Wettable Flank Plated PQFN

By Bradley Smith,
亚博棋牌游戏Allegro微系统有限责任公司

Background

塑料四扁平封装无铅（PQFN）或四扁平封装无铅暴露焊盘（QFN-EP）封装[a.k.a.QFN]没有容易焊接的、视觉上暴露的终端，因为封装设计在封装底部有用于焊接的终端。封装边缘在分离后露出终端的铜，并且由于铜的氧化不易湿焊。因此，无法通过光学或x射线目视确定包装是否有效焊接。电气测试是确定焊接终端电气连接的唯一方法。在某些应用中，所有终端的完整电气测试都很困难或不完整。因此，在高可靠性应用中，可能需要目视检查终端焊点的完整性。正是出于这些目的，可湿侧面，镀锡终端的QFN已被开发作为一种视觉援助，以确定板焊料操作的有效性。亚博尊贵会员

Figure 1: PCB Land Layout for 5 mm X 5 mm 28-Lead PQFN
图1:5 mm x 5 mm 28引线PQFN的PCB焊盘布局

Figure 2: PCB Land Layout for 4 mm x 4 mm 24-Lead PQFN

可湿侧翼板

在典型的QFN组装操作中，单个器件的封装体是单独的，并通过锯切操作与“砖”分离，锯切操作为每个终端留下暴露的铜侧壁。由于暴露在外的铜没有任何镀层，铜就会氧化。当设备焊接到电路板上时，这些侧壁区域焊接不一致。可湿性侧面镀锡是一个额外的工艺步骤，它将100%镀锡板（具有正常的镀锡厚度）放置在封装底部端子和暴露的侧面上。可湿侧面镀层保护铜，并允许在外部侧面区域进行焊接，以便进行光学检查，验证良好的焊角连接，从而验证良好的电气连接。

板焊料

A standard QFN board layout is used for sidewall plated parts with the addition of extended terminal lands 40 mil (1.00 mm) in length (see Figures 1, 2) and of 31.5 mil (0.80 mm) (refer to Figures 3 and 4). This allows additional solder paste to flow to the terminal wettable flank and form a solder fillet there which can be visually identified. Standard rosin or water-based no-clean flux lead-free (SAC305 or equivalent) solder pastes can be used with a nominal screen thickness of 0.127 mm (5 mil). Stencil land length should be extended an additional 0.20 mm. Therefore, 1.0 mm terminal lands should be extended to 1.2 mm, and 0.8 mm terminal lands should be extended to 1.0 mm. Reflow conditions should have a peak temperature no lower than 230ºC, typically 245ºC, and no higher than 260ºC, and use a nitrogen blanket or air. If nitrogen reflow is available, better wetting and consistency may occur, regardless of temperature, flux system, or alloy used. Typical package sizes are 5 x 5 mm (28L), 4 x 4 mm (24L and 20L), and 3 x 3 mm (10 leads--5 on two sides). Note that the above terminal land lengths are recommended, but customers may wish to use 20% shorter lengths to accommodate a particular solder paste/flux system and optical inspection tool preference.

图3:4 mm X 4 mm 20引线PQFN的PCB焊盘布局
Figure 3: PCB Land Layout for 4 mm x 4 mm 20-Lead PQFN

Figure 4: PCB Land Layout for 3 mm x 3 mm 10-Lead PQFN

图5：为可湿侧面准备的传感器IC
The wettable flank process makes a partial cut into the terminal edge to expose a side flank. In the next step of tin plating, the flank is plated with the full tin thickness. Image to the right shows the detail of the exposed flank, which is tin plated.

图6：显示与PCB板形成的可光学检查的固体焊料圆角的图示横截面

图7：显示与PCB板形成的可光学检查的固体焊料圆角的实际截面照片

Figure 8: Ideal Solder Fillet (230ºC, 40 mil pad length, 8-hr steam preconditioning), Side View, Optically Inspectable

Figure 9: Visually Inspectable Solder Fillet (245ºC, 40 mil pad length, 8-hr steam preconditioning)

焊料润湿的目视检查

图10：侧壁端接处焊料嵌条的X射线

Observation looking directly down on the device (90º) shows wetting of the lands and the wettable flank. In addition, x-ray can show (with an unobstructed board) that sidewall fillets have formed. A black band at the package edge shows the thick solder fillet (see Figure 10).

Refer to IPC-A-610 Spec Acceptability of Electronic Assemblies section 8.2.13 Plastic Quad Flat Pack-No Leads (PQFN) and note the maximum side overhang is 25% of termination width and the minimum end joint width is 75% of termination width. The minimum toe (end) fillet height up the termination sidewall is not specified by Allegro but is generally observed at more than 50% of the wettable flank height using nitrogen reflow. Refer to Application Information “Allegro产品的焊接方法（SMD和通孔）” at Allegromicro.com; Design Center; Packaging, for information on reflow profiles and considerations.

Allegro warrantees that the device will be reliable and electrically connected by the solder terminations under the package body regardless of the degree of sidewall soldering.

本文件中包含的信息不构成Allegro就本文件主题向客户作出的任何陈述、保证、保证、担保或诱因。所提供的信息不能保证基于此信息的过程是可靠的，或者Allegro已经探索了所有可能的故障模式。客户有责任对最终产品进行充分的鉴定测试，以确保其可靠并满足所有设计要求。