Traditionally, one of the main problems with board level pressure sensors is that most are thru-hole devices, and their installation is one of the last operations when a printed circuit board is populated. (Often an after process.) This is slowly changing as a number of new pressure sensors are being offered as true surface mounted parts, offered in tape and real, and are sold as able to withstand solder reflow operations.
However, the first alarm bells need to go off when you ask the manufacturer for a solder reflow thermal profile for the sensors, and they do not have that information or refuse to pass along that information. This is by design and not an accident. The idea of a surface mountable package is simply to conform to existing IC packaging to be able to drive the cost of the sensor down.
Pressure sensors are not made for solder reflow operations – at least not in the traditional sense. A couple of steps need to be taken so that they can be used in a solder reflow operation….
1) There are some really conventional packages being offered as surface mounted pressure sensors. A few examples are the GE/NovaSensor NPC-1210, NPX sensors, and Measurement Specialties 1451 and 1471 ceramic devices. GE’s parts use a traditional SOIC 14 package, while the Measurement Specialties is a none-standard ceramic housing. The parts are offered on tape and reel, can be used with most pick and place machines. But do not be mistaken: Pressure sensors are not integrated circuits. While many of the packaging materials, thermoset plastic, silicon, ceramic epoxy, and Pyrex, are well understood and documented to the nth degree for thermal profiles, pressure sensor die is applied using an RTV.
The RTV adhesive is used in attaching the pressure sensors die to the housing whether the housing is plastic or ceramic. (Typically a Dow Corning 730 series or equivalent types of adhesives are used for this adhesion process.) If the profile does heat the RTV up to 170 C, the RTV will start to soften. As it softens, the offset will change or even cause the unit to fail if the wire bonds are stressed too far. In order to come up with a thermal profile, one needs to ask what RTV, if any, are being used in production of the pressure sensor. Once this is known, the thermal profile can be established. But additional care needs to be taken before production can start.
2) RoHs and other lead free initiatives have found their way into any and all solder reflow process. This great for the environment, as most solder cleaning are now water-soluble and the cleaning process is now mostly water. Great for cleaning the solder flux off the IC’s, but unfortunately pressure sensor have an open hole to expose one or both sides of the pressure die. The water can destroy, or worse, cause a temporary failure which comes and goes. This problem is easily avoided by simply adding a cap over the pressure sensor port(s) during the cleaning process. Just make sure that any material added over the pressure ports does not melt or out-gas while going thru the thermal profile.
Other problem may creep-up which might need to be addressed like the location of the unused vent tube, which is sometimes located under the housing (Like the Measuring Specialties packages). This can be hard, as the vent-hole needs to be clear and not plugged. However, for the most part, following these two steps will allow most board level pressure sensors to be used in a solder reflow operation.
These are just ideas if one needs to absolutely have a solder reflow process. At the end of day, most manufacturers do not advise that pressure sensors through a solder reflow operation.