Protecting the DUT During PCB Assembly
The device-under-test or DUT site or sites is the most critical part of an ATE PCB. That’s where a component or chip is placed for testing. There can be one or more DUTs on a board, for instance, four DUTs as shown in Fig. 1. Others can be a dual-site board with two DUTs where a microprocessor (µP) or central processing unit (CPU) is to be placed for testing.
If the DUT is compromised during PCB assembly in any way, the ATE PCB becomes useless. Hence, details are of utmost importance. For example, nut fasteners referred to as PEM nuts (brand name Penn Engineering & Manufacturing Corp.) are used to keep the DUT socket in place. If they’re not properly installed, the DUT socket is not correctly attached. As a result, a chip or component to be tested cannot be loaded on to or tested on the DUT.
There are countless other special considerations that must be taken into account. Check out our article to get more expert details.
In the meantime, here are some tips and hints to get you more familiar with proper assembly steps to protect the DUT.
- Pogo or contact pins used instead of conventional soldering. This solder-less pin contact must remain pristine clean during assembly.
- ATE PCBs must be properly and comprehensively cleaned. A tiny piece of solder in the DUT area can be cleaned and cosmetically it may appear to be fine. But at high-speed levels of testing, the DUT may not perform well.
- Special care must be applied to avoid fingerprints, hand lotions, body perspiration and other contaminants from getting on an ATE board before it goes in the reflow oven.
- Aqueous cleaning is a must for ATE PCBs. This goes beyond de-ionized or DI water. This means using batch cleaning with chemicals used in a certain way to assure the ATE PCBs are extremely clean.
- A highly specific thermal profile is required to heat up the entire ATE PCB. Keep in mind an ATE PCB has 36, 40, 50 or 60 layers. This means there may be as many as 40 power and ground layers and 20 routing layers.
- Stencil design takes on new meaning with ATE PCBs. It must be precise. Omitting key steps in such a stencil design defeats the whole purpose of placing an ATE board on an automated assembly.
ATE PCB assembly takes on a completely different mindset than that of a conventional PCB. It involves placing special attention on several key assembly practices and procedures. That includes having a good handle on the correct thermal profiles, paying close attention to items relevant to ATE PCB assembly such as PEM nut installation, as well as a precise stencil design, and assuring ultra-clean surfaces, especially for the DUT.