PCBgogo

In the electronics manufacturing industry, poor soldering of through-hole devices (THD) has always been a pain point affecting product reliability. Recently, during the mass production of a certain model of industrial control board, our team encountered a batch of cold soldering issues with DIP-packaged connectors. Through systematic process improvements, the defect rate was ultimately reduced from 8.7% to 0.9%, with the characteristics of the PCB material from the supplier being one of the key variables.

I. Background and Diagnosis of the Issue

The batch of products used double-sided FR4 boards produced by PCBGOGO, and the following typical defects were observed after wave soldering:

• Insufficient solder climb height (IPC standard requires ≥75% board thickness)

• Solder joints with cold solder characteristics (dull and rough surface)

• Copper foil peeling at some vias

Laboratory analysis revealed the following:

• Material Factors: Differences in glass fiber fabric weaving density led to uneven local heat conduction.

• Process Factors: Manual rework soldering did not adjust the temperature profile according to the material characteristics.

• Design Factors: Some via-to-pin diameter ratios (via diameter/pin diameter) were designed at 1.3, lower than the recommended value of 1.5.

II. Targeted Improvement Measures

1. Temperature Parameter Optimization

Considering the glass transition temperature (Tg = 140°C) of the PCBGOGO boards:

The soldering iron temperature was adjusted from the original 350°C to a dynamic mode:

• Preheating stage: 280°C for 3 seconds (to activate the flux)

• Soldering stage: 380°C for 2 seconds (optimal measured solder penetration)

Using the HAKKO FX-951 soldering station with a chisel tip, the thermal recovery time was reduced by 40%.

2. Wetting Assistance Technology

Using a no-clean flux with an activity level of ROL1 (e.g., AMTECH NC-559):

Developed a "two-stage wetting method":

• First, use a desoldering braid to remove old solder.

• Apply flux to the via wall using a needle tip.

• Maintain a 30° angle between the soldering iron tip and the via wall during soldering.

3. Enhanced Process Control

• Established incoming material inspection records for PCBs (focusing on dielectric constant and Z-axis expansion coefficient).

• Implemented "three-zone temperature monitoring" for through-hole components:

• Pin root (target temperature 215±5°C)

• Via wall midsection (target temperature 195±10°C)

• Solder pad surface (target temperature 230±5°C)

III. Verification of Implementation Effects

After the improvements, 500 sets of sample machines were continuously produced:

• Cross-sectional analysis showed that the solder fill rate increased from 68% to 93%.

• The pass rate for thermal cycling tests (-40°C to 125°C) was 100%.

• Rework time per component was reduced from an average of 4.2 minutes to 1.8 minutes.

IV. Experience Summary

• Thermal parameters of PCB materials from different suppliers need to be documented separately. For example, the CTE of the material used in this case changed significantly above 270°C.

• For high-density through-hole layouts, it is recommended to use a stepped rework sequence: solder the ground pins first, followed by the signal pins.

• During manual rework soldering, the choice of soldering iron tip is more important than temperature settings. A micro-concave chisel tip is recommended.

Note: The process parameters mentioned in this article should be adjusted according to specific equipment, and it is recommended to consult the supplier's technical manual for special materials.