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Circuit board package BGA
Since Motorola introduced semiconductor components in the "ball-foot grid package" in 1995, it has swept the world with no rivals in the packaging field of high-pin count ICs, even Intel’s Pentium central processing unit (CPU), which has 320 pins. The QFP packaging method with four-sided extension pins had to bow down and change to the P-BGA-style Pentium II in line with the trend. Today's BGA package components have not only been reduced to CSPs with a pitch of only 0.5mm and 0.4mm, but also have been packaged from a single chip to a multi-chip overlapping package, and passive components and complex wiring have been added, which has become a condensed version. The SIP of the first-class system configuration still uses "Gray-type ball feet" to assemble on the boards of various main systems.
Comparison of traditional metal tripod type gull-pin package and organic substrate BGA ball-pin package
As this lead-free soldering is about to be fully developed, it is necessary for this important component that is most prone to problems to explain the difficulties it may encounter early, and find out how to respond and solve the problem.
The general trend of the semiconductor packaging industry's product progress and the three areas
1. IC with BGA package has three advantages:
It has the advantages of sturdy appearance, small size, high density, short path, and low noise and good electrical performance. The common P-BGA can withstand the power of 6-8W, and the heat sink can withstand up to 30W.
2. Among the various BGA packaged components, 16-64 pins account for more than half
Those with 208 feet or more accounted for only 5%. The foot pitch or ball pitch is usually between 0.65mm and 1.27mm. The ball pitch of the current mass-producers has approached 0.5mm and 0.4mm, and the current trial production has the densest one at 0.3mm, so it will be very difficult to mount on the PCB board.
3. The general ball diameter accounts for about 60% of the ball pitch
For those with close pitch, the ball diameter should also be fixed at 0.3mm. Most of the balls in the 1-3 rings of the ventral bottom are set as signal balls. One is easy to escape from the square because of the wiring, and the other is because of the better welding reliability, which reduces the failure of the function (but the four corners should not be equipped with the ball. 〕. The hard-to-sold inner ball can only be used for power supply, grounding, and heat dissipation without escape lines.
4. The current packaging form has become a template for various sub-system modules
A large complex interconnection carrier that can carry multiple chips (including stacked chips) and passive components, especially called MDS, and its large, high-functioning devices can reach as many as 1,700 feet. In order to facilitate the cleaning and insulation of the abdominal bottom, the frame height after assembly should be kept above 0.4-0.5mm.
Five, all kinds of BGA ventral base ball planting in N2
In order to ensure the stability of its volume and coplanarity (within 3-6%), high-viscosity flux is specially used as the adjuvant for temporary positioning and permanent welding, instead of using the metal which has more changes in height. paste. In fact, the carrier board in this ball planting stage is prone to warping, and the coplanarity of each ball is required to be 0.15mm on average. Therefore, the subsequent assembly of the BGA is best not to be placed on the center line of the PCB to prevent the assembly board from bending again and losing coplanarity. The current ball material is Sn63. Since July 2006, lead-free solder (mainly SAC305) must be used, and its self-recovery performance will deteriorate.
The ball feet used must not only be perfectly round but also within 3% of the size.
Only in this way can the necessary coplanarity be guaranteed after implantation.
6. On-chip wire bonding on the top surface of the carrier board
The working surface must be electroplated with nickel and gold. As a result, the ball pad on the bottom of the carrier plate had to be plated with nickel and gold. In order to prevent the brittleness of the gold during welding, the thickness of the gold should be 10 μm, and the width of the edge of the support pad on which the green paint is limited is about 0.1mm.
Seven, there are many difficulties in assembly and welding
For example, visual inspection, MSL of moisture sensitive water, heavy industry, cost, supply chain, and voiding etc. Once the P-BGA packaged device absorbs water, it will often bend and warp at high temperatures; otherwise, it will be prone to swelling, rice cracking, and cracking. Those with heat sinks on the back are more likely to bend, which often makes the corner balls high and the soldering is not good. It is even more dangerous above 260°C.
