1. Background
Wave soldering is applied and heated by molten solder to the pins of components. Due to the relative movement of wave crest and PCB and the “stickiness” of molten solder, the wave soldering process is much more complex than reflow welding. There are requirements for pin spacing, pin extension length and pad size of the package to be welded. There are also requirements for the layout direction, spacing and connection of mounting holes on the PCB board surface. In a word, the process of wave soldering is relatively poor and requires high quality. The yield of welding basically depends on design.
2. Packaging requirements
a. Mount components suitable for wave soldering should have welding ends or leading ends exposed; Package body ground clearance (Stand Off) <0.15mm; Height <4mm basic requirements.
Mount elements that meet these conditions include:
0603~1206 chip resistance and capacitance elements within the package size range;
SOP with lead center distance ≥1.0mm and height <4mm;
Chip inductor with height ≤4mm;
Non-exposed coil chip inductor (type C, M)
b. The compact pin fitting element suitable for wave soldering is the package with the minimum distance between adjacent pins ≥1.75mm.
[Remarks] The minimum spacing of inserted components is an acceptable premise for wave soldering. However, meeting the minimum spacing requirement does not mean that high-quality welding can be achieved. Other requirements such as layout direction, length of lead out of welding surface, and pad spacing should also be met.
Chip mount element, package size <0603 is not suitable for wave soldering, because the gap between the two ends of the element is too small, easy to occur between the two ends of the bridge.
Chip mount element, package size >1206 is not suitable for wave soldering, because wave soldering is non-equilibrium heating, large size chip resistance and capacitance element is easy to crack due to thermal expansion mismatch.
3. Transmission direction
Before the layout of components on the wave soldering surface, the transfer direction of PCB through the furnace should be determined first, which is the “process reference” for the layout of inserted components. Therefore, the direction of transmission should be determined before the layout of components on the wave soldering surface.
a. In general, the transmission direction should be the long side.
b. If the layout has a dense pin insert connector (spacing <2.54mm), the layout direction of the connector should be the transmission direction.
c. On the wave soldering surface, silk screen or copper foil etched arrow is used to mark the direction of transmission for identification during welding.
[Remarks] Component layout direction is very important for wave soldering, because wave soldering has a tin in and tin out process. Therefore, design and welding must be in the same direction.
This is the reason for marking the direction of wave soldering transmission.
If you can determine the direction of transmission, such as the design of a stolen tin pad, the direction of transmission can not be identified.
4. The layout direction
The layout direction of components mainly involves chip components and multi-pin connectors.
a. The long direction of THE PACKAGE of SOP devices should be arranged parallel to the transmission direction of wave peak welding, and the long direction of chip components should be perpendicular to the transmission direction of wave peak welding.
b. For multiple two-pin plug-in components, the connection direction of the jack center should be perpendicular to the transmission direction to reduce the floating phenomenon of one end of the component.
[Remarks] Because the package body of the patch element has a blocking effect on the molten solder, it is easy to lead to the leakage welding of the pins behind the package body (destin side).
Therefore, the general requirements of the packaging body does not affect the direction of the flow of molten solder layout.
Bridging of multi-pin connectors occurs primarily at the de-tinning end/side of the pin. Alignment of connector pins in the direction of transmission reduces the number of detinning pins and, ultimately, the number of Bridges. And then eliminate the bridge completely through the design of stolen tin pad.
5. Spacing requirements
For patch components, pad spacing refers to the spacing between the maximum overhang features (including pads) of adjacent packages; For plug-in components, pad spacing refers to the spacing between pads.
For SMT components, pad spacing is not only considered from the bridge aspect, but also includes the blocking effect of the package body that may cause welding leakage.
a. Pad spacing of plug-in components should generally be ≥1.00mm. For fine-pitch plug-in connectors, a modest reduction is allowed, but the minimum should not be less than 0.60mm.
b. The interval between the pad of plug-in components and the pad of wave soldering patch components should be ≥1.25mm.
6. Special requirements for pad design
a. In order to reduce welding leakage, it is recommended to design pads for 0805/0603, SOT, SOP and tantalum capacitors according to the following requirements.
For 0805/0603 components, follow the recommended design of IPC-7351 (pad expanded by 0.2mm and width reduced by 30%).
For SOT and tantalum capacitors, pads should be extended 0.3mm outward than those of normal design.
b. for the metallized hole plate, the strength of the solder joint mainly depends on the hole connection, the width of the pad ring ≥0.25mm.
c. For nonmetallic holes (single panel), the strength of the solder joint depends on the size of the pad, generally the diameter of the pad should be more than 2.5 times the aperture.
d. For SOP packaging, tin theft pad should be designed at the destin pin end. If the SOP spacing is relatively large, tin theft pad design can also be larger.
e. for the multi-pin connector, should be designed at the tin end of the tin pad.
7. lead length
a.The lead length has a great relationship with the formation of the bridge connection, the smaller the pin spacing, the greater the influence.
If the pin spacing is 2~2.54mm, the lead length should be controlled in 0.8~1.3mm
If the pin spacing is less than 2mm, the lead length should be controlled in 0.5~1.0mm
b. The extension length of the lead can only play a role under the condition that the component layout direction meets the requirements of wave soldering, otherwise the effect of eliminating bridge is not obvious.
[Remarks] The influence of lead length on bridge connection is more complex, generally >2.5mm or <1.0mm, the influence on bridge connection is relatively small, but between 1.0-2.5m, the influence is relatively large. That is, it is most likely to cause bridging phenomenon when it is not too long or too short.
8. The application of welding ink
a. We often see some connector pad graphics printed ink graphics, such a design is generally believed to reduce the bridging phenomenon. The mechanism may be that the surface of the ink layer is rough, easy to absorb more flux, flux in high temperature molten solder volatilization and the formation of isolation bubbles, so as to reduce the occurrence of bridging.
b. If the distance between the pin pads <1.0mm, you can design solder blocking ink layer outside the pad to reduce the probability of bridging, it is mainly to eliminate the dense pad in the middle of the bridge between the solder joints, and the main elimination of the dense pad group at the end of the bridge solder joints their different functions. Therefore, for pin spacing is relatively small dense pad, solder ink and steal solder pad should be used together.
Post time: Nov-29-2021