COB Bonding Issues. Application Notes

Transcription

1 COB Bonding Issues Application Notes ELAN MICROELECTRONICS CORP. Doc. # General Function AN-024 First Edition January 2005 i

2 Trademark Acknowledgments IBM is a registered trademark and PS/2 is a trademark of IBM. Microsoft, MS, MS-DOS, and Windows are registered trademarks of Microsoft Corporation. Document Number: General Function AN ELAN Microelectronics Corporation All Rights Reserved Printed in Taiwan, 01/2005 The contents of this document are subject to change without notice. ELAN Microelectronics assumes no responsibility for errors that may appear in this manual. ELAN Microelectronics makes no commitment to update, or to keep current, the information contained in this manual. The software described herein is furnished under a license or nondisclosure agreement, and may be used or copied only in accordance with the terms of the agreement. ELAN Microelectronics products are not intended for use in life support appliances, devices, or systems. Use of ELAN Microelectronics products in such applications is not supported and prohibited. ELAN MICROELECTRONICS CORPORATION Headquarters: No. 12, Innovation Road 1 Science-based Industrial Park Hsinchu, Taiwan, R.O.C Tel: Fax: Hong Kong Office: Elan (HK) Microelectronics Corporation, Ltd. Rm. 1005B, 10/F Empire Centre 68 Mody Road, Tsimshatsui Kowloon, HONG KONG Tel: Fax: USA Office: Elan Information Technology Group Miller Ave., Suite 100 Cupertino, CA USA Tel: Fax: Shenzhen Office: Elan (Shenzhen) Microelectronics Corporation, Ltd. Rm. 420/4Fl., Fuxing Garden Fuxing Rd, Futian Dist., Shenzhen Guandong, CHINA Tel: /6 Fax: ii Shanghai Office: Elan Electronics (Shanghai) Corporation, Ltd. 23/Bldg. 115, Lane 572, Bibo Road Zhangliang Hi-Tech Park Shanghai, CHINA Tel: Fax:

3 Contents 1.1 Faulty IC Physical Structure Known Cause Suggested Corrective Action Pad Peeling Known Cause Corrective Action Inadequate Adhesion of Wire Bond on Pad Illustrated Sample of Adhesion Problem Known Cause Corrective Action...6 iii

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5 1 COB (Chip on Board) Bonding Issues The most common problematic issues bothering COB bonding include abnormal fixed position, pad peeling, and inadequate adhesion in wire bonding with pad. The possible causes are stated below: a. Faulty IC physical structure (includes abnormal pad structure, FAB process drift, etc.) b. Inappropriate bonding setting parameters (include flawed PCB layout, fixed position shifted before bonding, residual glue on IC pad surface, bonding machine pulling force exceeds limit, etc ) c. Defective outsourced products (include substandard operational environment, etc ) d. Inferior storage condition (including high humidity storage environment, punctured vacuum (packaging probably during transportation), not using vacuum or nitrogen gas chamber storage for customer left-over ICs, etc ) Further details on each of the above mentioned issues are discussed in the following sections. 1.1 Faulty IC Physical Structure Known Cause IC s subjected to inappropriate bonding processes. As illustrated below, it is obvious that deviation from correct bonding location has been committed accompanies with metal chafing on pads peripheries. Breaching the critical bonding position parameters could result to VDD-VSS short circuits. It is evident that the commissioned bonding factory has been negligent in making proper adjustment to their bonding machines before performing the bonding job. COB Bonding Issues 1

6 1.1.2 Suggested Corrective Action 1) Check to ensure that the bonding machine is equipped with a clamping device with strong and rigid clips. Be sure that the clamping device is at its correct position and that it is properly locked with its locking screws tightly mounted with no gap for possible movement. 2) Check to verify that the BTO of bonding machine is at its proper fixed position (trial wire bond at PCB pad in order to confirm that the plus sign ( ) appears at the center of the pad as observed from display screen or as shown in the figure below). COB Bonding Issues 2

7 3) Check two PCB connectors and its corresponding two IC pads. Be sure they are at accurately aligned at its fixed positions before wire bonding. 1.2 Pad Peeling Known Cause 1) Pad structure: Problem can easily occur when processing a single metal designed pad structure. 2) Setting parameter of the bonding machine: The production yield loss is attributed to the inappropriate setting parameter of the bonding machine and of the PCB design. 3) Peeling major cause: Pad metal surface adhesion with lower level unable to withstand the pulling force of the bonding machine due to high throughput. COB Bonding Issues 3

8 4) COB (Chip on Board) layout design: The more pin counts there are, the probability of yield loss increases. This is because the PCB layout has to locate some connectors farther from the IC pads to accommodate the increases pin counts and the farther the distance between the IC pads & PCB connectors, the stronger is the bonding machine pulling force resulting to more yield loss Corrective Action 1) Use of appropriate bonding machine type and proper adjustment of its settings could help reduce the peeling yield loss. a. Try turning down the ultrasonic solder Power and Time settings but do not exceed the production spec limits. b. The bonding Force adjustment is for expert use only. Do not try to readjust it. Leave it at its original setting. c. Reduce the wire bonding speed (by increasing the time of travel between first bonding spot to the second one, or reducing the X & Y motor velocity). This should be recommended when peeling problem is received from customers in order to decrease bad yield. For example, replace automatic with semi-automatic or replace semi-automatic with slower speed. 1 st bond Pad nd 2 bond Chip d. The bonding head should travel in an arc from first bonding spot to the second one. However, limit the loop height of the bonding wire to as low as possible to avoid extending the wire length. Otherwise, short circuit could easily occur. 2) Increase the diameter of the aluminum soldering wire: That is, replace 1.0mil with 1.25mil diameter to increase contact area and improve resistance to pressure. 3) For ASM AB559 bonding machine, setting parameter should be: Power=85, Time=20, and Force=30. COB Bonding Issues 4

9 4) When pad peeling takes place, record the corresponding IC tray Lot Number and the rate of bad yield. Then notify the relevant staffs at ELAN immediately. Apply temporary remedy to reduce bad yields by adjusting the machine with the parameters mentioned above. ELAN QRA will deal with the problem as soon as problem report is received and keep the customer posted of the progress made to resolve the problem. 5) The following are the actions ELAN usually takes in dealing with pad peeling problem: a. Replace foundry to one which has a very minimum experience of, or has not encounter pad peeling. b. Improve cooperation between ELAN and foundry in resolving the pad peeling problem. 1.3 Inadequate Adhesion of Wire Bond on Pad Illustrated Sample of Adhesion Problem Known Cause 1) Pad oxidation due to bad storage condition: Inferior storage or poor shipping condition (including high humidity storage environment, punctured vacuum (packaging probably during transportation), no vacuum or nitrogen gas chamber storage of customer left-over IC s, etc ) 2) Presence of residual glue on IC s pad surface (resulting from customers dipping glue on dirty bamboo chopstick tip to pick up IC from IC tray) bamboo chopstick COB Bonding Issues 5

10 3) Improper bonding machine setting parameter Corrective Action 1) Adjust the bonding machine Time & Power parameters to reduce the failure rate: a. Time means the ultrasonic duration to of 1 st bonding. b. Power means the ultrasonic power of 1 st bond. 1 st Bond 2 st Bond 2) With the lack of adhesiveness condition, it is recommended that the Time & Power values are increased for the 1 st bonding. Then try the bonding task again and compare the current result with the previous (bad yield) one. 3) The left over ICs should be properly vacuum packed or stored in a nitrogen filled gas chamber. COB Bonding Issues 6