How to protect SMD under BGA chips during reflow

I need to do a reflow of two BGA chips and there are some surface mount components underneath them on their PCB’s. The components are very small capacitors, resistors and possibly inductors (look pictures attached)



. The problem is, the temperature on the underside of the PCB (where the SMD's are) reaches well over the melting temp of solder and stays there for over 4min. On my (not-so-similar) trial board, I tried to protect SMD's with Kapton tape but after about 200C the tape crambles and pulls the small components of the board. If I do not use the tape, according to my thermocouple, the temp underneath climbs even higher and stays high longer. So I am afraid that without tape, the only thing holding SMD's in place after solder melts will be its surface tension and I don't know if it will be enough. On the other hand, if I do use Kapton tape and it pulls the small components off, I will not be able to reattach them - not knowing which came from where (as I don't even know if same looking components are actually the same and many are too tiny for me to be able to do it anyway). The sizes of SMD's on my boards are: 1mm x 0.5mm; 1.5mm x 1mm; 2mm x 1.5mm and 3mm x 1.5mm. Therefore: 1. Do you think the surface tension of melted solder will be enough to hold SMD's in place on the underside of a PCB? 2. Are such small SMD's able to withstand temps of up to 245C with temps being over 217C for over 4min? 3. Is there a more temp resistant tape than Kapton? Something that will not crumble till after 250C? In other words, am I better off using the tape or not and if yes which tape?

Thank you , for contacting Digikey Tech forum we are looking into this , and hoping to have an answer for you as soon as we can.

Here are some links I found relating to this topic:

https://www.circuitnet.com/experts/88559.html
https://www.flason-smt.com/new/Component-weight-limit-on-PCB-underside-during-reflow-soldering.html

Ideally, a dot of heat curable adhesive is added to heavier components on the underside before they are reflowed, or they are reflowed using a higher melting point solder than the BGA side solder, but if you are doing rework, this may not be the case. However, it looks like, under most circumstances, with small smt components, solder surface tension is sufficient to hold them in place.

High Temp Masking Liquid - 2538 and 2538UV High-Temp Masking Liquid - 3M | DigiKey

Part Number: 19-2538-ND

Video: https://www.digikey.com/en/products/detail/3m/2538/13241410
Datasheet: Technical Data | July 2015 (digikey.com)

Thank you for clarifying the surface tension question.

Could you please give me some guidelines on likely heat tolerance of my small surface mount components (their dimensions given in the first post). Ie: what is the highest temp they are likely to tolerate and for how long? (With my equipment, I cannot keep the temp on the underside of the PCB above 217C for less than 2 min. With the peak temp hitting about 243C. Without any additional thermal protection, is this likely to kill them?).

Thank you for bringing the option of the High Temp Masking Liquid to my attention. However, it is very expensive and this cost doesn’t even take into account the cost of the apparatus to apply it. The cost makes this solution impractical for me. However, do you feel that aluminum tape would be a significantly more heat tolerant product than Kapton tape. Also, would a known brand of aluminum tape or similar product make a critical difference? If a well known tape product can last even to just 250C, without detaching or deforming - that would solve my problem.

Regarding a heat curable adhesive, could you recommend a specific product available from DigiKey (or elsewhere)?

Further regarding High Temp Masking Liquid, perhaps this product is available in smaller quantity and for different dispensing equipment? For example, a small rod the shape of a glue-stick that could be dispensed from a glue gun? This might be able to address the cost issue.

Unfortunately, the High Temp Liquid is only available in the brick form and not smaller qty’s. Here are some tapes that go to 260°C that you can review.

Link: Tapes, Adhesives, Materials | Tape | DigiKey

Hi Vic7,

The Loctite 3621, Digi-Key part number 1000-114-ND, is designed for such applications.

Regarding heat tolerance of your components, I really can’t say with certainty. You would have to read the respective documentation of the components used. However, as a general statement, the process of sending parts through a second reflow process to solder parts on the other side of the board is a common practice, so it would seem unlikely that failures would be common under such conditions. Obviously, keeping exposure time and temperature to the minimum required for successful soldering of the other side is desirable.