My last post dealt with IoT printed circuit board wire bond reliability and making sure wire bond connections are properly performed between a bare chip and the PCB or substrate. Beyond that particular subject, there is also so much more technology associated with IoT devices and reliability. Die shear strength is one area demanding increasing attention as IoT technology trends toward greater advances.
In tech talk, the term shear refers to forces or stresses. In the case with chips or dies, shear force runs parallel to the chip or die surface.
So what’s it got to do with IoT PCBs and dies?
IoT devices require more and more electronics functionality placed on extremely small PCBs like rigid and rigid-flex circuits. To comply with the reduced real estate, bare chips or dies without space-consuming packages are placed directly on the small board or a substrate on the board. These die attach or bare chips can be such types as chip-on-board, flip chip or others.
It’s important to note here that when referring to microelectronics or wire bonding, it deals solely with die attach and wire bond. The PCB industry is now trending toward taking into account both microelectronics/wire bonding and traditional surface-mount technology manufacturing on the same product line. This makes it more challenging in manufacturing newer handheld, portable and wearable devices.
There are three elements to deal with here. One is metallization between the die and the media. Two, the media can be adhesive, solder or epoxy. Third, there’s the media and the substrate.
A die or bare chip is attached to the PCB or substrate using either traditional solder or epoxy. What’s critical is assuring it is correctly and solidly attached so that die integrity can be verified. And that is done by applying die shear strength force. In effect, it verifies whether or not the die has a metallization problem. If there is a metallization issue between die and substrate, the result is a poor shear and joint integrity is compromised. This means the joint is not in accordance with Mil Standard 883, Rev G or Rev F.
A shear strength test is performed to check the integrity of the metallization between the die and the adhesive and the adhesive and the substrate. These are two distinct types of metallization that must be accurately performed for an optimal joint.
If the integrity is compromised when applying die shear force, the die will pop up, disintegrate or partially peel off. This tells the IoT PCB assembly technician that the die wasn’t properly assembled on the substrate or PCB. The amount of measurement calculated for applying the shear force depends on the size of the die. For example, a 100-mil square die requires a certain amount of force that needs to be applied. But if it’s a 150 by 100 mil rectangular die, it requires a different amount. The amount of force applied to verify the die strength is directly proportional to the size of the die.
A number of different die shear strength testers are available on the market for electronics manufacturing service providers performing IoT PCB assembly and wire bonding. There are variances among these different testers. However, the main points to be made here is each has a so-called “push gate” and a die contact tool. The push gate is applied at the side of a die to ensure the die remains in its position. The die contact tool then uniformly distributes force to the edge of the die to determine the validity of the joint.
Die shear strength testing like this is performed according to test conditions specified by application. The end result is the shear force specified in Mil Standard 883, Rev G or Rev F is met and the joint remains intact. Or, if the test fails, the die pops off and at times creates damage to the surrounding PCB area.
Caution must be exercised when an electronics manufacturing service provider lacks not only the proper die shear strength test equipment, but also the expertise to correctly administer die attach. Without verification tools like this, the IoT device OEM is taking a gamble. The possibility exists, therefore, for an IoT device chip or die to fall apart either in the lab or in the field, especially if the product is subjected to a rugged environment.
Considering today’s IoT device technology advances, die shear strength testing is of utmost importance to maintain a product’s long life and reliability.
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