Sn layer thickness is insufficient) or PCB pad surface contamination.

Percentage of soldering defect phenomenon: about 15%.

2.5. Thermodynamic imbalance

Specific performance: PCB heat capacity is too large or insufficient preheating, resulting in melting solder paste along the pins after climbing (core suction phenomenon).

Percentage of soldering defect phenomenon: about 4%.

3.  the systematic improvement proposal

Need to establish a ‘design - materials - process’ trinity of prevention and control system:

3.1. Material control to strengthen

Pin protection: use vacuum packaging for transport, complete the patch within 48 hours after opening the package, storage humidity ≤ 10% RH.

Solderability verification:

Thickness of pin plating layer ≥ 8μm (Sn layer), use Wetting Balance Tester (WBT) to detect contact angle ≤ 90°.

PCB pads are plasma cleaned, and surface energy ≥50mN/m is regularly tested.

3.2. Optimisation of process parameters

Printing process:

Adopt stepped stencil (thickness 0.12-0.15mm), with an opening area 10-15% larger than the pin.

Implement solder paste thickness monitoring (SPI), target thickness 60-80μm.

Solder profile:

Preheat zone temperature was raised to 120-140°C for an extended period of 120-150 seconds.

Peak temperature 245±5°C, liquid time >40 seconds.

Equipment Maintenance:

Clean stencil every 20 PCBs with special solvent and dust free cloth.

Weekly calibration of reflow temperature, ΔT≤5℃.

3.3. Inspection system upgrade

AOI+X-ray linkage:

AOI focuses on detecting solder joint wetting angle (qualification standard: <30°) and pin coplanarity.

X-ray detects the void rate of solder joints (IPC standard: <25%).

SPC process control:

Implement real-time control chart monitoring of key parameters such as soldering temperature and printing thickness.

Establish a database of soldering defect phenomenon and implement PDCA for continuous improvement.

3.4. Operation specification optimisation

Prohibit programming before soldering: prevent deformation of pins by force, and use non-contact tooling for programming operations.

Material flow control: adopt anti-static turnover car to avoid pin collision deformation.

4. the implementation of the effect is expected

Through the above improvement programme, it is expected to achieve:

Reduction of soldering defect phenomenon to less than 0.5% (excellent level in the industry)

Welding quality stability increased by 40%

Rework costs reduced by more than 60%

Product reliability (MTBF) increased by 30

It is recommended to give priority to the implementation of the solder paste printing process optimisation and AOI inspection upgrade, within 3 months to complete the process debugging and see significant improvements. Long-term need to establish DFM design rule base, from the source to eliminate the risk of soldering defect phenomenon.

Conclusion: soldering defect phenomenon is a key control point in the solder paste process, which needs to be systematically managed from the multi-dimensional aspects of materials, equipment, process and environment. Through strict process control and real-time monitoring, the soldering defect phenomenon rate can be significantly reduced to improve product reliability.