Application Note AN101
Reflow Soldering
1. Introduction
The following application note is intended to describe the best methods for soldering sensors manufactured by MeritSensor using automated equipment. All profiles should be evaluated and tested for the best performance.
More and more companies are moving away from the use of conventional tin-lead (Sn/Pb) solder in circuit board
manufacturing because of concerns over the safety of lead and new regulations prohibiting its use, such as theRestriction on Hazardous Substances (RoHS) Directive in Europe. The RoHS Directive bans the European sale of
new electrical and electronic equipment containing more than the specified levels of lead, cadmium, mercury,hexavalent chromium, polybrominated biphenyl (PBB) and polybrominated diphenyl ether (PBDE) flame retardants.
All pressure sensors from Merit Sensor are RoHS compliant and are mounted on ceramic substrates. The lead-freesolder pads are plated with AgPt to ensure a good solder joint for most PCB connections.
Soldering of Merit Sensor parts can be done using either Pb-containing or Pb-free solder processes. Thisapplication note is intended to assist end users with soldering Merit Sensor parts using either Pb-free solder or Pbcontaining solder.
In order to meet the RoHS directive, products should be soldered with Pb-free solder.

2. Soldering with Pb-free solder
Since Merit Sensor’s pressure sensors are fabricated on ceramic, it is import to choose the Pb-free solder that iscompatible with the solder pads. Merit Sensor suggests using solder alloys with SnAgCu that have a melting pointof 217-221°C. Table 1 below shows the Pb-free solder alloys in the SnAgCu family.
MetalSn (tin)Ag (silver)Cu (copper)

Mix Ratio
95.5 – 96.5%
3.0 – 4.0%
0.5 – 0.7%

Table 1: Pb-free solder alloys of the SnAgCu family
Please note that the surface of Pb-free solder alloys can look substantially different when compared to Pbcontaining solder (See Figure 1). A Pb-free solder joint will have a dull, or matte, finish, when compared to a Pbcontaining solder joint because when Pb-free alloys start to cool, the surface of the solder joint will become rough.
This is due to an increased volume contraction of the Pb-free alloys. The Pb-free solder joints are usually smaller
than Pb-containing solder joints but should have no impact on the reliability since these characteristics are onlycosmetic.

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Figure 1: Examples of a Pb-containing solder joint (left) and a typical finished surface of a Pb-free solder joint (right)

A reflow soldering profile for Pb-free soldering requires a higher melting point when compared to Pb-containingsolders. The process time for Pb-free solder is shorter than Pb-containing solder, so it is important that thetemperature differences on the board be minimized. For this reason, Merit Sensor does not recommend IR Reflow
systems for Pb-free soldering. To ensure a successful Pb-free reflow soldering, Merit Sensor recommends using
forced convection reflow systems.
Merit Sensor’s pressure sensors can be soldered with profiles based on the standard IPC/JEDEC J-STD-020C(January 2004). It is important to evaluate each process in order to find the best temperature profile. The besttemperature profile will be defined by the board and the solder paste used. The suggested profile according toIPC/JEDEC J-STD-020C can be seen in Table 2 and Figure 2 below.

Profile Feature
Pb-Free Assembly(IPC/JEDEC J-STD-020 Revision C)
Average ramp-up rate (Tsmax to Tp)
Preheat– temperature Min (Tsmin)– temperature Max (Tsmax)– time (min to max) (ts)Time maintained above– temperature (TL)
– time (tL)Peak temperature (Tp)
Time within 5°C of actual peak temperature (tp)
Ramp down rate
Time 25°C to peak temperature

Package thickness ≥ 2.5 mm or
Package volume ≥ 350 mm3

Package thickness ≥ 2.5 mm or
Package volume < 350 mm3

3°C/second max.
150°C
200°C
60-120 seconds
217°C
60-150 seconds
245 +0/-5°C

250 +0/-5°C
20-40 seconds
6°C/second max.
8 minutes max.

Table 2: Pb-free classification reflow profile according to IPC/JEDEC J-STD-020C

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Figure 2: Pb-free Classification reflow profile according to IPC/JEDEC J-STD-020C

Use of nitrogen – Due to the increased temperature and oxidation of Pb-free solder, it may be necessary to work innitrogen if air leads to unsatisfactory solder joints. Most Pb-free solder pastes however, can be used in air. Nitrogenmay be used if the solder joints do not have sufficient wetting.
Hand soldering – Merit Sensor does not recommend hand soldering. Pb-free soldering requires an excess amountof energy compared to Pb-containing solder alloys. The heat transfer to the solder joint is very critical and should
not be attempted with a soldering iron.
If a soldering iron is used, it is important to remember that Pb-free requires a fast heat transfer to achieve a
successful solder joint. It may require a longer solder time and/or an increase in tip temperature to of 360-390°C. Itis highly recommended to use solder stations of at least 80 watts of power. Pre-heating can be used to reduce theamount of heat imposed on surrounding components during hand soldering, as is done with reflow soldering.

3. Soldering pressure sensors with Pb-containing solder
If Pb-containing solder is used, then it is important not to exceed temperatures of 225°C for 30 seconds. Meritpressure sensors should be soldered with “no-clean” type solder paste which contains 62%Sn36%Pb2%Ag and has
a melting point of 179°C. This solder paste contains 2%Ag and significantly reduces silver migration from the AgPt
pad into the solder paste. We recommend that you do not use 63%Sn37%Pb solder paste. See Table 3 and Figure
3 for proper reflow profile for SnPb solder.

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Profile Feature
Sn-Pb Eutectic Soldering(IPC/JEDEC J-STD-020 Revision C)
Average ramp-up rate (Tsmax to Tp)
Preheat– temperature Min (Tsmin)– temperature Max (Tsmax)– time (min to max) (ts)Time maintained above– temperature (TL)
– time (tL)Peak temperature (Tp)
Time within 5°C of actual peak temperature (tp)
Ramp down rate
Time 25°C to peak temperature

Package thickness ≥ 2.5 mm or
Package volume ≥ 350 mm3

Package thickness ≥ 2.5 mm or
Package volume < 350 mm3

3°C/second max.
100°C
150°C
60-120 seconds
183°C
60-150 seconds
225 +0/-5°C
10-30 seconds
6°C/second max.
6 minutes max.

Table 3: SnPb classification reflow profile according to IPC/JEDEC J-STD-020C

Figure 3: SnPb Classification reflow profile according to IPC/JEDEC J-STD-020C

If the reflow process if followed accurately, then the solder joint should cover the entire solder pad of the ceramic
PCB (See Figure 4 - left). Soldering by hand, in most cases, will result in overheating of the device due to the highthermal conductivity of the ceramic. Too low of a temperature will result in incomplete soldering, resulting in a weakconnection to the PCB as can be seen in Figure 4 (middle & right). The middle solder joint is an example of asufficient solder. However, that solder failed to wet out and cover the entire pad. The joint on the right was exposed
to insufficient solder and low heat, resulting in both poor pad coverage and a weak joint as the solder balled up. It isrecommended to attach a thermocouple to the sensor to optimize the solder profile and make sure none of themaximum temperatures are exceeded.

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Figure 4: Example of good solder joint (left) and bad solder joints (middle & right).

4. Stress normalization delay for calibration
For best results prior to calibration, Merit Sensor recommends that any surface mount pressure sensor be allowed
to rest at room temperature for least 48 hours prior to calibration. The stress caused by reflow soldering will usuallynormalize within this period and help improve the calibration of the product.

Disclaimer Notice
Merit Sensor reserves the right to modify this document and assumes no responsibility for any failures that result
from the use of this information.
Merit Sensor Systems produces high quality products that perform within the parameters of the data sheet. Typicalpressure and temperature performance values are not tested 100% but they have been validated during
qualification. Merit Sensor cannot guarantee that the product will function properly after mounting and postprocessing by the customer. It is the responsibility of the customer to test and to qualify the function of the MeritSensor product in the final package. Customer is responsible for the required knowledge to handle product andMerit Sensor assumes no liability for consequential damages that may result in yield loss or field failures in the finalapplication.
THE INFORMATION IN THIS DOCUMENT IS PROVIDED TO YOU “AS IS” WITHOUT WARRANTIES OR
CONDITIONS OF ANY KIND, WHETHER EXPRESS OR IMPLIED. Merit Sensor Systems reserves the right tomodify this document and assumes no responsibility for any failures that result in the use of this information. All
information provided in this document is for illustration purposes only.

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