|Ask the Experts|
October 28, 2020
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?
|Expert Panel Responses|
When you are reworking double sided PCBs with BGAs, it is important to select a convection rework system that features dual zone bottom side heaters.
This type of heater setup will allow you to create rework profiles that are optimized for your assembly while allowing you to protect the delicate areas on the bottom side of your board from unintended reflow.
In addition, it should be easy to use and a nice feature would be automatic profile creation, this will make initial profiling much easier for your engineers and will get your process running sooner.
Regional Sales Manager
OK International Inc.
In my opinion, you have to accept that the lower BGAs will get quite close to reflow, due to position relative to the device above and also due via conduction of heat.
We use IR top and bottom so there is no vibration from moving air. Make sure board supports are touching PCB only. Also, be careful with pick-ups when they touch down... you need 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 lead free, probably aiming for a ramp rate of about 0.5 C/sec. Then gently heat component to reflow temp, again at a low ramp rate of about 1.2-1.4 C/sec.
The fact that you are asking this question means you care about your soldering processing and the impact they will have on your customer's assemblies. You are to be commended! For you to provide the best assembly process, your customer must be willing to help you with the needed components to perform this vital thermal profile.
There is no shortcut or "virtual" method to measuring the thermal profile 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 connection for any component is to place a T/C on or very near the points of interest.
In your case, BGAs on both sides of the board, this is not simple, but still very necessary. You should request an assembly from your customer to use for this purpose: 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 placed on 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 the thermal profile under the BGA, where the solder temperature profile must be achieved. Thermally equivalent, yet electrically defective, components can be used if the cost of the components is a deterrent.
Senior Project Engineer
Electronic Controls Design Inc
If the component layout of the boards is not yet solidified, consider locating passives on the opposing sides of BGAs to help distribute thermal mass and avoid back-to-back BGA configurations.
Profiling goes back to basics, instrument key components with properly sized thermocouples and reliable TC attachment methods. Data quality achieved is proportional to the time invested in instrumentation.
If there exists a primary component of interest, as in the case of Rework, utilize a profile test board or thermal representation. Instrument multiple solder interconnects,the die and the surface of the component closest to the heat source, along with both sides of the board tangent to the rework site to reveal thermal behavior for process optimization.
Double sided BGA applications demand precise thermal control. Whether component Rework or mass solder Reflow, ensure in advance that equipment on-hand is capable.
Regional Sales Manager
I'll assume that the BGAs are not back-to-back; if they are, that's another question entirely. If they are not back-to-back, then profiling best practice doesn't really change. You should base your profile shape on your paste manufacturer's recommendations.
Minimize the peak temperature and liquidus time while maintaining the recommended minimum peak temperature and time for the most mass-dense area of the assembly. Remember that 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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