While in South East Asia last summer, Indium's technical team and I had a chance to discuss flip-chip dipping processes with a major equipment vendor. One of the topics was device (bumped package or die) "pick-up" out of a flip-chip flux dipping tray, and how different factors affect whether the device is capable of being removed from the sticky flux. The same discussions are relevant to all flux and solder paste dipping processes, from flip-chip, to MEMS package assembly, to WL-CSP flux dipping.
为什么小设备比大设备更难从浸渍盘中取出，这一直是个谜。这似乎与逻辑相悖，因为较小的设备：
- 将更轻（质量更轻），因此这将使拾取更容易，而不是更困难
- for the same I/O size and density (number of I./O's per unit area), the effect of extensional viscosity ("tack"), as discussed previously, should scale linearly with the device area. So there should be no effect of flux contact area.
我们了解到，还有第三个方面需要考虑：设备与真空吸头对准的准确性。
This accuracy is a function of how carefully the center of the device aligns with the center of the pick-up nozzle. As can be imagined, the worst-case scenario is one where an inaccurate vacuum head placement must pick-up a package that has been allowed to move around in its overpack (tape & reel or waffle pack), and then dip it in the flux at a very high speed. In order to mitigate this effect, the vacuum head is always designed so that its contact perimeter (which is always smaller than the nozzle apparent diameter) will always be inside the die area. There is therefore a statistically-derived "keep out width" (KOW), which obviously has a greater negative impact as the package decreases in size.
使用圆形喷嘴的简单情况如下图所示。
对于方形封装，可以用简单的数学方法来表示封装宽度对效果的影响，即 "有效面积百分比"= 喷嘴面积/封装面积，如下图所示。请记住，将设备向上拉的唯一作用力是喷嘴内外的压力差乘以喷嘴内的面积。
Note that there are other factors that may also impact the ability to pick up small die and packages: all being related to a reduced pressure differential between the inside of the sealed package / nozzle cavity and the outside air:
- 封装/喷嘴接口处的微小泄漏对小喷嘴内部真空的影响比对大喷嘴内部真空的影响更大
- 气压可能会变化（海拔高度或天气条件）
The best chance for a nozzle to pick up a small or otherwise problematic die out of a dipping tray is, therefore, through implementing one or more of:
- Low "tack" flip-chip or MEMS flux
- 通过喷嘴设计和设备公差最大化喷嘴有效面积（降低 KOW）
- 从助焊剂浸渍盘中缓慢抽出
- Tighter tolerances on packaging "pockets" for tape and reel or waffle-packed die or packages
The tradeoffs here are that a low viscosity/tack flux may not be able to hold components in place prior to, or during, reflow, and that a slower, more precise, dipping and placement process reduces throughput.
Indium Corporation is currently expanding our range of dippable ultralow and near-zero residue fluxes and we will be teaching you more in coming months. Please feel free to share your findings with us.
感谢我的朋友 Hyoryoon Jo 博士创造了 "有效区域 "一词。
干杯安迪
