How best to probe QFP, SMD packs? - Page 1
Forum section: electronics / projects

author: iXod
author_rank: Frequent Contributor
timestamp: September 01, 2021, 06:56:32 am
content: Back when DIP packages were the norm it was common practice to use a 3M “nailhead” test clip, clamped onto the leads on each side of the device to provide test points for attaching scope probes, etc.What’s the equivalent technique for probing SMD devices, like 4-sided quad packs?Thanks.

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 01, 2021, 08:20:14 am
content: Solder thin wires to pads or some accessible spotsAnd when designing a board please include test pads

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 03, 2021, 08:55:24 pm
content: Something like the picture below may work if the pads are big enough, but I have not tried it myself.Another option is to solder wires directly to the pins like Miyuki mentioned.Thin "enameled" wire works best. (0.1mm or 0.2mm)You need to set your iron hotter then usual to burn through the isolation. Or even better, use two irons. One hot iron to tin the end of the wire, and a cooler iron to solder them to the PCB.Then also use a 90 degree 2.54mm male header, and glue that to the PCB in a convenient place. For example with hot snot, which can be removed if needed. Then solder the enameled wire between your pads and the connector. For the rest you can use "dupont" wires or similar.

author: tszaboo
author_rank: Super Contributor
timestamp: September 03, 2021, 10:15:10 pm
content: QFP is easy man. When you spend 30 minutes patching a board with QFN, because you have to solder 4 tiny wires to 4 pins on 0.5mm pitch, and the entire thing is held on by the soul and hearth of the baby Jesus, that is still fine. A BGA, that you are unable to route on 4 layers, that's when the issues begin. QFP... You can still see everything with the naked eye!

author: DEV001
author_rank: Contributor
timestamp: September 03, 2021, 10:24:17 pm
content: PCBite makes some really nice probes with a magnetic base.https://sensepeek.com/

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: September 03, 2021, 10:32:31 pm
content: As long as you are not referring to QFN packages, but rather the PLCC and QFP styles of package, there are test clips for complete 4-sided access. I have some here up to 200something pins. The larger pitches like 0.65 and 0.8 are rather affordable and I have several kits of them.But my favorite solution for the 0.5 and 0.65mm pitches, which is working very well as long as you do not need all pins at once are the so-called WEDGE PROBES. They are available in 2 to 16 pins and with a sensible combination of them you can run most tests that you'll ever need to.They are surprisingly robust and offer good signal integrity. Tek has another name for them.For the two-sided packages, complete clips are not only available for SOC, but also for the various TSOP and MSOP packages. It is just QFN which leaves you not much choice except a positioner.Names to look for (besides HPAK and Tek): Pomona, AP Products, Delta Probe, Emulation Technology, Winslow AdapticsSome pics should illustrate the mentioned versions..