Ball Grid Arrays: Questions & Answers

BGA Technology

Q: Why has BGA technology come to the forefront all of a sudden?
A: Due to fundamental limitations of wirebonding technology and pad pitch, PQFP/TQFP package option has run into a roadblock at pin count of 256 and above. The issue is that maximum wire lengths for currently available equipment is approximately 220-240 mils before significant yield loss occurs. Also, current standard leadframe stamping technology allows a best case lead frame finger pitch of 8.5 mils. To go below this finger pitch on lead frames is possible, but cost ineffective.

Q: If BGAs are so wonderful, why wasnt their use prevalent until recently?
A: Until 1993, the only BGA assembly was in captive assembly houses. With JEDEC registration in 1992 and licensing of the OMPAC process by Citizen, AMKOR, and others, the technology is catching on. As reliability data, advantages of the attachment process to printed circuit board, and the benefits of the space savings for system users become more evident, BGA packaging will catch on.

Q: How does a cross-sectional view of a BGA look?
A: Figure attached.

BGA Sectional View

Q: What is Ball Grid Array?
A: Plastic Ball Grid Array (PBGA) packages have increased signal density per unit area dramatically as compared to standard Quad-flat packs (QFP s). A high pin count array creates problems associated with routing signal traces out from the ball grid array device. The Perimeter Ball Grid Array improves the ability to route signals from the device, by depopulating the center balls and decreasing the pitch slightly.

Q: With regards to manufacturing, what does BGA buy me?
A: Improved Manufacturability:
· Relaxed lead pitch: 1.27mm compared to 0.4mm for a 208-pin PQFP.
· Coplanar issues absent, no lead skew concerns.
· Significantly improved manufacturing yields.
Reduced Size:
· Smaller footprint.
· PC board savings.
Improved Electrical & Thermal Performance:
· Excellent platform for enhanced thermal and electrical properties compared to PQFP.

Q: Can damaged balls be fixed or replaced?
A: Yes, BGA technology allows for damaged balls to be fixed and/or replaced. The rework involves removal of solder from the pad surface, then replacing with a new component using only liquid flux on the pad and allowing the ball on the new device to form t he interconnection.

Q: How many times can the package be desoldered before degrading the balls?
A: Typically, it can be done once.

Q: What should one look for during BGA assembly?
A: No device tweaking should be carried out after placement. Also, slightly misaligned parts will automatically align correctly during reflow. Grossly misaligned devices should be removed prior to reflow as they will have shorts if they are subjected to re flows.

Q: What are the most commonly used techniques in BGA rework/repair?
A: With QFPs, rework and inspection becomes more difficult as the pitch gets finer, hence newer techniques are needed. The technology and process knowledge to effectively and efficiently repair BGA s already exists. An added attraction for BGA packages is that they are compatible with existing SMT assembly processes and equipment: existing solder paste screen, pick-and-place reflow equipment can be used. Generally speaking, the percentage of non-repairable defects is significantly lower in BGA s compared to QFPs. There are two main approaches to BGA repair:
· Conductive heating: heat BGA & PCB to reflow solder balls.
· Convective heating: heat solder balls while minimizing heating of PCB and BGA.
Q: Are there any package performance comparisons between a PGA, BGA and a QFP?
A: Please refer to the table provided:

Various Factors	PGA	QFP	BGA
Size	43.5x43.5 mm	26x26 mm	21 x 21 mm
I/Os	196	256	324
I/O classification	Pins	Leads	Pads
Signal-Signal Capacitance	1.43 pF	-	0.40 pF
Signal-Gnd Capacitance	6.67 pF	1.78 pF	1.85 pF
Mutual Inductance	8.57 nF	20.31 nF	3.26 nF
Ground Inductance	-	-	0.76 nH

Package Performance Comparisons


Q: Is Intel pioneering BGA technology?
A: No, BGA technology has been around a while but is catching on fast now. Motorola, Compaq, Atmel, Citizen, AT&T, Blaupunkt, NEC, DEC and others have worked extensively with BGAs.

Q: What are the typical package dimensions for BGA technology currently?
A: The two most commonly used package sizes are: 33mil and 40mil.


Q: What kind of lead pitch does a BGA user have to deal with?
A: Typical lead pitch for BGAs is either 1.5mm or 1.27mm. This compares vary favorably with PQFP packages where the pitch is as low as 0.4mm leaving very poor tolerance margins.

Q: What is the total number of solder balls (pin count)?
A: The number varies depending on the package dimensions and pitch. However, up to 1000 balls can be accomodated on a 33mil package with a pitch of 1.5mils.

Q: Are the solder balls aligned or staggered?
A: Both configurations are being used presently. Typically the staggered configuration is used when the balls are distributed evenly on the underside from the center of the package to the periphery. Staggered configurations allows for easier routing. When the balls are distributed along the perimeter of the package, the aligned solder ball configuration is used.

Q: Can BGA solder joints be inspected after the reflow process?
A: Yes, by using Real-time X-Ray Inspection. It accurately helps identify:
· Solder shorts
· Popcorn delamination
· Gross misalignment
· Missing balls
· Foreign matter
· Excessive paste

Popcorning is defined as a delamination of a crack in a plastic package that reaches the outside of that package. It is caused by rapid expansion of moisture (water vapor) inside the package during solder reflow. This problem is particularly noted in surf ace mount packages which experience higher thermal and mechanical stresses due to exposure of the entire package to reflow temperatures. Various techniques have been used effectively to alleviate the Popcorning effect. Open thermal via holes under die pa d prevents popcorning during IR reflow. Package design improvements have reduced the problems in standard plastic packages. Also, improved baking steps and vacuum packaging are currently being used to reduce the popcorn phenomena.


Solder Shorts


Popcorn Effect

Summary
BGAs are the only easy-to-manufacture package that can meet the requirements of high pin count Integrated Circuits needed by various segments within the electronics industry. Lead spacing can be 50 mils, which are far easier to work with than fine pitch peripheral-lead packages. Recently, there has been an increase in the number of board manufacturing houses capable of handling ball grid arrays: a list of the top board vendors is provided as part of the document.

The Institute for Interconnecting and Packaging Electronics Circuits (IPC) is developing an industry standard on implementation of BGA technology. The effort, known as J-STD-013 will address design, assembly processes, technology choices, applications and reliability data. The IPC expects to have this document ready by September 1995.

BGA Board Houses

Contractor	Contact	Design Layout	Circuit Analysis	Thermal Analysis	BGA	Quality Systems
Anam/Amkor	Gil Olachea 602-821-5000	Yes	Yes	Yes	Yes	ISO 9002
SCI Systems	Larry Evans George King 205-882-4631	Yes	Yes	Yes	Yes	ISO 9002
Solectron	Gary Ogden 408-956-6689 Srinivas Rao 408-956-6478	Yes	No	No	Yes	ISO 9000
Valtronic USA	Joy Shanklin 216-349-1239	Yes	No	No	Yes	ISO 9000
S-MOS Systems	Pam Preston 408-922-0578	Yes	Yes	Yes	Yes	ISO 9002
Group Technologies	Brian Tracey 813-972-6242 Scott Mauldin 813-972-6134 Frank Mendoza 813-972-6941	Yes	Yes	No	No

Bibliography
1. ASICS & Ball Grid Arrays: A Question of Need
Advancing Microelectronics-July/August 1994, Chris Baumann.

2. Ball Grid Array Technology: Is it the Answer to Defect-Free Assembly? SMT-August 1994, Martin L. Barton.

3. BGAs making Micro Move Electronic Engineering Times-October 31, 1994.

4. Confab Eyes BGA Use Electronic Engineering Times-April 3, 1995.

5. Advanced Interconnect Technologies Vie for Dominance Assembly- February 1995, Scott Mauldin and Frank Mendoza.

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