Au-Au Thermocompression or Thermosonic attachment
Die can be presented with bumps facing down or bumps facing up. In order to ensure that the dies are presented correctly for flip chip assembly then bumps must be facing down. If the die is not presented in such a fashion then a flip chip tool is used to rotate the chip by 180degrees. Once the bumps are facing down then an automated pick and place machine, with a “look-up” and “look-down” camera is used to align the die onto the substrate prior to reflowing the solder bumps.
During the pick and place process it is crucial that the bumps are aligned accurately to the pads on the substrate to avoid shorting or open circuits. For high bump count and fine pitch applications it is important that high placement accuracy flip chip machine, with appropriate pattern recognition cameras, is used to ensure that bumps overlap the substrate pads.
Thermosonic (TS) or Thermocompression (TC) attach is used to attach gold plated or gold stud bumped die to a substrate by means of micro-welding technique. This uses a combination of heat and pressure in the case of TC attach processes and with the addition of ultrasonic energy for TS attach.
This is a flux free process and is widely used for MMIC, optical and sensor applications where flux can interfere with the device operation. It should be noted that it is expected that flux will have a negative impact on the reliability and performance on MMIC die.
In general the metallurgical joining provided by the gold-gold TC or TS attach processes is more reliable than conductive particles and adhesive joining. However, these attach processes are typically limited to low bump counts on the order of low hundreds due to the high force required, which scales with the number of bumps. As you start moving beyond a few hundred bumps the level of force required starts to risk potential mechanical damage to the die. In addition, the force level requirements start to move beyond the capability of many flip chip die bonders.
A thermo-compression process will operate at a temperature of around 250degC to compensate for the lack of ultrasonic energy and will require around 100g downward force per bump. For typical MMIC devices with 10-20 bumps this equates to a downward force of 1.4KG and 1.6KG.
A Thermosonic process can operate at lower temperatures in the order of 150degC and requires around 10g per bumps. This gives a downward force for typical devices of 140g and 160g respectively.
The downward force required for both MMIC devices and the thermo-sonic and thermo-compression processes are within the capability of the flip chip bonders at Optocap.
However, typical GaAs MMIC die used are thin in the order of 50-70um and GaAs is more brittle than Silicon. As a result the lower downward force used in a Thermosonic attach process is advantageous. In addition, the lower attach temperate means a lower zero-stress temperature which means a lower delta temperature operation. Once again looking at equation (1) a lower temperature delta means less stress on the bumps and hence better reliability.
However, we need to take into account that the ultra-sonic energy could increase the likelihood of bond pad cratering especially if the die experiences two ultrasonic events (one ultrasonic process for bumping the die and one ultrasonic process for attaching the die to substrate). In order to mitigate this we would look to bump the substrate instead of the die.
Au stud bumps and adhesive
The first option is to use an anisotropic conductive epoxy film (ACF). The ACF is placed between the bumped die and substrate and heat and force are applied. In some flip chip applications this has the advantage that the ACF, in addition to providing the electrical interconnection, also provides stress relief between the die and substrate in the same way as underfill. However, it worth noting that down-force required to make such an electrical interconnection is still high, on the order of 100g per bump, and hence the concern regarding damage to the thin GaAs MMIC chip, which was highlighted for the thermo-compression process, still applies. In addition, while an underfill is not used, it is anticipated that epoxy in between the die and substrate is not going to be beneficial to the high frequency electrical performance. As a result Optocap propose that such an approach is not evaluated further.
Another approach is with the epoxy applied directly onto the pads using a stencil printing or directly onto the bumps by dipping the bumps into epoxy or stamping the epoxy on to the bumps. The epoxy bumped die is then aligned to the substrate and lightly pressed against the substrate and the epoxy cured. The Au-stud bumps act as a mechanical stop, preventing the epoxy from over-spreading and causing electrical shorts between adjacent bumps. The uniformity of the Au stud bump is important when working with conductive epoxies for flip chip assemblies. If the Au-stud bumps are not of equal height above the chip’s surfaces, they may not reach far enough to contact the epoxy and ensure reliable electrical contact.
Optocap’s process for Au stud bump and adhesive approach is to precisely dip the bumped die, on the order of a few tens of microns, into an epoxy pot to achieve a very small amount of epoxy on the bump tip. The bumped die would then be aligned to the substrate bond pads and the electrical interconnection would be made by means of curing the epoxied bump to the substrate bond pad.
This process has a number of advantages. Firstly, the attachment process temperature would be determined by the selected epoxy. Epoxies exist which have cure temperatures on the order of 50degC. This would significantly reduce the zero stress temperature and hence improve the reliability. Secondly, the epoxy would provide addition compliance and stress relief without the use of an underfill and may allow be a good compromise between an underfill and non-underfilled option. Lastly, the down force required for such a process would low, likely on the order of a few 10’s of gram, and would reduce the chances of any mechanical damage to the chip.
A range of epoxies can be selected based on dispensability, pot-life, resistivity, cure times, cure temperature.
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