Scalloped edges - purpose?  - Page 1
Forum section: electronics / beginners

author: SteveThackery
author_rank: Super Contributor
timestamp: August 29, 2024, 08:40:58 am
content: This is probably a really silly question, but what is the purpose of those scalloped edges often found on microcontroller boards? I'm referring to boards with a row of holes along an edge which are typically used for those pin connector strips; as well as the THP holes, adjacent to each hole the PCB is "scalloped" and the copper wraps through the scallop to the other side.My guess is that it would allow the board to be pushed into some sort of pin array connector, but I've never seen such a connector.Anyone know? (And feel free to take the p**s!)

author: Phil1977
author_rank: Frequent Contributor
timestamp: August 29, 2024, 08:44:29 am
content: https://www.pcbbuy.com/news/What-Is-Castellated-Holes-in-PCB-Drilling-Process-with-4-Factors.html

author: gamalot
author_rank: Super Contributor
timestamp: August 29, 2024, 08:45:23 am
content: Allows you to use the entire board as a surface mount device.

author: newbrain
author_rank: Super Contributor
timestamp: August 29, 2024, 08:48:25 am
content: It's called "castellation" (or castellated edges/holes).Rather than for a connector, it's used to easily solder the board as an SMD, without the need of pin headers.Of course, castellated boards should only have components on top, as the Pico in your picture.

author: SteveThackery
author_rank: Super Contributor
timestamp: August 29, 2024, 08:50:45 am
content: Thank you, both. I'm still struggling to visualise it. So it allows the board to be soldered directly to another board. But is this edge-to-edge in the same plane (ie side by side), edge-to-edge but at right angles, or one board placed flat on top of another?

author: gamalot
author_rank: Super Contributor
timestamp: August 29, 2024, 08:51:47 am
content: like this

author: SteveThackery
author_rank: Super Contributor
timestamp: August 29, 2024, 08:53:20 am
content: Quote from: newbrain on August 29, 2024, 08:48:25 amSorry, our posts crossed. So it sounds like it is intended that the board (the Pico in my picture) can be placed flat on top of another board and soldered as if it were a giant SMD.

author: SteveThackery
author_rank: Super Contributor
timestamp: August 29, 2024, 08:55:54 am
content: Ah, yes, thank you! The picture makes it clear.My thought that the boards could be edge to edge but at right angles doesn't seem sensible because applying a force between them could lift the copper traces off the PCB due to the leverage.

author: SteveThackery
author_rank: Super Contributor
timestamp: August 29, 2024, 08:57:14 am
content: Thanks, everyone - it is clear now. I really appreciate your help - thank you again.

author: golden_labels
author_rank: Super Contributor
timestamp: August 29, 2024, 10:33:29 am
content: How reliable are solder joints on that kind of connection?I can imagine that those 1×2 cm PCBs don’t suffer much, but what about bigger ones? My guess would be, that joints may experience significant stress. The mother board is flexing, the daughter board experiences thermal expansion, and both have very different frequency response to vibration.

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 29, 2024, 12:54:52 pm
content: don't worry, think about automotive boards, i have projects with communication modules 35#35mm and more, we test those at -40 and +85 Celsius and they work, daughter board is kinda bga module,think of motherboard pads forces. btw, 85 C is the ambient, the ic goes to 110-115 before throttle the power...those esp07 modules for example have much bigger pads, lower operational temp (lot of smart devices on wifi/tasmota-like as industrial products are working fine for yeats now)br

author: ataradov
author_rank: Super Contributor
timestamp: August 29, 2024, 01:47:35 pm
content: Quote from: golden_labels on August 29, 2024, 10:33:29 amYou solder like to like material. Both PCBs will expand at the same rate.Microchip sells ARM MPU module like this, which is 40 mm x 38 mm with a lot of pins (https://www.digikey.com/en/products/detail/microchip-technology/ATSAMA5D27-SOM1/8536637), so it must be fine.

author: radiolistener
author_rank: Super Contributor
timestamp: August 30, 2024, 04:29:57 pm
content: you can use thin two sided adhesive tape to stick one pcb to another. It will dump mechanical vibrations.This way of solder one pcb module to another is often used for WiFi/BT modules.Also you can find FPGA modules on aliexpress in this form-factor.