Test board

 

The main challenge for soldering fine pitch components is bridgeless soldering. For some components with high heat capacity, through-hole filling may also be difficult. In order to understand the influence of different design parameters, test boards with different number of layers (2, 4, 6, 8 and 10 layers) were made to study the filling of solder through holes. This test board includes all the design features that may need to be supported to expand the welding process window. These features include solder masks, tin stealing pads, different vias and pad diameters suitable for five different pitches (1.00, 1.27, 1.50, 1.75, and 2.00 mm). Since the solderability is affected by the condition of the metal surface, this article compares two different metallization methods for dipping Sn and Cu OSP (organic solderability preservatives). The size of the test board is 285×184×1.6mm3. There are a total of 3 370 through holes on the assembly. In order to keep the substrate flat, a tray is used.

 

Equipment and configuration

 

The test was carried out on a selective point-to-point welding machine. This equipment has a high-frequency drip spray flux device, an infrared heater and a solder pot driven by a magnetic pump. The solder nozzle is one of the parameters studied. In the test, wetting nozzles and non-wetting nozzles were compared. To avoid bridging, a solder discharge regulator (SDC) is installed behind the solder nozzle. This device blows hot air between the leads of the connector. The gas temperature is much higher than the melting point of the solder, and the flow rate is fixed at 7 L/min. During the experiment, the solder temperature was 300 ℃, and the alloy was Sn3.0Ag0.5Cu. The power of the preheater is 45% and lasts for 51 s, so that the upper surface temperature of the substrate is 120 ℃.

 

Experimental design-through hole filling

 

The first experiment focused on filling the vias. A full factorial experimental design with five different parameters was selected, which has historically proven to have an impact on solderability. These factors include flux and welding parameters. Choose the amount of flux and the type of flux on two levels. Drag welding speed (contact time), nozzle type and solder angle are other factors. Although the solder temperature is another factor that affects the filling of the vias, a constant 300°C is maintained here. The preheat setting also remains constant (see data above).

 

The soldering area of ​​the test board has different pitches, through-holes and pad sizes, copper layers, tin stolen pads, and solder resist removed. The purpose of this experiment is to provide engineers with the right tools to design the circuit board and process conditions, so that the fine-pitch connectors on the high-heat-capacity circuit board can achieve sufficient through-hole filling. According to the requirements of IPC-A-610, count the number of well-filled through holes. 360 leads are welded on each printed board.

 

The substrate is designed to show the difference. It is very difficult to achieve 100% through-hole filling on a 10-copper printed board, and it may not be feasible for all components. The purpose of this test board is to find the limit. Different double-row pin connectors of 1.00, 1.27 and 2.00 mm are welded, and a single-row pin connector with 10 leads of 1.50 mm is welded.

 

Two factors have a great influence. The amount of solder and the number of copper layers. There should be enough flux to support the flow of solder into the copper vias. The heat capacity of the copper layer absorbs so much heat that the solder solidifies before it reaches the top of the substrate. Conclusion: A higher heat capacity requires a higher activity flux. There is also an interaction between the flux and the copper layer. Figure 3 shows that with sufficient flux activity, all through holes will be well filled. Even 10-layer copper through holes.