Thursday, August 25, 2016
VOLUME -24 NUMBER 9
Publication Date: 09/1/2009
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ARCHIVE >  September 2009 Issue >  Partnering > 

How Partial Underfilms Help BGAs
0.009-in. partial underfilm component in standard EIA-481 compliant carrier tape and SMT feeder.

Lead-free assembly has created a difficult hurdle for the PCB assembly process, especially for CSP and BGA packages. Current soldering technology requirements for RoHS compliance requires brittle alloys, which along with miniaturization, is reducing joint sizes and prematurely fatiguing soldered assemblies. The end result is product failure, especially when end product is subject to vibration and/or shock.

Lead-free solder can create brittle solder joints. Current technologies for improving reliability for mechanical and thermal fatigue in these packages include capillary flow underfill; pre-dispense "no-flow" underfill; solid underfill; wafer-level solid underfill and many others. While many of these techniques have generated significant results for reliability, the cost of achieving these results has been very high.

Partial Underfilm
Partial Underfilm Technology involves bonding the edge or corner of an IC package to the PCB, resulting in improved solder Ball/Joint reliability. This is done by laying a solid film onto the PCB before placing the BGA/CSP. The film is only placed in selective areas — usually where it will come in contact with the edge of the BGA/CSP package — that will yield the best results. By utilizing this format, the solid film will reflow and bond to the IC as well as to the PCB, forming a strong mechanical bond between the two. The film is a thermoplastic that performs well in the typical reflow profiles for these assemblies. Although it is rigid during the pick-and-place process, during reflow, the underfilm softens and acts as an adhesive between the package and PCB, reducing stress and improving the overall performance of the solder joints. The most common areas for bonding are along the edges of the device, known as edge bonding.

Lower Cost
Studies have shown that the dispense technology can introduce significant increase to the reliability with regard to thermal and mechanical fatigue manufacturing process but at a considerable cost to the manufacturing process. The investment for equipment can range from $100,000 to $500,000 per SMT line.
Outline and dimensions of 132-pin BGA.


Much of the resources required to manage extra equipment required to store, handle and process underfill materials can be completely eliminated through the use of Partial Underfilm Technology. There are no additional costs to the Partial Underfilm Technology as it uses existing equipment and processes available to all surface mount assemblers.

To further review the capability of the Partial Underfilm Technology, we conducted testing similar to the underfill testing that has been presented in various studies.
Board layout with "Tacky Pads" and partial underfilm locations outlined in red.


By placing a boundary of underfilm material down onto the PCB after solder paste dispense and prior to placement of BGA and/or CSP components, the underfilm and solder paste areas, will provide a durable lead-free solution that is activated by the heat of reflow. This underfilm technology does not require additional equipment for placement or storage, simply a tape-and-reel feeder at the SMT placement machine. The underfilm material is a low cost plastic that has been employed and field tested on millions of handheld devices. It is a benign, non-toxic material.

Tape-and-Reel Packaging
The underfilm units are supplied on EIA 481 tape-and-reel packaging. After the solder paste has been dispensed, an arrangement of linear films are dispensed from tape-and-reel and placed at the boundary areas of BGA and CSP component placements atop "tacky pads". Then the components are placed atop the underfilm and solder paste — the underfilm making contact with non-electrical edge areas of the BGA/CSP device. The heat of the reflow process causes the underfilm material to adhere to the component body and to the PCB. In doing so, it acts as a dampening device for any physical shock, thereby improving the durability of the product. No additional cure cycles or fume extraction is necessary when using this material.

First, a test vehicle was designed that contained one board mounted with ten 132-pin BGAs. A standard 0.005-in. solder screen was placed. Five of the BGAs would receive no additional support and five would receive a boundary pattern of partial underfilm of 0.009-in. thickness.

The boards were reflowed utilizing a standard SAC 305 profile. Once the boards were finished, a 32-gram weight was attached to the top creating the Test Vehicle for reliability analysis.

These boards were repeatedly dropped from a height of six feet onto a steel gage block. After each drop, a continuity test was conducted to ensure that the product was still in working condition using the traces at T+ and T-.

After the first few drops, all of the unsupported BGAs failed and completely fell off of the Test Vehicle. As for the Partial Underfilm Technology BGAs, although not process optimized, 40 percent had continuity after 100 drops. Unlike the unsupported BGAs, none of them fell off of the Test Vehicle.

The results are significantly different between the unsupported BGAs and Partial Underfilm supported BGAs. Although there is no standardized test for lead-free components and mechanical shock, this same test was used in several other underfill studies with similar results.

Although testing is still being done to optimize the yield, it is probable, with better optimization of solder ball and solder paste combined height after reflow, the Partial Underfilm Technology can be completely successful throughout the testing. We know that the costs for implementation, training, maintenance, equipment and general overhead are insignificant when compared to that of traditional underfill technology. The individual films cost pennies in high volume. Some of the other benefits are the complete migration of the technology from prototype to production facilities. OEMs can move the manufacturing process without concern for capability with underfill technology.
Unsupported BGA failed and broke free of the PCB.


By utilizing the underfilm technology, lead-free assemblies can be greatly improved without investing in additional equipment, factory space or PCB space. Through careful design consideration, the process can be optimized to improve the yield and performance of active environments for hand-held devices.
 

Contact: Alltemated, Inc., 3196 North Kennicott Avenue, Arlington Heights, IL 60004 866-255-8362 or 847-394-5800 fax: 847-394-5805 E-mail: steveh@alltemated.com Web:
http://www.alltemated.com

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