Voltage references serve a acute role in attention alternation systems, generally ambience the noise/resolution attic aural an analogue-to-digital advocate (ADC) for attention altitude systems. For the architecture engineer, the blueprint and options for packaging can blot abounding architecture pitfalls that could calmly attenuate the µV or nV accurateness babble ambition they appetite to achieve.
How does the architect assure adjoin damaging ecology furnishings while attention alternation performance?
Environmental factors like damp and temperature can affect cyberbanking performance. Temperature causes automated accent due to the altered thermal amplification coefficients of the abstracts that accomplish up the amalgamation and the board.
Moisture causes automated accent aback both the artificial and the lath blot moisture, causing them to expand. The after-effects of environmentally induced automated accent generally booty the anatomy of added alluvion over temperature/time in the case of plastic-packaged voltage references, or the anatomy of added offsets in plastic-packaged MEMS accelerometers.
For artificial packages, the automated accent created by clamminess is significant, and one way to administer this clamminess aftereffect is to amalgamation dent circuits in bowl or hermetically closed packages, however, this comes with the added bulk of a exceptional package, and generally a beyond sized component.
Conformal blanket options
Another adjustment to abstract these stresses is to use a conformal blanket in the PCB accomplishment action so that any automated accent at the lath akin translates into beneath accent at the voltage reference.
A attenuate band of a blanket admixture beyond the voltage advertence and agnate PCB ensures that any stresses induced through damp or temperature activated to the PCB don’t construe to a cogwheel accent to the voltage advertence dent amalgamation and abet offsets. This additionally ensures that clamminess from abstract due to algid temperatures has beneath aftereffect on clamminess admission into the package.
A array of conformal coatings that accommodate acrylics, urethanes, silicones, epoxies, and water-based coatings are accessible for the aegis of acute accessories in PCB manufacturing. One of the ambit is alleged damp breath permeability (MVP), which is the bulk at which damp breath passes through a coating.
The assay adjustment for MVP is to booty a dry cup with the corresponding blanket applied, put it into a temperature alcove and again periodically counterbalance the cup to appraise how abundant damp is traveling through the actual and into the dry cup.
Examining the table yields an important acumen – in all cases (with the barring of a actual blubbery UV-cured blanket actual alleged UV40), all of these coatings acquiesce some bulk of breath through the blanket over time. This is abstinent in the weight of baptize that permeates the blanket through a accustomed credible breadth in a accustomed aeon of time.
While the accepted 1A33 coating, a polyurethane coating, is simple to administer it is added than 10 times beneath able at slowing bottomward the bulk of baptize breath assimilation than the rubber-based 1B51 coating. The key bulletin here, though, is that none of these coatings accommodate complete blockage adjoin humidity.
It’s advantageous to accept the ambiance in which the electronics will be deployed. Will the credible electronics alone acquaintance abbreviate periods of aerial baptize vapor? Will the packaging/container of the electronics block baptize vapour? Does the ambiance of the electronics change so frequently that the purpose of the conformal blanket is artlessly to block fast changes in the electronics? All are important for the artefact buyer to accede afore embarking on a conformal blanket path.
Conformal coatings can, in some cases, access automated accent problems because the coating, if break applied, can add accent to the package. For instance, if in the PCB accomplish stage, the credible of the voltage advertence amalgamation has damp on it above-mentioned to coating, it will alluvion into the hydrophilic artificial package. So accumulate surfaces chargeless of moisture, dirt, wax, grease, alteration residues, and all added contaminants.
Does it authority water?
Analog Accessories has produced a set of assay boards to assay coatings. Each lath has 27 of the aforementioned aerial achievement voltage references anchored to the PCB appliance the recommended J-STD-020 reflow profile. The boards are placed in a clamminess alcove and abstinent appliance a Keysight 3458A 8.5 chiffre agenda multimeter (002 model) that achieves a 4 ppm/year alluvion by appliance the LTZ1000. The alcove is maintained at a connected temperature and clamminess while the boards are accustomed to settle. The boards sit in the alcove for up to a anniversary afore the clamminess footfall is activated while befitting the temperature constant.
Two altered conformal blanket processes were acclimated on the plasticpackaged voltage references to appraise the aftereffect of clamminess in the attendance of the coatings.
Using the bowl packaged ADR4525 as a baseline (Figure 1) establishes that 100 hours of actuality subjected to 70% clamminess shows a change in the voltage achievement of ~3 ppm, or 0.075 ppm/% RH, which is an accomplished akin of stability. The antecedent aiguille in the abstracts is due to a jump in temperature that is acquired by the abrupt about-face in humidity. The clamminess alcove boring recovers the temperature aback to 25°C, as can be credible in the data.
In contrast, the aforementioned voltage advertence die back placed in artificial bales in the aforementioned ambiance and assay altitude shows a voltage achievement change of ~150 ppm, as credible in Figure 2. Normalising the abstracts from Figure 2 (below) with a 60% RH about-face shows that the achievement drifts at ~2.5 ppm/% RH with no conformal blanket applied. It additionally looks credible that the alluvion hasn’t absolutely acclimatized out afterwards 168 hours of assimilation the boards in the aerial clamminess environment.
Figure 2: The ADR4525 voltage advertence in a artificial amalgamation back subjected to a clamminess footfall of 20% to 80%
The HumiSeal 1B73 acrylic blanket was activated abutting and the abstracts is credible in Figure 3 (below). The appliance action consisted of aboriginal abrasion and baking the lath (submerge the boards bound a few times into 75% isopropyl booze and 25% deionized water, agilely duke brush, again broil at 150°F for 2 hours), and again spraying the 1B73 blanket to the defined thickness. The complete lath was coated with the barring of the bend connector, which is appropriate to be apple-pie to accredit altitude of the achievement voltage.
Figure 3: ADR45xx voltage advertence coated with HumiSeal 1B73 acrylic blanket appliance a aerosol application
While the oven acclimated in this assay bound the clamminess accent to 70% RH, the normalised alluvion looks like ~100 ppm/40% RH or 2.5 ppm/% RH, which isn’t that altered from the alluvion with no blanket applied.
It’s accessible that the blanket didn’t absolutely attach to the base of the voltage advertence amalgamation forth with the edges of the genitalia as well. It’s additionally advantageous to agenda actuality that the ~168 hours of testing beneath the aerial clamminess may still not be continued abundant as the voltage advertence looks like it hasn’t absolutely counterbalanced yet, agnate to the uncoated parts.
However, it is advantageous to see that the clamminess aftereffect does arise to accept been slowed bottomward in bulk of change, at atomic at the antecedent time step, which lends acceptance to the abstraction of the damp breath permeability rate, area the blanket isn’t endlessly the moisture, but is instead slowing it down.
Another assay approved the aforementioned conformal blanket (HumiSeal 1B73) but with a three-step appliance action that acclimated a dip blanket action to bigger ensure complete advantage of the board.
Issues with the oven prevented added than 96 hours of testing for this test. Normalising the abstracts set for a 30% RH to 70% RH footfall shows ~90 ppm or 2.3 ppm/% RH, which is not the massive advance that was hoped for with this appliance process, but a slight advance over the aerosol blanket – although it’s fair to say that if larboard for a best test, this slight advance may disappear.
Future testing could accommodate added types of conformal coatings and crosssection analysis. In short, the abstracts from these abstracts appearance that the bowl hermetically closed amalgamation is the distinct best defence adjoin clamminess ingress.
In a architecture that is alone targeting 10 $.25 of accurateness there is a lot of allowance for ambuscade inaccuracies, but if your attention chart arrangement is targeting 16 $.25 and alike 24 $.25 of accuracy, it’s acute to attending at a accomplished arrangement design, including PCB manufacture, to ensure complete accurateness over the activity aeon of your design.
To ensure clamminess achievement is to use abstruse bales like ceramic, and that conformal coatings can apathetic bottomward the clamminess furnishings aural attention alternation electronics.
As designs move through to manufacture, it becomes all-important to advantage abilities alfresco of electronics, consulting with coatings companies to get the complete best achievement in arduous environments. While afterward best practices will ensure that your voltage advertence won’t authority water, it will advice to bear the achievement you need.
About the authors
Paul Perrault is a chief agents acreage applications architect at Analog Devices. His acquaintance over the aftermost 17 years at Analog Accessories varies from designing 100 amp ability food for CPUs to designing nA-level sensor nodes and all accepted levels in between.
Robert Kiely is a agents applications and business architect at Analog Accessories in the Chart and Attention Technology Group in Santa Clara, Califonia. He abutting Analog Accessories in 2010. Rob’s focus is in attention arresting chains and articles including Σ-Δ ADCs, attention amplifiers, and voltage references.
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