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June 13, 2018

Profiling for Double Sided BGA

We are about to produce double sided BGA boards - BGA components on both sides. What advice would you give for profiling? What advice would you give for reworking?

B.O.

Expert Panel Responses

Whenyou are reworking double sided PCBs with BGAs, it is important to select aconvection rework system that features dual zone bottom side heaters. This typeof heater setup will allow you to create rework profiles that are optimized foryour assembly while allowing you to protect the delicate areas on the bottomside of your board from unintended reflow. In addition, it should be easy to use and a nice feature would be automaticprofile creation, this will make initial profiling much easier for yourengineers and will get your process running sooner.

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Edward Zamborsky
Regional Sales Manager
OK International Inc.
Mr. Zamborsky serves as one of OK's technology advisers to the Product Development group. Ed has authored articles and papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead Free Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.

In my opinion, you have toaccept that the lower BGAs will get quite close to reflow, due to positionrelative to the device above and also due via conduction of heat. We use IR topand bottom so there is no vibration from moving air. Make sure board supportsare touching PCB only. Also, be careful with pick-ups when they touch down... youneed to make sure to not bend the PCB at any point. Preheat slowly and with lower power to gently bring PCB to about 180 C for leadfree, probably aiming for a ramp rate of about 0.5 C/sec. Then gently heatcomponent to reflow temp, again at a low ramp rate of about 1.2-1.4 C/sec.

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Roger Gibbs
Managing Director
PDR
Twenty years ago Mr. Gibbs, Managing Director of PDR, produced the first rework system based on Focused Infra-Red light energy. PDR now has a 3,500-strong global installation list with top OEMs and EMS providers.

The fact that you are asking this question means you care aboutyour soldering processing and the impact they will have on your customer'sassemblies. You are to be commended! For you to provide the best assemblyprocess, your customer must be willing to help you with the needed componentsto perform this vital thermal profile. There is no shortcut or "virtual" method to measuring the thermalprofile that your reflow oven imparts to the solder connections of an assembly.The only way to know and document the thermal profile of the solder connectionfor any component is to place a T/C on or very near the points of interest. Inyour case, BGAs on both sides of the board, this is not simple, but still verynecessary. You should request an assembly from your customer to use for thispurpose: to perform the thermal profile. T/Cs must be installed under the BGAs(near the center and 2 corners if possible) in question. The T/Cs can be placedon the board before the BGAs are placed and then soldered through your process.The BGAs on this test assembly do not have to solder well to measure thethermal profile under the BGA, where the solder temperature profile must beachieved. Thermally equivalent, yet electrically defective, components can beused if the cost of the components is a deterrent.

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Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 39 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes and well as moisture sensitive component storage solutions.

If the componentlayout of the boards is not yet solidified, consider locating passives on theopposing sides of BGAs to help distribute thermal mass and avoid back-to-backBGA configurations. Profiling goes back to basics, instrument key componentswith properly sized thermocouples and reliable TC attachment methods. Dataquality achieved is proportional to the time invested in instrumentation. Ifthere exists a primary component of interest, as in the case of Rework, utilizea profile test board or thermal representation. Instrument multiple solder interconnects,the die and the surface of the component closest to the heat source, along withboth sides of the board tangent to the rework site to reveal thermal behaviorfor process optimization. Double sided BGA applications demand precise thermalcontrol. Whether component Rework or mass solder Reflow, ensure in advance thatequipment on-hand is capable.

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Al Cabral
Regional Sales Manager
Finetech
Al Cabral is Regional Sales Manager for Finetech and Martin rework products. His expertise includes through-hole, surface mount and semiconductor packaging with an emphasis on soldering and heat transfer. Al has been a significant contributor to the development and optimization of reflow and rework processes and systems, particularly lead-free transitions and microelectronic applications.

I'll assume that the BGAs are not back-to-back; ifthey are, that's another question entirely. If they are not back-to-back, thenprofiling best practice doesn't really change. You should base your profileshape on your paste manufacturer's recommendations. Minimize the peaktemperature and liquidus time while maintaining the recommended minimum peaktemperature and time for the most mass-dense area of the assembly. Rememberthat the longer and hotter your profile, the more work the flux has to do(twice for the first-side BGAs). If you run out of flux activity, you'll windup with oxidized, misshapen BGA joints.

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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.