Patent Application
20130011941
The present invention relates to the field of semiconductor assembly and packaging, and more particularly, to the attachment of a semiconductor chip or die onto a substrate using adhesive.
The semiconductor die-attach process is one of the steps involved in semiconductor device manufacturing, and it involves attaching semiconductor dice to specific bond pads on a lead frame. The attachment is usually achieved by first dispensing an adhesive material (e.g. epoxy) onto the bond pads, and then pressing the dice into the adhesive material with a certain pressure.
Thermal treatment with oven cure is thereafter performed to solidify the adhesive and secure the dice onto the lead frame after the die-attach process. The cured dice are then electrically coupled to the bond pads via connecting bonding wires between the dice and the conductive leads on the lead frame. The cured die and the bonding wires are finally encapsulated in a protective case using a molding material, such as thermoplastic resin or ceramic, to complete the packaging of the semiconductor device.
As the adhesive 102 is used for attaching the die 101 onto the lead frame 103, the bond line cannot be too thin. After the die 101 is cured so that the adhesive 102 hardens, the die 101 is still subjected to thermal expansion and contraction at subsequent packaging procedures. If the bond line is too thin, since the thermal expansion and contraction may happen at different rates between the die 101 and adhesive 102, there may not be sufficient adhesive 102 under the die 101 to cater for such expansion or contraction of the die 101 and adhesive 102. This may lead to fractures and cracks in the die 101. The die 101 may also become detached from the adhesive 102 in some severe cases.
On the other hand, the bond line cannot be too thick either. If too much adhesive 102 is present, the adhesive 102 may seep out and contaminate the surface of the die 101. Amongst other things, this may result in poor wire bonding quality when electrical wire connections are made between the die 101 and the lead frame 103. Moreover, the problems described above inevitably deteriorate the reliability and performance of the packaged semiconductor device. Hence, the bond line thickness has to be carefully controlled within an appropriate range during the die-attach process.
Accurate measurement of bond line thickness is necessary in order to achieve precise bond line thickness control. A conventional bond line thickness measurement method is cross-sectioning, which requires a cured die to be cut open along a line. Then, the cross sectioned die and adhesive are put under a microscope for measuring the bond line thickness. Cross-sectioning is a destructive method, and the cutting procedure makes it time-consuming.
As online feedback is difficult to achieve, there have been approaches proposed to control the bond line thickness by designing special lead frames. In US Patent Publication number 2009/0115039 A1 entitled “High Bond Line Thickness for Semiconductor Devices”, it is proposed to create boundary features at the edges of a bond pad on a lead frame. When adhesive is dispensed onto the bond pad, the adhesive is confined within the pad area defined by the boundary features and accumulated. This ensures that the pad area has enough adhesive to create a certain thickness for the bond line. Further, U.S. Pat. No. 5,214,307 entitled “Lead Frame for Semiconductor Devices having Improved Adhesive Bond Line Control” describes a similar approach. Four bumps inside the bond pad area are used instead of the boundary features at the edges. When the die is bonded onto the lead frame, the die would contact the bumps and a specific bond line thickness would be achieved. With the aid of such special lead frame designs, the bond line is guaranteed to be at least of a certain thickness. This prevents the problems faced when a thin bond line is too thin. However, there is still no control on avoiding a bond line that is too thick.
It is thus an object of the invention to seek to provide an online bond line thickness measurement method, so that the bonded die samples do not need to be removed from the die-attach platform in order to measure bond line thickness.
It is a related object of the invention to seek to utilise the online measurement results to control a thickness of a bond line during die attachment.
According to a first aspect of the invention, there is provided a method for manufacturing a semiconductor package comprising the step of attaching a semiconductor die to a substrate on a process platform, the step of attaching the semiconductor die onto the substrate further comprising the steps of: dispensing an adhesive with a dispenser onto the substrate; bonding the semiconductor die onto the adhesive which has been dispensed onto the substrate with a bonding tool; and thereafter measuring a bond line thickness between a bottom surface of the semiconductor die and a top surface of the substrate on the process platform using a measuring device.
According to a second aspect of the invention, there is provided a die-attach apparatus for manufacturing a semiconductor package, the die-attach apparatus comprising: a dispenser for dispensing an adhesive onto a substrate; a bonding tool for bonding a semiconductor die onto the adhesive which has been dispensed onto the substrate; and a measuring device for measuring a bond line thickness between a bottom surface of the semiconductor die and a top surface of the substrate.
It will be convenient to hereinafter describe the invention in greater detail by reference to the accompanying drawings. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention as defined by the claims.
An example of an apparatus and method in accordance with the invention will now be described with reference to the accompanying drawings, in which:
The die-attach apparatus 201 includes a process platform 202, an adhesive dispenser 203, a die bonding tool 204 and a measuring device such as a laser displacement sensor 205. The adhesive dispenser 203, die bonding tool 204 and laser displacement sensor 205 are located at different locations on the process platform 202. Thus, a conveyor is operative to transport a substrate successively to the respective locations during a die-attach operation.
During the die-attach operation, the substrate, which may be in the form of a lead frame 103, is transported along the platform 202. First, the lead frame 103 is positioned at the location of the adhesive dispenser 203 for dispensing an adhesive 102 onto the lead frame 103. Next, the lead frame 103 on which the adhesive 102 has been dispensed is forwarded to a location of the die bonding tool 204 for bonding a semiconductor die 101 onto the adhesive 102 which has been dispensed onto the lead frame 103. Then, the bonded lead frame 103 is moved to a post-bond location, whereat the laser displacement sensor 205 is mounted. The laser sensor 205 is operative to measure a difference in height between the die surface and the lead frame surface in order to measure the bond line thickness or BLT between a bottom surface of the die 101 and a top surface of the lead frame 103.
Thus, the laser displacement sensor 205 is employed at the post-bond location of the die-attach apparatus 201. The displacement readings of the die surface and the lead frame surface are measured immediately after the die 101 is bonded. The BLT can then be calculated via a processor, such as a microprocessor 206, which is electrically connected to the laser displacement sensor 205, adhesive dispenser 203 and die bonding tool 204. Based on the results of the BLT measurement, process parameters are adjusted online in order to control the BLT within an appropriate range.
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It should be appreciated that the die-attach apparatus 201 according to the preferred embodiment of the invention allows the BLT to be controlled in real time. There is no need to remove the bonded die for measuring the BLT or to design special lead frames for the purpose of BLT control. Hence, the said die-attach apparatus 201 helps to improve the yield and quality of the die-attach process.
The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the above description.
