State of art substrate manipulation gonzales istfa 2011

ISTFA 2011, Proceedings from the 37th International Symposium for Testing and Failure Analysis, November 13-17, 2011 San Jose, CA, USA

Copyright © 2011 ASM International®.

All rights reserved. www.asminternational.org

State of the Art Substrate Manipulation As a Tool for Enhancing Product Performance Michael A. Gonzales, Jose Cabanillas Qualcomm, San Diego, CA, U.S.A. michaelg@qualcomm.com, (858) 651-8605 This method can be incorrectly referred to as a “Package FIB” in reference to the “Focused Ion Beam” based circuit editing that incorporates high vacuum and charged ion beam chemistries for silicon die layout edits. Unlike focused ion beam modifications, substrate modifications can be performed in the open air and use traditional lasers mounted on a microscope as shown in Figure 2.

Abstract Substrate modifications on the Integrated Circuit (IC) package provide opportunities for the Failure Analyst (FA) to troubleshoot a routing failure or allow a design engineer to create new routing possibilities for a prototype device.

The results can mean the difference in finding the root cause of the problem and being early or late to market.

This paper describes a variety of methods to open sections of the package circuit board to access and cut I/O traces interwoven throughout the package substrate. It also describes the use of conductive epoxies for connecting traces, vias and solders bumps. Restoring the solder mask with an ultraviolet (UV) light curing conformal coating is also discussed. This method was used to characterize ground sensitivities and simulate inductance effects on the package. The flexibility and fast turnaround time this method enables has already enhanced product performance.

Introduction Microcircuit analysis and repair have a long history in microsurgery technology [1]. There are many references to accessing the chip [2] directly but relatively few on accessing the chip via the package substrate. The package substrate serves as the link between the IC circuitry and the application/phone Printed Circuit Board (PCB). It is here that an opportunity to effect the routing of the circuitry is easiest by virtue of being the most accessible. Multichip Module substrate packages with two or more die lying in the same plane also provide access to the substrate circuitry between the dice.

Figure 1: An example of a modification on the package substrate involving the cutting of two signal lines. The exposed area will be insulated to prevent reshorting during mounting and testing.

The good news is: Package substrates do not follow Moore’s law [3]. Package board geometries have not scaled down proportionally with the transistors in the die.

It is in the relatively large substrate package where there is still access to make cuts, jumpers or combinations of both cuts and jumpers, as well as providing locations for probe access. [Figure 1.] Despite recent developments in throughsilicon-stacking which will introduce thousands of inter-die connections, the board-to-package connections are still relatively abundant.

Typical trace geometries are tens of microns wide while metal interconnects in the silicon die can be tens of nanometers wide.

Figure 2: Laser mounted microscope with controls for wavelength, power and navigation.

35

Substrate circuit editing began as simple requests from Failure Analysts who wanted to quickly troubleshoot the circuitry in question.

By selectively shorting or disconnecting selected copper traces, we could test possible sources of failure or determine if the change would preserve, re-create or alter the status of the failure thus narrowing the field of possible causes of the “condition”.

It became apparent to designers that these package/board design modifications could simulate design changes before committing them to the package or silicon. They could also alter routings whose geometries cause undesirable induction effects. Finding such bugs early creates a savings in time and money that are measureable, improve confidence for the next product revision, and ultimately lower the time to market for a new product.

Signal trace cutting and substrate removal Laser mounted microscope Figure 3: Mechanical milling tool with x,y sweep and z control with interchangeable drill bits for different material removal.

Access to the traces can be made with either a small drill, milling machine or by chemical dissolution. However, the most successful method in accessing the substrate circuitry comes from a laser capable of cutting copper traces, vias and pads as well as fiberglass embedded in organic resin.

We use a continuous laser that is incorporated in the optical path of a four objective high power microscope. [Figure 2] This selectable “green and UV wavelength” laser microscope uses a 10X, 20X, 50X and 100X objectives.

The lower two objectives are for site location while the 50X and 100X are for simultaneously lasing and viewing of the sample. The narrow beam of the 100X objective results in increased power density. Power is variable for bulk vs. thin layer removal.

Mechanical milling and chemical etching Mechanical milling [Figure 3], drilling and selective chemical etching [Figure 4] provide another means of accessing the substrate trace lines. For example, Metal 1 die to die level traces that are covered with over-mold compound [Figure 5] can be accessed by mechanically milling off material without damaging the dice followed by a selective acid etch to dissolve any remaining overmold compound and/or solder mask. The acid used is composed of fuming nitric, fuming sulfuric acids or mixtures of both. The main consideration here is to know that nitric acid readily dissolves copper while sulfuric acid does not. It then becomes a balancing act to provide enough nitric acid to dissolve any remaining mold compound or solder mask that was not removed by milling.

This mixture is usually 1:1. Premilling allows the selectivity and the short etch times to fully expose the copper traces without damaging them.

Figure 4: Jet etcher used for chemical removal of mold compounds and solder mask with sulfuric acid, nitric acid, or mixtures of both.

An example of milling the overmold compound followed by selective acid etching is shown in Figure 5.

Approximately 50 traces were cut to isolate the dice and enable the Failure Analysis engineers to investigate leakage from individual die in the package. This is a rare opportunity to access M1 copper in the substrate material due to the fact that the space between the die offered a unique window of opportunity. Advances in stacked die and through silicon via (TSV) products may soon close this window.

36

Insulative Conformal Coating For package substrate modification, a UV cure conformal coating is used to repair any of the conformal coating or solder mask to prevent reflow solder from entering an access point and re-shorting the traces to each other. The coating is applied with a microprobe tip to ensure accurate placement. It is then treated with a UV source for approximately 40 seconds to fully cure it. This material is capable of withstanding multiple attachments to boards and solder ball re-flows.

Figure 7 shows multiple applications of the UV cure on the package substrate. The solder mask has been removed in selected areas, allowing access to the metal 1 traces. Both cuts at metal 1 isolated those pads from the ground plane, and enabled the signals to communicate. The UV cure is seen on top and is used to protect the cut locations from solder reflowing into the gap. More than 100 samples were created and yielded a 66% success rate.

Figure 5: An example of milling access in the package between two die to laser cut traces at metal 1 of the package substrate.

Signal Trace Addition and Insulation Conductive Epoxy for New Signals In addition to cutting signal traces, it is often necessary to add connections to re-route power and signals. This is often more challenging than eliminating signals on the package substrate. A two part conductive epoxy is mixed in equal parts and carefully placed to make contact with the desired surfaces. The epoxy contains silver particles and cures in 15 minutes at 100ยบC. The result is a conductive path that does not shrink and can withstand reflow temperatures. [Figure 6].

Figure 7: This area of the substrate has 2 cut locations at package M1. Each of these cut locations are insulated with UV cure material to prevent solder reflowing into the space.

Case study Low noise amplifiers are prone to oscillate if a feedback path between its input and output exists. These feedback paths can exist on the die but also on the package. Parasitic capacitances or interaction between inductors are typical examples of on-chip elements that can introduce spurious oscillations. There are also package elements than can cause unwanted oscillations; package interaction between the different grounds of an amplifier are a typical case of that. The plot in figure 8 shows the small signal gain of an amplifier with two different package connections. Initially, the small signal gain showed unacceptable gain peaking as corresponding to a quasi un-stable amplifier. After

Figure 6: The image shows the proximity of a newly created path to several existing solder pads. Here, the silver conductive paste is in a lower plane than the solder pad next to it and is covered with a layer of insulating conformal coating.

37

modifying the ground connectivity [Figure 9] of the package through a package modification, the gain peak was removed, and the stability of the amplifier (as well as its performance) was considerably improved.

particular region of interest (ROI).

Also substrate modifications typically deal with four metal layers (M4 to M1) alternately sandwiched between fiberglass and epoxy substrate layers.

Programmable Lasers Problems to Overcome

Navigation Since navigation is a major component of successful substrate modifications, it would seem that the next logical progression would be to make these modifications using a programmable laser. Laser decapsulators are available in today’s market that allow us to administer the necessary laser cuts in a sequential and repeatable manner. Each progressive cut could be selectively programmed to handle the variety of substrate materials, including metal pads, and traces of varying thickness.

For now, computer software aids in visualizing the spatial relationships of the substrate layers.

CAD-to-image software may be useful when we update to a programmable laser for navigation and improved selectivity of the material being removed.

Figure 8: Shows the gain correction after modifying the ground connection.

Endpoint detection Endpoint detection however could still be a major concern for a programmable laser. Despite good programming, it cannot make on-the-fly judgment calls to keep from lasing too deep in alternating layers of metal and fiberglass.

Especially challenging would be to mill through vertical vias.

Repeatability on multiple samples The ability to make consistent repeatable cuts on a large sample set is a programmable laser’s best advantage.

However, substrate edits come in typically one, or two-ofa-kind sample sets. The advantages of a programmable laser may be offset by a typically small sample size. A definite requirement for repeatability with a programmable laser would be the inclusion of a consistently positionable sample stage.

Figure 9: Shows the ground connection modifications.

Key Points to Ponder

Insulation deposition

With package substrate modifications, there are no submicron features that need to be resolved for alignment.

The 50X and 100X objectives are sufficient to provide the necessary resolution. A typical FIB is working close to potentially thousands of connections, whereas the substrate modification is dealing with only a dozen or so in any

Currently, it is difficult to reinsulate a trace when it is near a proposed conductive pathway. For now, we try to preserve substrate material that serves to protect a conductive path. In the future, it would be advantageous to find a way to selectively lay down an insulator as easily as

38

it is to remove it. This is definitely an advantage that FIB has over this process.

In addition, as focused ion beam (FIB) tool resources become stretched, substrate modifications could ease the load by reconfiguring some of the routings on the substrate.

Conclusion

Acknowledgements Michael DiBattista Bernardo Diaz del Castillo

Whether it is Failure Analysis troubleshooting, design modification/correction or projecting future design possibilities, substrate modifications have improved our ability to make these changes with a minimum turnaround time and a savings in mask re-spin iterations. As word spreads, more and more designers and engineers will see the many advantages this technique offers. This method lends itself well to design for test (DFT) designers to make more accessible trace routings and include internal probe pads and fuse like links that could make the cuts easier especially in the proximity of neighboring traces, vias or solder bumps. Whatever future this technique holds, it provides real opportunity for quick, opportunistic modifications that will improve our time to market or advance the possibilities for an innovative design engineer.

References [1] Wills, K.S., Pabbisetty, S.V., “Mircosurgery Technology for the Semiconductor Industry,” Microelectronics Failure Analysis Desk Reference, Fourth Edition. 1999.

[2] S. Perungulam, K.S. Wills, “Chip Access Techniques”, Microelectronics Failure Analysis Desk Reference Fifth Edition, ASM International, 312-322(2004).

[3] G.E. Moore, Electronics, Volume 38, Number 8, April 19, 1965.

39