Electronics Assembly Knowledge, Vision & Wisdom
Soldering Improvement for SMT Connectors
Soldering Improvement for SMT Connectors
This paper covers experiments for improving the SMT process for a few types of difficult to solder surface mount connectors.
Production Floor

Authored By:
Tho Vu, Anil Kumar, Raymond Tran, Stephen Chen, Zhen (Jane) Feng Ph. D., Greg Ruiz, Murad Kurwa
FLEXTRONICS International Inc.
Milpitas, CA USA
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Transcript
As Original Design Manufacturers adopt the use of finer pitch connectors with increased pin counts on PCB assemblies, it has become challenging for EMS companies to build to a very low defect rate.

In this paper the authors share experiments for improving the SMT process with two connector types.

Number one - Samtec's SEARAY connectors with 500 leads which have a unique solder charge design. The leads were on a 50 x 50 mil pitch spacing from row to row.

Number two - Press-fit SFP Cages with different lead lengths, one with lead protrusion and one with no lead protrusion on an 18 layer fab.

So what were the conclusions?

First, Samtec's SEARAY connector.

There was signification yield improvement for Samtec's SEARAY connector with a new stencil design and oven profile.

A new stencil, stepped down 6 to 4 mils, reduced defects to .5% from 15%.

Modified peak temperature was approximate 243C. Reflowing time above melting point of 220C was approximately 65 seconds. And soak time was approximately 100 seconds with a ramp rate of 2.12 degrees C per second.

The authors realized an SMT process time saving of 3 minutes for each board.

Second, Molex and Tyco Press-fit SFP cages.

Part retention improvement was successful, and passed structural test, with the use of a wave solder process implement.

This included a modified wave fixture and add flux on the top side of PCB then wave soldered as normal process.

Plus no modified wave fixture and add flux on the top side of PCB then wave soldered as normal process.
Summary
As Original Design Manufacturers (ODM) adopt the use of finer pitch connectors, with increased pin count on PCB assemblies. It becomes challenging for Electronic Contract Manufacturing Services (EMS) to build with very low or zero defects in the Printed Circuit Board Assembly (PCBA) operations.

In this paper we will share our experiments for improving the SMT process with these connector types:
  1. Samtec's SEARAY TM (AEAM/AEAF Series) connectors with 500 leads which have a unique solder charge design. The leads themselves are on a 50 x 50 mils pitch from row to row.

  2. Two Press-fit SFP Cages with different lead lengths, 1 with protrusion and 1 with no lead protrusion on an 18 layer fab (2.5mm thickness).
Case1. Samtec's SEARAY TM (AEAM/AEAF Series) connector The connector leads have a solder charge (pre-tin), and the minimum stencil thickness requirement is 6 mils. However the assembly supports a mixture of component technology for this product, where many components need the use of a 4 mils stencil thickness. The fab thickness is 40 mils. There are two main SMT process improvements which we did to eliminate defects:
1. Use 6 mils stencil thickness with a Step-Down to support the 4 mils thickness requirement of other components on the assembly, and replaced the use of a Mini-stencil for the connectors to solve operator handling issue that have been causing damage to the solder charge and others;
2. Based on experimental data, we also adjust the profile for optimization of the solder joints of the connector. With new stencil and oven profile, the defects reduced from 15% to < 0.5% for the connector.

Case2. Two Press-fit SFP cages with different lead lengths Because there were issues with these Press-fit SFP cages failing mechanical drop test. The customer requested us to add solder to the peripheral row of pins of the SFP cages, for a stronger retention to the fab. We couldn't make all pins have a good solder joint with a Non-modified wave fixture, and wave as a normal process. Therefore, we have new process designs (a. Modified wave fixture, add flux on the top side of PCB, and wave as a normal process for the 2 different vendor's components; b. A non-modified wave fixture and add flux on the top side of PCB and wave as a normal process; c. Modified wave fixture and wave as normal process). All these Selective Wave process methods are working: these cages now have good retention with the fab, passing mechanical drop test, and no defective pins for current boards were building. We use 2DX with tilting angle detector to check the solder joints of the cages.
Conclusions
1. Samtec's SEARAYTM (AEAM/AEAF Series) connector
There is signification yield improvement for Samtec's SEARAYTM connector with new stencil design, oven profile.
  • New stencil (step-down 6 to 4 mils) reduced defects to 0.5% from 15%.

  • Modified Peak Temperature is about 243C. Reflowing time above melting point, 220C, is about 65 seconds. And soak time is about 100 seconds. Ramp rate is 2.12 C per second.

  • SMT process time saving 3 minutes for each board (3.5 per board); and the defective pins reduced to 0.5% from 15%.
2. Molex and Tyco Press-fit SFP cages
Part retention improvement was successful, and passed structural test, with the use of the wave solder process implement for all three methods: modified wave fixture and add flux on the top side of PCB and wave as normal process; or not modified wave fixture and add flux on the top side of PCB and wave as normal process; or modified wave fixture and wave as normal process.
Initially Published in the IPC Proceedings
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