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HDC3021-Q1: Low-Pressure Molding

Ognyan Dimitrov
Prodigy 20 points
Part Number: HDC3021-Q1
Other Parts Discussed in Thread: HDC3021

Tool/software:

Dear TI Support Team,

In one of our products, we are using the HDC3021-Q1 humidity and temperature sensor. As specified, the device comes with a polyimide tape that covers the opening of the humidity sensor element.

We are planning to protect the electronic components on the PCB using Low-Pressure Molding (LPM) technology. The design is such that the molded material will encapsulate the PCB with a thickness ending approximately 1 mm below the top surface of the sensor element. Our intention is to keep the original polyimide tape in place during the LPM process in order to protect the sensor opening.

Before proceeding, we would like to clarify the following:

  1. Thermal impact – During LPM, the molding material (typically polyamide) is injected at temperatures around 180–220 °C. Could this temperature negatively affect the sensor element itself (for example, by drying or otherwise altering the internal gel of the sensing structure)?

  2. Polyimide tape durability – Could the high temperature and pressure conditions of LPM damage, deform, or reduce the protective capability of the polyimide tape that covers the sensor opening?

In short, is it safe to expose the HDC3021-Q1 with its protective polyimide tape to LPM encapsulation, or should we consider an alternative method to protect the sensor element?

Your guidance on this matter would be very valuable for us before moving forward with our design.


Best regards
Ognyan Dimitrov

  • 0
    TI__Expert 3615 points

    Hi Ognyan, 

    The protective tape cover can withstand high temperatures, so I think that application temperature is okay. From what I see online, usually LPM is solvent-free, which is good; it's best to prevent exposure to volatile chemicals such as solvents or other inorganic compounds that could affect the sensor accuracy. I'm not sure about the low-pressure though. I can double-check with our technologist to get more detailed guidance here. Let me know if there's a specific LPM vendor/part number you plan to use, so we can take a look.

    Best,

    Sakeenah 

  • 0 in reply to Sakeenah Khan
    TI__Expert 3615 points

    Hi Ognyan, 

    A couple follow-up questions: 

    1) How long is the 180-220C exposure? The high temperature will dry out the sensor but won't affect its structure or protective tape. You should rehydrate the part (following datasheet recommendations) after assembly/manufacturing to restore the RH accuracy. 

    2) What is the pressure used? 

    Best,

    Sakeenah

  • 0 in reply to Sakeenah Khan
    Prodigy 20 points

    Hi Sakeenah,

    The process details are:


    Injection pressure: 2kg/cm2
    Injection time: 4 sec.
    Holding time: 5 sec.
    Cooling time: 30 sec.
    Injection speed: 400 rpm.
    Holding speed: 500 rpm.
    Tank temp.: 205°C
    Gun temp.: 210°C

  • 0 in reply to Ognyan Dimitrov
    TI__Expert 3615 points

    Hi Ognyan, 

    This LPM process sounds fine. As a good practice while bringing up the process, I suggest to put a few HDC3021 though the LPM and then compare them to some control units (remove the tape and optically inspect) just to double-check that nothing has entered into the sensor cavity; I don't really have major concerns though and this is more for due diligence. 

    At the end of assembly after all the high-temperature steps complete (such as soldering and LPM), the sensor polymer may be dried out so please follow the Rehydration procedure to restore RH accuracy (as outlined in the datasheet): 25°C and 50%RH for 5 days. Following this rehydration procedure allows the polymer to correctly settle after reflow and return to the calibrated RH accuracy.

    Lastly as always, humidity sensors are susceptible to chemical contamination so just ensure that none of the materials contain any problematic volatile chemicals or solvents (see a non-exhaustive list of chemicals to avoid in the HDC302x Silicon Users Guide). 

    Otherwise, I don't have any major concerns with the stated assembly process. Good luck and feel free to come back with any additional questions. 

    Best regards,

    Sakeenah 

  • 0 in reply to Sakeenah Khan
    Prodigy 20 points

    Hi Sakeenah,

    Many thanks for the detailed reply! From our side, the topic is resolved and the thread may be closed.