A Reliable Interconnection Solution for Absorbing Large CTE Mismatches

 

Ever since the inception of printed boards and electronics interconnection, the industry has struggled with the dilemma of CTE (Coefficient of Thermal Expansion) mismatches. Essentially, when you combine a whole range of different materials and then bake them in an oven, they all expand or contract at different rates.

Even after the initial stress of manufacturing, many components are exposed to harsh environments, which can result in failure after repeated cycles.

The problem was greatly exac- erbated with the introduction of lead free solders, which made the reflow temperatures rise from around 125 degrees C to 160 degrees C and greater.

BGAs have their limitations in harsh environments such as excessive thermal excursions or high vibration or shock. CCGA (Column Grid Arrays) are often used in these applications to dis- sipate the heat/shock along the length of the column. This article examines a novel manufacturing solution for assembling CCGA (Column Grid Arrays).

 

Introduction

CCGA’s or CGA’s are not new. They first appeared around 1970 with a wire and cast made pillar from IBM.

Through the years, they have evolved into many different forms including a spiral copper wrapped version from Raychem (1980), a column interposer
rom NGK (1990), a copper plated ver- sion from IBM (2000) and a Micro-Coil Spring from NASA (2010).

CCGA were developed as an alterna- tive to BGAs (Ball Grid Arrays) to reduce the stress between large array ceramic packages and the substrate, usually FR4 or polyimide. Ceramic has a CTE of ~7ppm per degree of change. FR4 has a CTE of ~17ppm, a difference of 10ppm per degree of change up or down.

A BGA uses a smaller deposit of solder and exerts more stress between the solder ball and the substrate or between the solder ball and the ceramic. Cracks can form in the intermetallic and the solder ball will delaminate.

A CGA spreads that stress along the length of the column, dissipating the heat transfer and protecting the device. The CTE mismatch also causes physical stress which the column grid array is better able to withstand and compensate, when compared to the BGA.

Typical life cycle testing oscillates between -40 degrees C and 125 degrees

C. In industry papers from Jet Propulsion Labs,1 CCGAs survived 1,000 to 2,000 temperature cycles versus large size ceramic BGAs which only survived 100 to 500 cycles.

Figure 4 shows a highly-magnified photo of a Pb80 Sn20 column with copper ribbon wrap. The core of the column is a high temperature melting solder and the gaps between the copper windings is filled with regular Sn63 Pb37. The copper ribbon adds additional structural support to the column.

In 2010, Micro-coil Springs were invented by engineers at NASA Marshall Flight Center. This increased the reli- ability of CCGAs by enabling them to withstand an exceptionally high shock of up to 50,000G. In laboratory conditions the Micro-coil spring column survived 8 cycles before failure versus the plain solder column Pb90 Sn10 which survived only 4 cycles.

Figure 5 shows a Micro-coil column grid array with up to 1752 I/Os on a ceramic substrate and Figure 6 shows the physical dimensions of the spring on a one cent coin.

Further evidence of the reliability of CCGAs was demonstrated in a drop test at 1,500G. The photo in Figure 7 illustrates the positive and negative deflection of the FR4 board.

Table A illustrates the size and tem- perature range, highlighted in Green, where it is safe to use a BGA device. In the Yellow area it is generally recom- mended to use a CCGA as a BGA would risk delamination. The Orange zone must use a CCGA to prevent delamination.

As a ‘rule of thumb’ ceramic BGAs greater than 27mm square will delami- nate when mounted on FR4.

In Table A the CTE mismatch is set at 10ppm (the difference between ceramic and FR4). The temperatures shown are ± temperature swings from ambient.

 

Low volume column assembly

A novel manufacturing method has been introduced for assembling low volumes of CCGA using a graphite tool and pre-packed cassettes of columns. The cassettes are pre-loaded with different types of columns ranging from plain Pb90 Sn10 to copper wrap or Micro-coil spring, enabling the manufacturer to choose which columns to use, depending on the application.

  • STEP 1: Place the Land Grid Array (LGA) into the lower graphite jig and apply solder paste using stencil and squeegee.
  • STEP 2: Inspect the paste deposits then place the top graphite jig over the base using the location pins.
  • STEP 3: Place the Pin-Pack column array onto the top graphite jig.
  • STEP 4: Peel off the tacky backing film from the pins.
  • STEP 5: Check all the columns fall into the graphite mold.
  • STEP 6: Reflow. Use a vapor phase oven for best results.

 

Conclusion

Pin-Pack offers a versatile alternative for low-volume CCGA applications ranging from 240 ± 2,577 I/O and a package size of 23mm ± 52mm. Harsh environment such as space, military, down-hole, automotive and medical electronics are typical applica- tions for this novel technology.

Solder columns are COTS (Commercial Off The Shelf) components.

They are not controlled by ITAR and they do not require an export license. Pin-Pack is manufactured by Topline Corporation.

REFERENCES

1. Ghaffarian, R., ”Reliability of CGA/LGA/ HDI Package Board/Assembly”, NASA JPL Jet Propulsion Laboratory (2013).

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